What does a plug-in solar kit actually save?

The number you want is a single figure. We are not going to give you one, and the reason is worth thirty seconds.

An 800 W kit angled south generates somewhere around 550 to 830 kWh a year depending on where you are — a spread of nearly 50% between Lerwick and Brighton before anything else is considered. Mount the same panel vertically on a railing instead and it drops by roughly a quarter. Face it north and it falls by three quarters.

Then comes the part that decides the answer: how much of that you are actually at home to use. Because a plug-in kit earns nothing for exported electricity, the difference between using half your generation and using nearly all of it is the difference between two quite different investments.

So: put your own numbers in, and watch what the assumptions do.

How this is calculated

Three inputs, one of which is honest guesswork, and we would rather say which.

Generation

The UK industry method is the one in MCS MIS 3002: annual AC output equals the panel rating in kWp, multiplied by a location-and-angle figure called Kk, then by a shading factor. We use that method.

The Kk figures are ours, not MCS’s. MIS 3002 restricts reproduction of the standard, so rather than republish their tables we generated our own from the same underlying source their tables come from — the European Commission Joint Research Centre’s PVGIS — at the same 20% system losses their 0.8 factor represents. Every figure is published in the table on this page, so you can check any of them against PVGIS yourself.

There is no shading factor in the calculator. Shading is site-specific and we cannot see your balcony; if a wall or a tree shades the panel for part of the day, reduce the result accordingly.

Self-consumption — the number that decides everything

A plug-in kit earns nothing for what it exports. Every unit generated while nothing in the house is drawing power is simply lost, so the saving is generation multiplied by the share you happen to use.

That share depends on when you are in, what is running, and how the output lines up with it across a year. MCS publishes the authoritative UK method in guidance note MGD 003, which works from occupancy patterns — home all day, in half the day, out all day — with annual consumption as a proxy for household size.

The default range is now 40–56%, taken from that method rather than from our own judgement: for a kit generating in this range on a household using 2,500 kWh a year with no battery, MGD 003 gives 40% if the property is empty on weekdays, 48% if it is empty half the day and 56% if somebody is home all day. We do not reproduce its lookup tables, for the licensing reason set out above, and the document is free to download.

This default used to be 50–90%, and that was our invention. It was above what the source supports, in the direction that flattered the kit. We corrected it on 17 August 2026 and have written up how it happened on self-consumption, because picking this number high is exactly what we criticise other calculators for doing.

Move it if you know better than the archetype: your own half-hourly data, which your supplier holds and will give you, beats any lookup table.

Money

Two Ofgem figures, both dated, both editable:

• 26.11 p/kWh, the price cap unit rate for direct debit customers for 1 July to 30 September 2026. The cap changes quarterly.
• 2,500 kWh a year, the typical medium household electricity consumption from Ofgem’s review decided on 27 May 2026 and in force from 1 July 2026.

If you are on a fixed tariff or your usage is nothing like typical, replace them. The result is only as good as those two numbers.

What this deliberately does not do

It does not model inverter clipping. A compliant device caps output at 800 VA while the specification permits up to 2000 W of panels. On a bright day the inverter limits what those panels deliver, and the loss depends on the specific kit. We have no source for it, so above about 1000 W of panels the calculator warns you that its figure is a ceiling rather than an estimate.

It does not include an export payment, because there is not one to include. See our SEG page for why.

It does not tell you whether to buy. It tells you what the generation arithmetic looks like. Payback depends on what you paid, and we do not track kit prices.

What we could not confirm

We could not find PVGIS’s usage conditions in a form we could read and quote — the Joint Research Centre describes it as free and open access, and the Commission’s general position on reuse is attribution-based, which is why every page and the dataset itself carry the acknowledgement. If we have that wrong we want to know.