This is intended to be a fairly detailed run through my own domestic solar PV project in the UK.
Four years ago I visited the "Solar & Storage Live" exhibition at Birmingham NEC.
This is an annual event mostly for the trade but open to the public.
It gave me an opportunity to see most of the major brands of inverters and panels in the flesh, to ask some questions and to identify some of the wholesalers who supply the installers and in some cases also the public.
Overall impressions: Solar PV was a mature technology and improvements are mostly incremental. Batteries are the new big thing. The industry seemed to be a bit down at the time. Feed in tariff had ended and demand for new systems was down.
I parked the idea for a couple of years and went back to the show last year, with my engineering head on, and specifically looking to shortlist the panels, inverters and battery that I might buy. I had no intention of doing the job myself but I would shortlist the kit and get quotes from installers who could supply and fit it.
From the show, solar panels that interested me :
Inverter manufacturers :
Batteries :
I was looking simply for high quality product, indication that the manufacturers were large and reputable, and for some degree of integration - not just a random collection of bits.
I was also interested in the solar panel mounting hardware because one of the real risks I anticipated was poor installation leaving me with a leaking roof and unsecure panels. I came away understanding that there are high quality roof mounts available and cheaper + less well made alternatives.
Shortlisting the components
Panels
Modern solar panels pretty much evolved from the space program decades ago. These panels use so called "P type" silicon. If you ever heard of a PNP and NPN transistors in school physics, it's the same idea. If not, don't worry and just follow along.
P type silicon proved to be the most long lived, and has been developed over the years. The latest flavour being called PERC. This is used to make the vast majority of panels and an excellent guarantee is 25 years plus a performance guarantee that the panel will still be achieving 85% of it's original output after 25 years in service.
A panel is simply a metal frame, an impact resistant glass top sheet covering typically 60 individual solar cells all connected by thin metal tracks that are printed, not actual wires, and a backing sheet of some type. A panel has a DC+ and DC- connection on the back and you wire several panels up in series (just like batteries in a toy) to make a "string".
There are some functional and cosmetic differences between panels - like the all black look and different ways of arranging the (printed) electrical connections that connect all the cells together in the panel. These can make small improvements in efficiency and make the wiring unnoticeable from a distance. The majority of modern panels are very similar in size and performance. Differences are subtle. The main thing is to buy from a reputable company that achieve consistently high reliability.
In recent years, studies have been done on the other type of solar cell. "N type" silicon. N type doesn't last as well in space because it suffers more damage from cosmic rays, but on the ground that doesn't matter and it is better than P type in other ways.
The upshot is that a small number of companies make premium panels out of N type silicon, and these can be bought with longer guarantees (25 years) and higher guaranteed outputs (90% of original output after 40 years !).
I decided that I wanted this technology. In a decade it will be mainstream but for now the choice is limited : REC Alpha (Norwegian), Jinko Tiger N (Chinese), LG Neon R (Korea) or Sunpower Maxeon 3 (USA + Mexico). The long life and high performance matched my lifetime buy objective and a price premium of 15% ish felt acceptable.
In practice, global supply chain was a nightmare this Easter. No one had any of the good stuff. I first shortlisted the Tiger N panels for value and while we were looking for them, I discovered that a lot of Chinese solar manufacture is tainted by reports of forced labour. Not OK.
I chose LG Neon next as this is a reputable Korean company, but LG withdrew from solar panel manufacture this summer !
Eventually my chosen installer stumbled on a supplier with Sunpower Maxeon 3 panels in stock, took 3 days to work up the courage, then suggested them to me. Maxeon 3 are the best made panels in the world. The company is Californian and the panels made in Mexico. I omitted them from my original list simply because I knew of their reputation and assumed they would be unaffordable. They cost me about 50% more than a good PERC panel but having realised how tough the supply chain problems were, I said yes. They have turned out to be the most impressive part of the system. Simply superb.
I'll explain the choice and layout of the panels later, but for now, I'll tell you that someone needs to consider the physical size and shape of the roof and work out the best way to arrange panels on it. I did all this for my roof and ended up with a simple, clean layout. 8 panels wide, 3 panels tall giving 24 panels in total. The maxeon 3 panels just fit my roof leaving enough space above and below. 375 W apiece x 24 = 9kW peak output.
Inverter
Solar panels output their energy in DC (direct current). 40 or 50Volts per panel is common. My Maxeon 3 panels make over 70V DC apiece. Panels are wired in series as "strings" that can be 400V, 600V, even more.
The inverter is a box of electronics that converts DC from the panels to 240V AC mains and feeds it to your consumenr unit through a circuit breaker - typically 20 or 30 Amp rated.
The AC from the inverter is exactly synchronised with the electricity coming in from the grid so that they play nicely together. It's just at a slightly higher voltage which allows the inverter to push it's energy into your home or back into the grid if you can't use it. The AC voltage from the inverter constantly adjusts, more sun, more power generated, slightly higher voltage to push that power out.
The inverter can be running 16 hours straight over a long day, handling several kWatts, and doing it 365 days a year for a decade or more. It's a pretty demanding job and the electrical eng in me thinks that a good quality, well designed and built piece of electronics matters if you want a long life and reliability. I was absolutely looking for something built to a "commercial" or "industrial" standard, not something that felt like a domestic appliance that you would expect to last 3 to 5 years.
Batteries
You have to consider batteries and inverter together.
When adding a battery to a solar PV system, there are two ways to do it.
First is to use an AC coupled battery which simply wires in to any new or existing system using 240V AC. The battery includes a charger. This approach is simple but has the advantage that an AC battery can be added to any solar setup, including ones already in service. Also, any battery can be matched with any inverter so you have freedom to mix and match. Tesla Power Wall is an AC coupled battery.
Second is to use a DC coupled battery matched with a specific inverter.
Batteries store power as DC. Solar panels make DC.
For maximum efficiency and maximum intelligence, the inverter can double up as a battery charger. It takes DC power from the panels and feeds it still as DC to charge the battery. There is no need to convert to AC in between and this improves efficiency. The battery pack doesn't have a charger built in because the inverter does this task.
The inverter draws DC power either from the solar panels and / or from the battery and converts this to AC to supply the needs of the house. This type of inverter is smart and has full visibility of the power in and out of the battery which a simple inverter with an AC battery does not. It is the better system if you are putting in a new, solar + battery system. The downside is that you need the battery and inverter to be designed to work together like this. That limits the available choices.
Battery size
How deep are your pockets ?
Our average daily electricity consumption is very typical for our size of home and family of 3-4.
Measured over the last couple of years it's 13kWh a day. We have gas hot water and central heating
The amount we use while the sun is down is maybe half that. We often use the oven for an evening meal.
We could choose a battery that held enough power to take us through the average "night" so just 6kWh say.
We could choose a battery big enough to carry us through an entire average day, so 13kWh
In the event, I decided that I wanted a battery that could feed a 6kW load - this is our oven plus a kettle in the evening or oven plus microwave. whatever.
It's a bit arbitrary, but small batteries can't supply that much power. and that 6kW load figure meant that I needed a battery with 16kWh of capacity. A bit more than one entire, average, day.
That's a lot of storage. About £10k and almost half the cost of the entire installation.
It means that we will be self sufficient - able to move the power generated by the solar to when we want it - almost 100% - for about 10 months of the year. Only in the two coldest months of the year is our solar generation predicted to fall to 8kWh a day. For the rest of the year, the solar will supply our average daily needs or much more and the battery will hold more than the average day's demand and deliver it when we need it. Overnight or during the next cloudy day.
If we have a run of bad weather we need the grid so never 100% but well up there.
Without the battery our self consumption will be hard pushed to reach 50%, 25-50% is more likely.
But ignoring inflation and Net Present Value calculations, I think the battery will take over 10 years to pay back and I genuinely don't know how long it will last for us. Potentially long enough and maybe much more, but this comes down to what we're willing to pay to be more green. We're willing to take the risk that this system will be more expensive than the grid in the end, in order to do our bit and not be so much at the mercy of governments and energy companies in future.
Battery Chemistry and Life
I didn't know as much about batteries when I chose mine as I do now, but reading around trends in EV battery tech is helpful.
For cars, people tend to want range and the greatest amount of energy storage for the least amount of weight. The batteries that fit that bill in 2021 and 2022 are Lithium Nickel, Manganese (?) Cobalt. So called NMC technology. This tech is expensive, uses Cobalt which is in short supply and a humanitarian car crash, more prone to fires, and lastly has a poor cycle life - around 1,000 charge discharge cycles before capacity drops to 80% of new. That could be just 3 years. Maybe 6yrs if you take it down to 60% of new capacity.
It's light weight but sitting in your house, who cares ?
For houses, Lithium, Iron (aka Fe), Phosphate (aka PO4) is a better tech. LFP batteries are more stable, less fire risk, much longer lasting (4,000 maybe 6,000 and max perhaps 8,000 charge discharge cycles) and cheaper. It's far heavier but improving. LFP batteries are used in buses and will be going into the base model Tesla3 made in China and many more new EVs. LFP batteries going in cars hold about 2x as much energy per Kg as the ones I'm still waiting for to put in the house. My battery stack weighs 1/4 ton, 250 odd Kg.
A huge factor in this is trying to guess what lifespan my storage battery will have. Cost divided by lifetime energy throughput will give a very real "pence per kWh" figure that has to be costed into all the energy that is temporarily stored in the battery.
LiFePO4 battery warranty is typically 10 years.
At one full cycle per day that's 3,650 cycles. A LFP battery should do that. An NMC battery is unlikely to be doing well after that many cycles.
LFP in fact may well be usable even at 6,000 cycles. At once per day, that's nearing 20 years as long as your battery was a bit oversized to start with.
20 years is somewhere near the calendar age limit of lithium batteries. They die of old age even if never cycled at all.
Putting all this into a hat and shaking it up, I figured my own eventual selection of batteries have a chance of lasting closer to 20 yrs than 10, and will cost me somewhere near 12p / kWh stored over that lifetime. A long way from free, but a lot less than the power company charge if I can't store my daytime generation and have to buy electricity in the evenings.
Inverter and Battery Selection
In the end, I had only two serious candidates because I wanted a "hybrid DC" inverter and battery solution described above.
One was "GivEnergy" - a very new company based in Stoke on Trent, UK but with overseas investors and partners. They are growing very fast because they offer inverters, batteries and an intelligent control that works with some of the most innovative energy company agile tariffs. The snag is that their inverter is 5kW and battery 8kWh. I would need two inverters and two batteries in my system and even then it was only rated for 5kW maximum discharge from the pair of batteries. It's a new company, unproven.
The second, and the option that I chose, was to pair inverters made by Fronius in Austria with batteries made by BYD in China.
These are separate companies, but they are cooperating to make their products work together to deliver a solution. BYD batteries are tested to work with Fronius inverters.
Fronius are a family owned firm that specialise in power electronics. They make just 3 groups of product: welders, forklift truck battery chargers and solar inverters. The core competence in power electronics is the same for all three.
They have an excellent reputation and I own one of their Magicwave AC DC Tig welders. It's design and build is very, very good. Up there with Kempii and Miller, arguably better than both. I've also dealt with their technical support and found them good. All this gave me confidence that the inverters should be good.
BYD was a real unknown, but if a respected supplier chose to partner with them they might be OK. It turns out BYD are the largest manufacturer of batteries for industry / traction application in China and I think the world. They make more batteries than Tesla and their batteries are the ones that power the new electric bus fleet in Sydney, Aus. Their batteries are LiFePO4 type prismatic cells that are better suited to long life needs of domestic energy storage than the car battery technology used in the Tesla Power Wall. Only time will tell if this is right, but an Australian facility that has been testing solar storage batteries and making the results public has tested the BYD battery and it lost very little capacity. The results give me some hope that battery could manage a 20 year useful life, maybe more,
- I'm a retired electrical engineer in my early 60's.
- I'm as concerned as everyone else about rising fuel costs
- We would like to be more "green". After travelling some 1.5 million miles in the course of my working life I've certainly done enough damage to the planet and agree that we need to consume less. This project wasn't done on the basis of financial returns.
- Our house has a relatively large SE facing roof for the size of the place. Over the years we've felt how the sun can heat up the roof and have long felt that it would be well suited to solar energy.
- We don't intend to move house - at my age I'm looking at this as a lifetime buy.
- We were in a position to afford solar and any delay just meant we would get less benefit from the spend so now was the time to do it.
Four years ago I visited the "Solar & Storage Live" exhibition at Birmingham NEC.
This is an annual event mostly for the trade but open to the public.
It gave me an opportunity to see most of the major brands of inverters and panels in the flesh, to ask some questions and to identify some of the wholesalers who supply the installers and in some cases also the public.
Overall impressions: Solar PV was a mature technology and improvements are mostly incremental. Batteries are the new big thing. The industry seemed to be a bit down at the time. Feed in tariff had ended and demand for new systems was down.
I parked the idea for a couple of years and went back to the show last year, with my engineering head on, and specifically looking to shortlist the panels, inverters and battery that I might buy. I had no intention of doing the job myself but I would shortlist the kit and get quotes from installers who could supply and fit it.
From the show, solar panels that interested me :
- Hanwha Q cells - Korea - PERC technology - very nicely built with good warranty
- Jinko Tiger N - Chinese - N type silicon - good reliability scores
- LG Neon R - Korean - N type silicon - good reliability scores
- REC Alpha - Norwegian owned - N type silicon -
- JA Solar and Trina - both Chinese - also had decent panels on offer but not as refined as the others above.
Inverter manufacturers :
- Fronius - Austrian - premium brand, very high quality design and build
- SMA - German - makers of the reputable Sunny Boy inverters - very well built
- GivEnergy - UK - innovative and intelligent (as in automated) set of products but very new company
- Solar Edge - German - well established and proven - broad set of products that work together and quite intelligent - have a very good solution for roofs subject to shadows - build quality looks much lower than Fronius or SMA to me
Batteries :
- GivEnergy
- There were a number of battery suppliers there including the majors like the Tesla Powerwall, but I found the products hard to assess and came away unconvinced. Few impressed me.
I was looking simply for high quality product, indication that the manufacturers were large and reputable, and for some degree of integration - not just a random collection of bits.
I was also interested in the solar panel mounting hardware because one of the real risks I anticipated was poor installation leaving me with a leaking roof and unsecure panels. I came away understanding that there are high quality roof mounts available and cheaper + less well made alternatives.
Shortlisting the components
Panels
Modern solar panels pretty much evolved from the space program decades ago. These panels use so called "P type" silicon. If you ever heard of a PNP and NPN transistors in school physics, it's the same idea. If not, don't worry and just follow along.
P type silicon proved to be the most long lived, and has been developed over the years. The latest flavour being called PERC. This is used to make the vast majority of panels and an excellent guarantee is 25 years plus a performance guarantee that the panel will still be achieving 85% of it's original output after 25 years in service.
A panel is simply a metal frame, an impact resistant glass top sheet covering typically 60 individual solar cells all connected by thin metal tracks that are printed, not actual wires, and a backing sheet of some type. A panel has a DC+ and DC- connection on the back and you wire several panels up in series (just like batteries in a toy) to make a "string".
There are some functional and cosmetic differences between panels - like the all black look and different ways of arranging the (printed) electrical connections that connect all the cells together in the panel. These can make small improvements in efficiency and make the wiring unnoticeable from a distance. The majority of modern panels are very similar in size and performance. Differences are subtle. The main thing is to buy from a reputable company that achieve consistently high reliability.
In recent years, studies have been done on the other type of solar cell. "N type" silicon. N type doesn't last as well in space because it suffers more damage from cosmic rays, but on the ground that doesn't matter and it is better than P type in other ways.
The upshot is that a small number of companies make premium panels out of N type silicon, and these can be bought with longer guarantees (25 years) and higher guaranteed outputs (90% of original output after 40 years !).
I decided that I wanted this technology. In a decade it will be mainstream but for now the choice is limited : REC Alpha (Norwegian), Jinko Tiger N (Chinese), LG Neon R (Korea) or Sunpower Maxeon 3 (USA + Mexico). The long life and high performance matched my lifetime buy objective and a price premium of 15% ish felt acceptable.
In practice, global supply chain was a nightmare this Easter. No one had any of the good stuff. I first shortlisted the Tiger N panels for value and while we were looking for them, I discovered that a lot of Chinese solar manufacture is tainted by reports of forced labour. Not OK.
I chose LG Neon next as this is a reputable Korean company, but LG withdrew from solar panel manufacture this summer !
Eventually my chosen installer stumbled on a supplier with Sunpower Maxeon 3 panels in stock, took 3 days to work up the courage, then suggested them to me. Maxeon 3 are the best made panels in the world. The company is Californian and the panels made in Mexico. I omitted them from my original list simply because I knew of their reputation and assumed they would be unaffordable. They cost me about 50% more than a good PERC panel but having realised how tough the supply chain problems were, I said yes. They have turned out to be the most impressive part of the system. Simply superb.
I'll explain the choice and layout of the panels later, but for now, I'll tell you that someone needs to consider the physical size and shape of the roof and work out the best way to arrange panels on it. I did all this for my roof and ended up with a simple, clean layout. 8 panels wide, 3 panels tall giving 24 panels in total. The maxeon 3 panels just fit my roof leaving enough space above and below. 375 W apiece x 24 = 9kW peak output.
Inverter
Solar panels output their energy in DC (direct current). 40 or 50Volts per panel is common. My Maxeon 3 panels make over 70V DC apiece. Panels are wired in series as "strings" that can be 400V, 600V, even more.
The inverter is a box of electronics that converts DC from the panels to 240V AC mains and feeds it to your consumenr unit through a circuit breaker - typically 20 or 30 Amp rated.
The AC from the inverter is exactly synchronised with the electricity coming in from the grid so that they play nicely together. It's just at a slightly higher voltage which allows the inverter to push it's energy into your home or back into the grid if you can't use it. The AC voltage from the inverter constantly adjusts, more sun, more power generated, slightly higher voltage to push that power out.
The inverter can be running 16 hours straight over a long day, handling several kWatts, and doing it 365 days a year for a decade or more. It's a pretty demanding job and the electrical eng in me thinks that a good quality, well designed and built piece of electronics matters if you want a long life and reliability. I was absolutely looking for something built to a "commercial" or "industrial" standard, not something that felt like a domestic appliance that you would expect to last 3 to 5 years.
Batteries
You have to consider batteries and inverter together.
When adding a battery to a solar PV system, there are two ways to do it.
First is to use an AC coupled battery which simply wires in to any new or existing system using 240V AC. The battery includes a charger. This approach is simple but has the advantage that an AC battery can be added to any solar setup, including ones already in service. Also, any battery can be matched with any inverter so you have freedom to mix and match. Tesla Power Wall is an AC coupled battery.
Second is to use a DC coupled battery matched with a specific inverter.
Batteries store power as DC. Solar panels make DC.
For maximum efficiency and maximum intelligence, the inverter can double up as a battery charger. It takes DC power from the panels and feeds it still as DC to charge the battery. There is no need to convert to AC in between and this improves efficiency. The battery pack doesn't have a charger built in because the inverter does this task.
The inverter draws DC power either from the solar panels and / or from the battery and converts this to AC to supply the needs of the house. This type of inverter is smart and has full visibility of the power in and out of the battery which a simple inverter with an AC battery does not. It is the better system if you are putting in a new, solar + battery system. The downside is that you need the battery and inverter to be designed to work together like this. That limits the available choices.
Battery size
How deep are your pockets ?
Our average daily electricity consumption is very typical for our size of home and family of 3-4.
Measured over the last couple of years it's 13kWh a day. We have gas hot water and central heating
The amount we use while the sun is down is maybe half that. We often use the oven for an evening meal.
We could choose a battery that held enough power to take us through the average "night" so just 6kWh say.
We could choose a battery big enough to carry us through an entire average day, so 13kWh
In the event, I decided that I wanted a battery that could feed a 6kW load - this is our oven plus a kettle in the evening or oven plus microwave. whatever.
It's a bit arbitrary, but small batteries can't supply that much power. and that 6kW load figure meant that I needed a battery with 16kWh of capacity. A bit more than one entire, average, day.
That's a lot of storage. About £10k and almost half the cost of the entire installation.
It means that we will be self sufficient - able to move the power generated by the solar to when we want it - almost 100% - for about 10 months of the year. Only in the two coldest months of the year is our solar generation predicted to fall to 8kWh a day. For the rest of the year, the solar will supply our average daily needs or much more and the battery will hold more than the average day's demand and deliver it when we need it. Overnight or during the next cloudy day.
If we have a run of bad weather we need the grid so never 100% but well up there.
Without the battery our self consumption will be hard pushed to reach 50%, 25-50% is more likely.
But ignoring inflation and Net Present Value calculations, I think the battery will take over 10 years to pay back and I genuinely don't know how long it will last for us. Potentially long enough and maybe much more, but this comes down to what we're willing to pay to be more green. We're willing to take the risk that this system will be more expensive than the grid in the end, in order to do our bit and not be so much at the mercy of governments and energy companies in future.
Battery Chemistry and Life
I didn't know as much about batteries when I chose mine as I do now, but reading around trends in EV battery tech is helpful.
For cars, people tend to want range and the greatest amount of energy storage for the least amount of weight. The batteries that fit that bill in 2021 and 2022 are Lithium Nickel, Manganese (?) Cobalt. So called NMC technology. This tech is expensive, uses Cobalt which is in short supply and a humanitarian car crash, more prone to fires, and lastly has a poor cycle life - around 1,000 charge discharge cycles before capacity drops to 80% of new. That could be just 3 years. Maybe 6yrs if you take it down to 60% of new capacity.
It's light weight but sitting in your house, who cares ?
For houses, Lithium, Iron (aka Fe), Phosphate (aka PO4) is a better tech. LFP batteries are more stable, less fire risk, much longer lasting (4,000 maybe 6,000 and max perhaps 8,000 charge discharge cycles) and cheaper. It's far heavier but improving. LFP batteries are used in buses and will be going into the base model Tesla3 made in China and many more new EVs. LFP batteries going in cars hold about 2x as much energy per Kg as the ones I'm still waiting for to put in the house. My battery stack weighs 1/4 ton, 250 odd Kg.
A huge factor in this is trying to guess what lifespan my storage battery will have. Cost divided by lifetime energy throughput will give a very real "pence per kWh" figure that has to be costed into all the energy that is temporarily stored in the battery.
LiFePO4 battery warranty is typically 10 years.
At one full cycle per day that's 3,650 cycles. A LFP battery should do that. An NMC battery is unlikely to be doing well after that many cycles.
LFP in fact may well be usable even at 6,000 cycles. At once per day, that's nearing 20 years as long as your battery was a bit oversized to start with.
20 years is somewhere near the calendar age limit of lithium batteries. They die of old age even if never cycled at all.
Putting all this into a hat and shaking it up, I figured my own eventual selection of batteries have a chance of lasting closer to 20 yrs than 10, and will cost me somewhere near 12p / kWh stored over that lifetime. A long way from free, but a lot less than the power company charge if I can't store my daytime generation and have to buy electricity in the evenings.
Inverter and Battery Selection
In the end, I had only two serious candidates because I wanted a "hybrid DC" inverter and battery solution described above.
One was "GivEnergy" - a very new company based in Stoke on Trent, UK but with overseas investors and partners. They are growing very fast because they offer inverters, batteries and an intelligent control that works with some of the most innovative energy company agile tariffs. The snag is that their inverter is 5kW and battery 8kWh. I would need two inverters and two batteries in my system and even then it was only rated for 5kW maximum discharge from the pair of batteries. It's a new company, unproven.
The second, and the option that I chose, was to pair inverters made by Fronius in Austria with batteries made by BYD in China.
These are separate companies, but they are cooperating to make their products work together to deliver a solution. BYD batteries are tested to work with Fronius inverters.
Fronius are a family owned firm that specialise in power electronics. They make just 3 groups of product: welders, forklift truck battery chargers and solar inverters. The core competence in power electronics is the same for all three.
They have an excellent reputation and I own one of their Magicwave AC DC Tig welders. It's design and build is very, very good. Up there with Kempii and Miller, arguably better than both. I've also dealt with their technical support and found them good. All this gave me confidence that the inverters should be good.
BYD was a real unknown, but if a respected supplier chose to partner with them they might be OK. It turns out BYD are the largest manufacturer of batteries for industry / traction application in China and I think the world. They make more batteries than Tesla and their batteries are the ones that power the new electric bus fleet in Sydney, Aus. Their batteries are LiFePO4 type prismatic cells that are better suited to long life needs of domestic energy storage than the car battery technology used in the Tesla Power Wall. Only time will tell if this is right, but an Australian facility that has been testing solar storage batteries and making the results public has tested the BYD battery and it lost very little capacity. The results give me some hope that battery could manage a 20 year useful life, maybe more,
Last edited:
