James Thinks
writing is a kind of thinking
Solar panels are now a regular sight on rooftops, at least where I live in Wiltshire. We've had them nearly three years, so I'll share our experiences of installation and use.
Installation
We asked two companies to quote for our solar install, both based in the Bristol area. Solarsense and 1World Solar. The latter are not installing in the Bristol area anymore. We asked about battery options, but decided to go just for the panels as the cost of batteries was high and they didn't have all the features we would've liked.
Both seemed pretty good and gave similar quotes around the £5000 mark for a 3.7kW array of 12 panels. These days I believe newer panels can deliver more power than that.
In the end we went for 1World Solar as they gave slightly better answers to our early questions.
We discussed the options of using optimisers so that the partial shading from trees we get early morning would not cause the overall efficiency to drop by too much. In the end we took the advice that the additional cost of optimisers would be better spent on more powerful panels resulting in more total energy out.
They arranged all aspects of the install including sourcing all materials, wiring and scaffolding. They installed a GroWatt inverter, arguably not the best in some ways, but it has done the job.
The inverter was installed in our loft and has a small screen on the front which provided current and total energy displays as well as a couple of other numbers.
When I realised that there was little to no data on output available to me, I asked 1World Solar and they sold me a WiFi dongle for around £40. This was fiddly to set up, but provided access to a website showing a graph of recent generation. The site was rather awkward and partially translated from Chinese and there was apparently no way to download a CSV/JSON file. So I had no way to keep records of the data. What's more the dongle stopped working after about three months and while 1World Solar said they would replace it, they didn't reply to my further emails, so I shrugged and got on with my life.
Living with solar panels
Savings
Undoubtedly, solar panels will save you money. The typical payback period is 6 to 10 years. There are calculators by Solar Wizard and the Energy Saving Trust. However, I don't think the values are likely to be very accurate. For one thing, it seems Solar Wizard overestimates the cost of install compared with recent quotes.
With both money and carbon, it's difficult to quantify exactly how much you're saving.
For carbon, again you will certainly be producing less carbon per kWh of energy by using solar power, but it is not zero. As solar panels have some embodied carbon from their construction and transport, there's also a carbon payback period, which is said to be shorter at 1 to 4 years (PDF). To know exactly what you're saving, you'd need to know how carbon intensive your grid is at the times you're using your own solar. You could use a service like Carbon Intensity, but bear in mind that it relies on the UK and EU assumption that biomass is zero emissions, which is rarely the reality, especially when old-growth trees are being imported from North America to fuel it.
For money, it depends on how much electricity you're using during the day. If you mostly heat your home with gas, the savings will be modest. Perhaps you use an electric oven, but this is often in the evening when there is little or no solar power. It will also depend on your daytime electricity tariff and how much you get paid for exporting.
Unfortunately the little screen on our Growatt inverter stopped working after a couple of years, so the only information I have on solar panel performance is from our smart meter, which will show a negative value if we're using less than the panels are generating. However, this is always a net figure - there's no way to separate generation from consumption.
Energy generation
So how much do we get?
First, a small note on the units I'm using.
- Watts (W) are a unit of power - how much is being generated at any given moment. kilowatts (kW) is 1000 watts.
- kilowatt-hours (kWh) are a unit of energy, or power over time. So producing (or consuming) 1kW for an hour is 1kWh.
It seems that anytime there is daylight the panels will generate something. When the sky starts to lighten before dawn that something might only be 25W. Under a blue sky, that rises to the full 3.7kW by 11am. Although our panels face South-East, they are partly shaded until roughly 9am in the summer and 10am in the winter. About 2pm the total output starts to drop as the sun moves to the other side of the house, but on a good summer's day, we might still be getting 1kW at 4pm from indirect sunlight. In total, the very best days of summer might produce 22kWh of energy, while the shortest, dullest days of winter might only yield 1kWh.
The above chart shows export, not generation; lower values may be cloudy days or days when we consumed more energy.
Note that even on dark, rainy days, the solar panels do generate some power, it's just less than a bright sunny day. In summer at midday during heavy rain, we might get 300W of power, which is a lot better than nothing.
Curiously when the sun is behind the trees, as it is for us in the early morning, I've often observed more power from the panels when the sky is slightly hazy or overcast than when it is cloudless. It seems that hazy sunshine scatters and relfects off light clouds and still provides significant energy.
It is also noticeable that output is slightly reduced on extremely hot days. Solar panels are less efficient as the temperature rises so, when it's 30 degrees or more, output appears drop to 90% of what I'd otherwise expect.
During summer months we're typically exporting more than we import, in spite of having an air-source heat pump which uses electricity for hot water and central heating.
Usage habits
Domestic solar installations are typically set up so that any energy you generate is consumed within the home if you have sufficiently powerful appliances switched on. Any excess is exported. If the solar panels are generating less than you're consuming at any moment, the difference is made up by importing from the grid.
To be paid for export you will need a smart meter, ideally SMETS2.
For a long time we were on a low 4.1p/kWh export tariff before we could convince Octopus that we had a smart meter capable of providing half-hourly values. Now we're on a 15p/kWh tariff and our summer export earns us £40-£50 a month.
On our previous tariff, importing electricity from the grid during the day was costing us around 34p/kWh, while using our generated solar, instead of exporting it, cost 4.1p/kWh (in missed export earnings). This meant it made good financial sense to use as much power during the day as possible, so I'd make a game of checking the weather forecast and if it was bright, putting on washing, using the bread machine, etc. The tricky part of this was that if the sun was to go behind a cloud, the remaining power might not cover all our appliances and we'd be importing the difference at the high daytime rate. Appliances often have "spiky" output, consuming maybe 150W for most of a washing cycle, rising to 2kW when heating the water, so it can be hard to make efficient use of solar while avoiding all import.
As a result, during winter I guessed it to be more cost-effective to run these appliances in the small hours when our import tariff was 15p/kWh than to try to make use of a few hundred watts of solar and end up importing a fair bit at peak rate.
Later we managed to swap onto a better tariff, "Cosy Octopus", with export paid at 15p/kWh and two cheap periods at 11p/kWh, one during the day, the other at night. The thinking is that these are the times during which you would run your heat pump and the house would hopefully be well-insulated enough to retain that heat during the peak period in the early evening. Indeed this is how I have the heat pump programmed and we seem to be using very little peak-rate electricity. Interestingly, it also creates some counter-intuitive incentives. One of the cheap periods is in the early afternoon, during which I might want to run intensive appliances. However, if the energy use is fully covered by our solar power, no import will occur, meaning I'm not making use of that 11p/kWh rate, but losing the 15p/kWh export. As a result I would be better off, financially, running the dishwasher during the nighttime cheap rate and exporting as much as possible during the day.
Perhaps I'm over-optimising because I happen to find these things interesting. It's certainly not necessary to put this much effort into analysing electricity usage and timings. Users who don't change their behaviour will still benefit, just not so quickly. For my part, I consider the consciousness of where my electricity is coming from to be beneficial in making me appreciate it more. No longer do I take for granted that there will always be abundant cheap energy whenever I need it. I'm thinking not only about efficiency, but sufficiency; how much energy do we really need?
Conclusions
For our house, solar panels have been an excellent investment and certainly brought our energy costs and emissions down significantly. This is even more significant since we also installed a heat pump, replacing most of our gas use with electricity. Until electricity prices align properly with gas prices, heat pumps are only going to save users a little money. Solar panels make them much more cost-effective. It's especially useful having a hot water storage tank which can act like a battery, heating up when the electricty is cheap and storing it until it's used. Some people use a solar diverter to achieve this, but if you have a heat pump it provides additional efficiency.
From an environmental stanpoint, however you use the energy, increasing the renewables on the grid is a good thing. Solar and wind should displace fossil fuel energy sources such as gas. However, if energy demand continues to grow, partly as a result of more data-centres, it may be difficult to retire any energy sources and decarbonise the grid.
More information
For more on the various options on solar panels, I'd recommend the YouTube Channel Gary Does solar.
Alternative options
If you want to invest in renewables for environmental or financial reasons, solar panels on your roof are not the only option. You may lack suitable roof space, have insufficient funds for the upfront investment or maybe you're planning to move house soon. One alternative is to invest in a Ripple wind or solar farm and get money off your bills in proportion to your investment. You can take this investment with you if you move house.
If you're keen to learn and have the time, do-it-yourself solar options are described on the Low-Tech Magazine.
If you'd like to connect to and control the various devices in your home and like a bit of tinkering, consider the open-source Home Assistant. This can measure solar panel and battery performance and control a wide variety of smart appliances.
James Bradbury
I write about whatever is on my mind. I do so mostly to help me think more clearly. If other people find it interesting that's good too. :-)
Undoubtedly, solar panels will save you money. The typical payback period is 6 to 10 years.
During summer months we export more than we import, in spite of having an air-source heat pump
To be paid for export you will need a smart meter, ideally SMETS2