Last February, when the world was a very different place, Spirit installed SunPower panels in the roof of my new home - read all about the installation process here.
So it’s been just over a year now that I’ve been living with solar panels. Although not the best of years, it has been nice to work from home to maximise the use of my own solar energy. But how is my PV system doing? Has it performed anywhere near what my quote promised? And how have my behaviour, bills and carbon footprint changed?
Let’s start with kWh. My monitoring methods are reading the generation meter annual total, which is the most accurate way of monitoring production, and the Growatt online portal, which is based on inverter calculations. How did these compare?
- Generation meter: 4,644 kWh.
- Growatt total: 4,638 kWh.
So despite being at the cheaper end of the market, the Growatt monitoring seems to correlate surprisingly well, being only 6kWh off the correct annual total!
How does this compare to annual predictions?
The PV quote is based on MCS estimates, which account for orientation, pitch and postcode, giving an annual output of 4,242 kWh.
PV Sol, which factors in the 1% shade reduction from a small parapet on my roof, gave an annual output of 4,051 kWh.
So in real life, the system generated nearly 10% more than estimated. My system is 10 degrees off south at 45 degrees, which may explain how the system is performing so well with SunPower 350W panels.
We do hear of worries that in-roof panels can potentially decrease annual output, due to decreased air flow making cells operate at a warmer and less efficient output, but based on my own kWh totals I have not found this problem reflected in the performance.
What was the best generation day?
There were many good days from March - July, but the top output was April 20th at 28.7kWh in a day - not bad for a 4.2kWp system! Cold days give a better output, so the peak days will require a nice crisp cold day with clear sunlight. You notice a significant loss of power on days where it is very hot, over 25-30°C.
And the worst? December 18th produced a mighty 0.1kWh. This is why annual kWh is so important; it eliminates the day to day variance and gives a surprisingly accurate assessment of output, allowing you to compare how one PV system performs compared to another regardless of daily weather variation.
Interestingly, my system does hit 4.2kW output on some days in spring and summer, as it is facing south, but only for around half an hour or so when the sun is in the correct position at midday. You can see our blog about how kW relates to kWh and kWp, and why kWh is all that really matters.
Octopus took a while to fit my smart meter due to Covid disruption, so I do not have annual data from my smart meter on units consumed on site compared to those exported. Based on February 2021 production, I exported around 60% and consumed around 40%, so extrapolating this over the year means my annual bill savings based on 4,644 kWh would be:
- Bill savings @ 15p per kWh: £278.64.
- Export payments @ 5.5p per kWh: £153.25.
- Total expected annual savings: £431.89.
I think basing it on February figures will increase the on site consumption, but we’ll need another year of generation with export figures from my smart meter to make a more accurate assessment.
The export payments are tallied and credited automatically to my online account by Octopus each month.
So how has my behaviour changed?
When the solar panels generate, on site loads are powered first, with any excess energy going back to the grid at 5.5p/kWh with my smart export guarantee. So to get the best 15p/kWh payback on each unit of energy, I should try and use energy when it is sunny. The smart meter gizmo in my kitchen tells me what the ‘live’ output of my solar panels is so I know when I have extra energy to use. I can also see live production (but not export) on the Growatt app on my phone.
I usually find I am compelled to turn on any electric devices such as the dishwasher, washing machine etc. during the late morning or daytime - never at night. I am a lot more proactive on hoovering/mowing the grass when it’s sunny, where before I would probably have left it another week. I think if you are the electric bill payer you will be more motivated to police yourself compared to a family member that may not pay it...
I try not to run too much at the same time and try to run things consecutively to keep the base load lower, so use the coffee machine then use the toaster afterwards rather than turn everything on at once.
I turn on office 1 or 2 kW fan heaters when it’s cold and sunny rather than the gas central heating. This is a good way to use the excess PV energy as it’s instant and allows you to stay warm in one room you occupy as opposed to gas heating the whole house. Air conditioning would also be a great way to use excess energy in the summer if it is too hot. The added benefit of working at home is I can use PV energy during the day to power computers, lights etc.
Note that despite my best behaviour, I can only hit around 40% on site consumption in February. Going above 50% is really difficult without a battery or electric car to run the energy into. I never feel too bad as the excess green energy I generate runs to my neighbours instead, so it’s not lost energy, but if you want to go self sufficient you won’t be able to do it on solar PV alone. The UK is just not sunny enough to do it, and your household energy usage profile will never perfectly match the PV output on a daily basis.
Some days you cannot use your solar too; if you need to go to the office, you can’t use PV energy at home. If you want a cup of tea at midnight, you’ll have to accept you’ll drain from the grid, not your panels.
PV generation is great in spring and summer, OK in autumn and dreadful in winter, so you really notice your electric bill nosedive in summer and creep back up as winter gets closer. The odd day might be good or bad, but 6 hours of daylight with low sun and drizzle won’t compare to a sunny crisp day in April.
My next car will definitely be electric once my petrol one finally gives up. This means I’ll probably add a 32A V2 zappi in my garage to trickle charge at least 1.4kW of excess solar PV into the car. This is a great way to use excess solar, especially when I work at home and the car is parked on the drive during the day.
You cannot yet drain from car batteries; this is vehicle to grid (V2G) technology which is still in testing and not commercially available, but I am sure it will become more commonplace as we all get electric cars over the next few years.
If I had frequent power cuts, or my electric usage was large, I would be tempted to add a 13.5kWh Powerwall which gives power cut protection, maximises self consumption of the solar energy and lets you off-peak charge. It also allows access to the Tesla Energy Plan which gives 8p or 11p flat rate with no standing charge. I just don’t use enough electricity over the year to justify adding a battery.
Adding solar has encouraged me to improve the other elements of the house too. I think it’s important to remember solar PV and battery storage is not the only thing you can do to improve your property and go more green. Adding insulation, draft excluders, LED light bulbs, caulking windows etc. are all practical ways to reduce your energy bill too.
Overall I am really pleased with the system. Aside from the inverter clicking quietly for less than a minute in the garage when starting up and turning off as it gets dark, you wouldn’t notice it is there. The all black panels look great being integrated into the roof line, and it’s great knowing on a nice sunny day your electric bill is getting chipped away as you generate clean energy for the next 24 years (at least)!
It will save around 50,655kg of CO2 over its life, the same as 290 trees or 76 flights from London to New York.