What would a rooftop solar system generate and save?

Two numbers decide what a solar system is worth, and only one of them is about the sunshine.

The first is how much it generates, which depends on where you are and which way your roof faces. The second is how much of that you are at home to use — because a unit you use is worth your import rate, and a unit you export is worth a fraction of it.

Put your own figures in. The gap between the two ends of the range is the whole argument.

How this is calculated

Generation

Panel rating in kWp, multiplied by a location-and-angle figure from our own dataset. The method is the UK industry one set out in MCS MIS 3002; the figures are ours, generated from the European Commission Joint Research Centre’s PVGIS at 20% system losses. Every value is published in the table on our plug-in solar calculator, so you can check any of them against PVGIS directly.

There is no shading factor. Shading is site-specific and we cannot see your roof; if a chimney or a tree shades part of the array for part of the day, reduce the result.

The split that decides the answer

This is the part that separates an honest calculator from a sales tool.

A unit you use as it is generated saves you your import rate — 26.11p under the current price cap. A unit you export earns your SEG rate. The best untied rate in Ofgem’s latest annual report was 12p, and eleven of the nineteen untied tariffs still open at year end paid 4.1p or less.

So the same kilowatt-hour can be worth 26p or 4p depending only on whether somebody was in to use it. That ratio is why self-consumption matters more than system size, and why a calculator that quietly assumes a high self-consumption figure will always flatter a large system.

The default range comes from MCS’s guidance note MGD 003, the authoritative UK method, which works from occupancy patterns with annual consumption as a proxy for household size. For this calculator’s default scenario — 2,500 kWh of consumption, an array generating around 3,700 kWh, no battery — it gives 14% if the property is empty on weekdays, 19% if it is empty half the day and 23% if somebody is home all day. Hence 14–23%.

Two warnings attach to that. Self-consumption falls as the array grows relative to demand, so the default stops applying once you change the size or your usage, and the calculator says so when you do. And a battery changes it substantially: on MCS’s figures, 3.1–4.1 kWh of usable storage takes the home-all-day case from 23% to 50%.

This default used to be 25–60%, which we had invented. The floor of our old range sat above the ceiling of what the source gives, so the calculator was overstating the value of a system — the exact failure this page accuses other calculators of. We corrected it on 17 August 2026 and the write-up is on self-consumption.

We do not reproduce MGD 003’s lookup tables, for the licensing reason set out on our other calculator. It is free to download.

Where the money figures come from

Both Ofgem, both dated, both editable:

• 26.11 p/kWh import — the price cap unit rate for direct debit customers, 1 July to 30 September 2026. Changes quarterly.
• 2,500 kWh a year — typical medium household consumption, decided 27 May 2026 and in force from 1 July 2026.
• The export rate defaults to the best untied tariff in our SEG table, recomputed whenever that table changes. Untied because it is the only kind you can switch to without moving your import supply.

The trap this calculator is designed to expose

Once a system generates more than the household consumes in a year, the surplus can only be exported. No amount of behaviour change gets past that ceiling, and the surplus is worth the export rate rather than the import rate.

A 4 kWp array in the south of England generates around 4,000 kWh. A typical medium household uses 2,500 kWh. So for that household the system is already producing more than it can possibly use across a year, and the calculator says so rather than quietly valuing every unit at 26p.

That is an argument for sizing to your consumption, for shifting usage into daylight hours, or for storage — not an argument against solar. But it is an argument you will rarely see on a page that is trying to sell you panels.

What this does not do

Costs and payback. We do not track installation prices and will not estimate them.

Batteries. A battery changes the split by storing what you would otherwise export, which is exactly the lever this page shows the value of. Sizing one is a separate calculation and a separate page.

Degradation, inverter replacement or maintenance. All real, none modelled here. Treat the annual figure as year-one output.

Whether your roof can take it. Structure, condition, shading and roof area are site questions. Roughly 2 kWp per 10 m² is a planning rule of thumb, not a survey.