Tuesday, 5 April 2022

Ten years of rooftop solar power - no decline in output can be seen

Today is the tenth anniversary of the installation of our mains connected rooftop solar panels. In total the inverter reports that 33935 kWh of electricity have been generated in ten years, an average of 3935 kWh per year since they were installed.

After ten years we still can't see any decline in output

Output in kWh per year. Note that there's considerable variation depending on the weather. The lowest output was 3126 kWh in 2018-2019 and the highest 3516 kWh in the second year. Last year was also higher than average, within 0.1% of the highest.

A rough calculation before installation suggested that we could expect around 3478 kWh of electricity each year, but our supplier suggested that in practice we should expect around 3150 kWh per year. The guarantee with the panels said that a decline in performance of 10% over the first 10 years was within normal bounds. This year's 3392 kWh is below the ten year average, but as it's higher than four of the previous years (including the first year) it's also far from abnormal. I don't see evidence of a decline in output due to aging.

We still produce more electricity than we consume

Blue and red bars show production and consumption per month. The yellow line shows the cumulative difference which we've exported to the grid. We started consuming less electricity five years ago, but over the last year we've consumed slightly more than the previous four years.

I painted our house last year, but I didn't clean
the solar panels. That doesn't seem to matter at all

Over the ten year period we've produced about 10% more electricity than we've consumed, but actually for the first five years our consumption was close to the production (for reasons explained previously) so it's more accurate to say that we've produced about 20% "too much" over the last five years. In other words, our overproduction over five years is roughly the same as the annual consumption of a household like ours.

Cooking with electricity changes the pattern

You'll notice that the graph showing the cumulative difference between our production and consumption has actually leveled off a bit over the last year. The reason for this is that we've switched to cooking with electricity instead of gas, this being one of the ways we've been trying to further reduce the footprint of our already low carbon diet. There's no new kitchen, that'll have to wait for a while. We've been using a small portable electric hob resting on top of the gas appliance as an experiment, which has worked out very well. An induction hob would probably work better. Anyway, it's nice to see that despite this increase in electricity consumption the yellow line is still heading upward at a faster rate than it did on any year before 2016.

Thoughts about home storage

A smart meter was installed four years ago so we now have three full years of smart meter readings. Though we have a single tariff contract so pay the same for electricity any time night or day, the smart meter nevertheless separates out the low and high rates of electricity which roughly correspond to day and night. Overall through the year, 39% of the electricity that we use comes directly from our own panels while 61% comes from the grid and we use 38% of what is generated by our panels while exporting 62%. Obviously a battery comes to mind immediately when looking at these figures, but I'm still not convinced that it's worth the investment. How much could it change these figures ? I think by less than we might hope:

Home storage batteries have capacities of around half of a day to a whole day of typical usage. It's enough to reliably keep your refrigerator going overnight during summer, but not remotely enough to span the seasons.

In the summer months such a battery would fill up in the first couple of days and then it would stay nearly full for weeks. The battery would allow us to consume our own electricity overnight, so that for periods of nice weather all of the electricity we use would be "our" electricity from our panels, which is of course an attractive idea. During that best case scenario we would increase from the average 60% of our electricity that currently comes from our own panels during summer to 100%. For the six best months each year we produce on average between 1.7x and 2.1x as much electricity as we consume so even with a battery working as well as it possibly could we could still only capture some of our own electricity for our usage and doing this would only actually increase the percentage of our own electricity that we consumed from the current still 30-35%% to about 50-60%. Also, this is Northern Europe. There are periods even during the nicest Northern European summers when the solar panels don't produce as much as we'd like, so the battery will run down and we would still have to buy some of our electricity from the grid.

New, huge, windturbine in Drenthe
And then there's winter: In the winter months there's far less sunlight falling on our panels and we possibly produce enough electricity to cover our own usage without annexing our neighbours' roofs. Because there's so little sun in the winter what comes from our panels currently covers only about 10-15% of our consumption and during these dark months that means we are already consuming 70-80% of our own electricity just with normal daytime usage. During the winter a battery would therefore be empty or near empty most of the time. While a battery would mean we would reliably consume all of our "own" electricity during winter it wouldn't win us much because there's not much to store: It would reduce the proportion of our electricity that we buy from the grid from the approximately 90% during winter now to around 85% with a battery.

Financially I don't see the benefit of a battery at all. It's an expensive gadget which will change very little. Let's also remember that the electricity leaving our home is not wasted. It's used elsewhere, reducing demand for other, on average less green, generation. So what exactly is gained by trying to keep our electricity to ourselves ? It seems more likely to benefit the ego than the environment.

So to summarise, I'm still not convinced that there is actually any real point in home storage using batteries. I think we'd possibly get some real benefit from installing a few more panels on the roof as while these could of course not do anything about our nighttime usage they would mean that we'd cover more of our own consumption during the day and if we get around to installing an electrical heat-pump for heating (instead of the old gas boiler) then balancing the resultant greater electricity consumption with more panels would make sense. I also have doubts about how wise it is to encourage people to install large and potentially highly flammable lithium batteries in their homes. They might be fine when new, but what happens in 20 years time when they've not seen maintenance engineers in many years and they're failing in various unexpected ways? I think it is a better idea to install batteries as large scale devices at substations or next to large solar parks or wind turbines.


Our business is also powered by these solar panels. We sell practical bike parts which help people, especially those in other countries where such parts are not so easy to find, to reduce their impact by cycling. Bicycles are the most efficient vehicles on the planet. We don't use motor vehicles so every shipment begins with at least the first few km on my bicycle, using nothing but human power.

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