How to calculate your solar savings without a salesperson

You do not need anyone’s calculator to get a defensible estimate. You need four numbers you already have and about twenty minutes. Here is the method, in the order that makes each step easy, with a worked example running alongside so you can check your own arithmetic against it.

Doing it yourself matters for a specific reason. Every input below is an assumption somebody else would otherwise make on your behalf, and the one that moves the answer most is the one they are least likely to state.

Step one: your annual consumption

Find your electricity use for the last twelve months, in kilowatt hours. Sources, in order of reliability:

• Your annual statement, which states it directly.
• Your supplier’s app or online account.
• Meter readings twelve months apart, subtracted.

Write it down. Call it your annual usage. For the worked example, we will follow a household using 3,600 kilowatt hours a year.

Step two: your daytime share

This is the number that decides most of the answer, so it is worth more than a guess.

If you have a smart meter: request your half hourly data. Most suppliers provide it through their app or a data portal. Add up consumption between roughly 10am and 4pm across a representative fortnight, and divide by total consumption over the same period.

If you do not: estimate from your routine. An empty house on weekdays with occupied weekends is very different from someone working at home five days a week. Be honest rather than optimistic; this is the number that gets flattered.

Our example household is home some weekdays and lands on 40%, so 1,440 of its 3,600 kilowatt hours happen in daylight.

Step three: your two rates

From your tariff:

• Import rate, pence per kilowatt hour. If you are on a time of use tariff, note the rate that applies during daylight, not the overnight one. On the current cap this is 26.11p for direct debit customers.
• Export rate, pence per kilowatt hour, from whichever export tariff you could realistically take. This need not be your current supplier: Octopus Outgoing pays 12p to its import customers, and other suppliers publish theirs.

The example uses those two: 26.11p in, 12p out.

Step four: an honest generation estimate

You need the annual output your roof would produce, in kilowatt hours.

Use a location specific estimate rather than a rule of thumb, and make sure it accounts for your orientation, your pitch and your shading. The European Commission’s PVGIS tool will produce one free for any location and orientation. If you have quotes, each will carry a figure; ask which tool produced it and what shading assumption was used. Where two quotes disagree, prefer the more conservative one for your own sums.

The example household is quoted a 4kW system and takes a 3,400 kilowatt hour estimate, which matches our model’s UK average of 850 kilowatt hours per kilowatt for a south facing roof.

Putting it together

Now the arithmetic. Using your own figures:

1. Self consumed units equal generation multiplied by your daytime share. One bound to respect: self consumed units can never exceed your actual daytime consumption from step two, so cap them there if the multiplication exceeds it.
2. Avoided import equals self consumed units multiplied by your import rate. This is money you no longer spend.
3. Exported units equal generation minus self consumed units.
4. Export income equals exported units multiplied by your export rate.
5. Annual saving equals avoided import plus export income.

The worked example:

1. Self consumed: 3,400 times 40% is 1,360 kWh, safely under the 1,440 kWh of daytime consumption.
2. Avoided import: 1,360 kWh at 26.11p is about £355.
3. Exported: 3,400 minus 1,360 is 2,040 kWh.
4. Export income: 2,040 kWh at 12p is about £245.
5. Annual saving: roughly £600.

Against a 4kW system at the government cost data’s 2025/26 median of £1,780 per kilowatt, about £7,100 installed, that £600 is a simple payback a little under twelve years, before the corrections a full payback calculation adds.

The reason steps two and three stay separate is that the two rates are usually very different: the example’s avoided import is worth more than twice its export income per unit. Blending them into one average rate is the most common way these calculations go wrong, and it always errs in the optimistic direction if the blend leans on the import rate.

Sense check what you get

Two quick tests:

Against your bill. Your annual saving cannot sensibly exceed your annual electricity spend plus your export income. The example household spends around £940 a year on units at capped rates, so a claimed saving of £600 is plausible; a brochure promising it £1,200 would be arithmetically impossible.

Against the self consumption assumption. Redo steps one to five with your daytime share a third lower. The example at 27% instead of 40% drops to about £240 of avoided import and £298 of export, roughly £540 a year, and payback stretches past thirteen years. If a one third change in one estimated input moves your answer that much, that input is carrying the result and deserves measuring properly rather than estimating.

Then extend it over time

A single year is a starting point. For a full picture you also need panel degradation, an inverter replacement at some point, and battery capacity loss if storage is included: our calculator models each of these explicitly. Keep everything in today’s money rather than compounding an assumed energy price rise, so the total remains a figure you could actually compare against other uses of the same capital.

Why your version beats theirs

Not because the arithmetic is better. Because you know which assumptions you made, and you can change the one you are least sure of and watch what happens. A savings figure handed to you as a single number cannot be interrogated, which is precisely why it is presented that way.

When you want the year by year version with degradation, replacement and finance handled properly, put the same four inputs into the ROI calculator: it runs the projection with the assumptions stated, so your twenty minutes of arithmetic becomes a twenty five year curve you can still argue with.