The useful starting point
The value of solar is the electricity you use directly plus the payment for eligible exports. Your roof and your pattern of use determine the split.
Start with annual generation
An installer’s generation estimate should relate to your location, roof orientation, pitch, shading and proposed system. Rated kWp is not the same as the annual kWh you will receive. Ask which assumptions are included and how the estimate was prepared.
A single annual figure is useful for a first calculation, but it hides seasonality. The home may export heavily in summer and import much more in winter. Do not assume annual generation cancels annual consumption unit for unit.
Split the generation into two values
For a fictional system generating 3,400 kWh annually, assume 40% is used directly. That gives 1,360 kWh of avoided imports and 2,040 kWh of exports.
| Part of the value | Example calculation | Annual amount |
|---|---|---|
| Electricity used at home | 1,360 kWh × £0.27 | £367.20 |
| Electricity exported | 2,040 kWh × £0.12 | £244.80 |
| Combined energy value | £367.20 + £244.80 | £612 |
The tariff rates and usage split are illustrative. Only the export element is income in this example; the rest is expenditure avoided. Standing charges remain unless your account arrangements change.
What changes when you use more solar directly?
Using energy while the panels generate can increase direct consumption. But shifting an appliance’s time of use only helps to the extent that it replaces grid energy you otherwise would have bought. Running extra loads simply to use the solar is a different decision.
A battery may move surplus to later, but it has purchase costs, losses and limits. Credit the battery only with the additional benefit after the export income given up. Read the solar-battery guide before adding all discharged energy to a savings forecast.
Test three cases
Run a lower, central and higher case with your installer. Vary self-use, generation and tariff rates separately so you can see what causes a change. Then ask what happens if electricity prices fall or an inverter needs replacement.
For a £7,000 illustrative installation and £612 first-year value, simple payback is about 11.4 years. It is a quick ratio, not a lifecycle forecast. Finance, maintenance, degradation and future prices are excluded.
Make the next estimate more personal
Bring a year of electricity use, your current unit rates and a realistic description of when the home is occupied. If you expect an EV or heat pump, show those as an additional scenario rather than quietly adding them to the base case.
Use the solar scenario tool to change the inputs and download your result. For a tariff change, compare the whole electricity bill rather than the export rate alone.
Published information supports your research. Your installer should confirm the exact product, suitability, scope and terms for your property.
Suggest a correction ↗Your next useful step
Put the information to work.
Take the questions that matter to your project, explore the numbers and decide what to ask next.