Fourteen years of rooftop solar power

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The solar panels on the roof of our home were installed on the 5th of April 2012. For fourteen years they've produced electricity every day, and though they were expensive to install all that time ago they've more than paid for themselves in reducing our energy costs. The total amount of energy generated by that system over fourteen years stood at 47220 kWh on the 5th of April 2026. On top of that we now generate a considerable amount from extra panels installed on our garage.

Solar power production from our rooftop panels (blue) and additional panels (orange) per year. The total output for 2025-2026 is lower than the year before because we sometimes switch them off to avoid exporting at a negative price

We switched to a dynamic tariff last year. Sometimes the prices go negative and it can be better to switch solar panels off than to continue to supply the grid. That's the reason why the graph shows the panels as having produced a little less electricity than in previous years - they were switched off at times when they would have produced a lot of kWh, but we would have had to pay to send that unwanted electricity to the grid (see later in this article as I'm changing how we do this).

Total energy bill and dynamic tariff
Our total energy bill for the year, from April 2025 to March 2026, was €128.47. That's the total that we paid over the entire year, including the fixed fee for a connection, taxes etc.. Everything in our home runs on electricity. We do not have a gas connection or associated bill.

With a dynamic tariff the amount paid each month varies quite a lot. The largest single month payment was €187.41 which covered our usage in January. Most of the electricity was consumed by our heat pump, and our solar panels were covered in snow for a significant part of that month. The largest negative bill (we received money from the energy company) was €88.42 for July when we exported a lot of electricity. There are more positive months than negative months. We received a payment for eight months and had to pay the energy company for four months.

Making negative prices work for us
This year our panels should be switched off less often than they were last year. For most of last year my software was switching off the panels whenever the price went below 0 c / kWh. That's actually too aggressive and this year they will switch off far less often.

For a start we need to ignore negative prices which are not very negative. Just say that the current price is -0.1 c / kWh. If we switched off our solar inverters for an hour at that price we would prevent paying to export 4 kWh, which would save us  0.4 c. But if we switch off the inverters and use electricity during that hour we'll have to pay the retail price for what we consume including taxes which works out as around 13.5 c / kWh. i.e. we'd have to consume less than 0.1 kW in order to not end up paying more for what we import than we'd lose from exporting the balance.

There's also the issue of how slowly inverters restart. It takes more than a minute for them to come back online and rise back to maximum output. So if you try to switch them on whenever the consumption rises, for instance when you put a kettle on, they'll almost certainly miss the peak. You could consume 0.5 kWh before the inverters get going again, and then have to switch them straight off again once the kettle is boiled.

It makes more sense to only switch off the inverters when the supply is very negative.

There are two reasons for this: first, I realised that I was being too aggressive in switching off the panels last year, which led to us sometimes having to import electricity in order to power things that could have been supplied by our panels, and second that I now have more devices which can be switched on as required to absorb extra electricity.

The problem with switching inverters off is that they take minutes to come back to full power output again, which means we can't accurately match usage. But if we can instead "dump" excess electricity elsewhere, e.g. into resistive underfloor heating elements, these can be switched off immediately when we need the electricity elsewhere. So we now have some resistive underfloor heating, which also makes a difference to our comfort in winter, and I've modified the control of the water heater to also switch more responsively.

This year compared with last year
The changes made to the software have already produced a result. Our bills for April and May 2025 when we were aggressively switching off the inverters whenever the price per kWh went negative were -€43.86 and -€46.66. With the new software, which tries to ensure that we still power as much as possible from our own solar panels our bills were €-62.27 and -€71.41. That's an extra €43 for us due to not switching off our solar panels so aggressively, but allowing some production with a slightly negative price.