How Much of Your Electricity Can a Solar PPA Cover?

Self-consumption decides what a solar PPA saves. How to estimate the share of your demand an array can cover, why summer and weekends matter, and how to size it.

Two numbers decide what a solar PPA saves, and they are easy to confuse. Coverage is the share of your annual electricity demand the array supplies. Self- consumption is the share of the array's output you actually use on site. They pull in opposite directions: the bigger the array, the more of your demand it covers — and the more of its output it spills to the grid.

Why self-consumption is the number that pays

Under an on-site PPA, every unit you use on site displaces a grid unit at your full delivered rate. Every unit exported earns only an export rate — and because the provider owns the array, export income is only yours if the contract assigns it to you. So the saving tracks self-consumption far more closely than it tracks array size. That is why our solar PPA calculator asks for your self-consumption share rather than the size of your roof alone, and excludes export income unless your contract assigns it to you.

The seasonal shape

Solar output is heavily seasonal. PVGIS puts an optimally tilted array in London at about 120 kWh per kWp in June and 38 kWh per kWp in December — a threefold swing — with about two-thirds of its annual output arriving between April and September. A site whose demand is flat through the year will therefore use almost everything the array produces in winter and spill a growing share in summer.

London monthly output from PVGIS v5.3 (EU Joint Research Centre) — PVGIS-SARAH3 radiation data, 1 kWp crystalline silicon, 14% system loss, optimised angle: June 120.1, December 38.3 kWh/kWp; April–September 66.3% of 1,022.9 kWh/kWp a year. Queried 26 Sep 2026.

The weekly shape

The week matters as much as the year. Solar keeps generating at weekends and on bank holidays; many offices, schools and single-shift factories do not keep consuming. A site closed on Saturdays and Sundays loses its load on two days in seven, which matters most in summer, when those days produce the most. Warehouses with refrigeration, hospitals and continuous-process plants sit at the other end, with demand that runs through the weekend.

Estimating it properly

You do not need to guess. If your site is half-hourly metered, your supplier or data collector can release twelve months of consumption data. Lay an hourly solar profile for your location over it — PVGIS provides one — and count, for each half-hour, the smaller of generation and demand. The total of those overlaps, divided by annual generation, is your self-consumption; divided by annual demand, it is your coverage. We are not aware of an official UK statistic for commercial self-consumption, which is exactly why your own data beats any rule of thumb.

Sizing to what you use

The practical rule follows from the shapes above: size the array to your daytime base load, not your annual total. An array matched to the load you carry on a summer weekday afternoon will be used almost entirely on site; one sized to your whole annual demand will export much of its summer output. If the roof could take more, the extra capacity only pays if you can use its output — through a second shift, battery storage, or electrified heat and transport that shifts demand into daylight hours.

Once you have a realistic self-consumption figure, put it into the calculator with the tariff you have been offered. It is the input that moves the answer most, and the one most often left at an optimistic default.

The share you can use matters most on a behind-the-meter PPA, where every unit you consume displaces a delivered grid unit.