It is frequently asserted that solar doesn’t need sunshine; technically this is true, but in practical terms this is a barefaced lie. If you are looking at investing into solar, whether proper rooftop, or the new plugin, here are some real Scottish numbers for you.

TL;DR: in Scotland solar alone, in any form, is a loss making investment over anything less than a decade; for it to make a modicum of economic sense, your setup must include a sizeable battery, and you have to have an EV to have access to an EV-only tariff.

As promised, some numbers. We have 6.8kW worth of rooftop solar, distributed on south and west facing roofs in about 40/60 ratio. This is ideal, as we are able to generate electricity from sunrise to sunset, with the system being able to provide the full 6.8kW in the middle part of the day, and around 3kWh in the early late parts of the day. In the summer, this can add up to 30kWh / day.

But we don’t get many cloudless days in Scotland, and this time of the year, with there being about 12h of daylight, on a good sunny day we produce around 20kWh / day. Based on that, and being told ‘solar doesn’t need sunshine’, you might be surprised to learn that on an overcast wet day, like today, we will be lucky to produce 2kWh.

Now, if you spend say £10k on the solar, then for it to pay off in a decade, you would need to make £2.74 / day; we get 12p / kWh, which means we would have to produce just shy of 23kWh / day, which I don’t think is achievable in Scotland. The official estimate is we will average 13.7kWh / day over the course of the year (but already in September, with 12h of daylight, we are only averaging 12.8 kWh), which means almost 17 years to break even on the initial investment.

The math for plugin solar is even worse. Say you get a 500W panel for £500. On the official estimates you could generate on average 1kWh / day; assuming you can actually consume it when it is being produced (and that’s a big if), you will be saving around 30p / day, which means a 4.5 year to break even on the investment.

The only way to shorten the time it takes to break even on the initial investment is to include a sizeable battery (enough to cover your needs over each 24h period), and have access to a cheap overnight tariff. You can then run the house on the cheap tariff (through the battery) and sell any solar you generate. How much you can save this way depends on the nighttime tariff and how much electricity you use, for ourselves the saving presently works out about 22p / kWh compared to the variable tariff.

But, of course, the battery will add up the the initial investment costs, and, worst of all, in the UK cheap night time tariffs are only available to EV owners, which means another substantial investment …

There are other reasons to invest into solar (environmental concerns, degree of energy independence), but it all assumes you can spare a large lump of cash to start with — as I am sure I have said before on this blog, the sort of environmental policies pushed by the likes of the Scottish Greens are by the affluent for the affluent and, necessarily, at the expense of the less well off, and they make very little difference to the environment.

The only way to accomplish fair domestic decarbonisation is to slash the cost of electricity so it matches that of gas, anything else is but a plaster on the consciences of the chattering classes, and pissing into the wind.

PS: Please do not take this to mean I am against domestic solar, or even unhappy with our own setup. I do think solar is important, and rooftops are where it belongs. But the economic realities around it currently suck, and the powers to be don’t care.