Solar calculator assumptions UK: what numbers to use

Why the assumptions matter

Solar calculators can be very helpful, but they can also flatter a quote if the inputs are too generous. The main job is to make the moving parts visible, not to produce a perfect prediction down to the pound.

The five inputs that matter most are installed cost, annual generation, import unit rate, export rate and self-consumption. If those are sensible, the estimate becomes useful. If one of them is badly wrong, the payback can move by years.

Installed cost

Use the full installed solar-only quote, including panels, inverter, mounting, scaffolding, electrical work, monitoring, certification and VAT treatment. If a battery is included, ask the installer to split the battery cost out.

Energy Saving Trust currently describes an average home solar panel system as around £6,100 and generally around 4.5kWp. That is a benchmark, not a target. A difficult roof or higher-spec design can cost more for legitimate reasons.

Annual generation

Annual generation is measured in kWh. If you only know the system size, a rough early planning range is 800 to 1,000kWh per kWp installed each year.

Replace that shortcut with a roof-specific estimate when you can. PVGIS and installer modelling can take location, orientation, pitch and shading into account. If a sales quote assumes unusually high generation, ask why.

Import unit rate

The import unit rate is what you pay for electricity from the grid. The calculator default is rounded to 26.1p/kWh because Ofgem's average Direct Debit electricity unit rate for 1 July to 30 September 2026 is 26.11p/kWh, including VAT.

Use your own tariff if you know it. Some households pay more or less than the cap average, especially on fixed or time-of-use tariffs. A few pence per kWh can make a visible difference over thousands of kWh.

Export rate

The export rate is what you are paid for surplus solar sent to the grid. Ofgem says SEG suppliers set their own tariff rates and terms, and the rate must be above zero.

Don't use the best rate you have seen online unless you know you can access it. Check MCS or accepted equivalent paperwork, export MPAN requirements, smart meter requirements, payment timing and whether the tariff depends on taking import supply from the same company.

Self-consumption

Self-consumption is the share of solar electricity used in the home. For many solar-only homes, a cautious starting point is often better than assuming the household will use most daytime generation.

Daytime occupancy, EV charging, smart hot water, appliance scheduling and batteries can all improve self-consumption. Strong export tariffs can also make exporting less of a problem, so the right number depends on both routine and tariff.

Battery assumptions

A battery should not be hidden inside one blended solar payback figure. Add the battery cost separately, then test how much extra self-consumption it might create.

Energy Saving Trust says battery storage costs vary by type and size, with a 5kWh system around £4,600 and wider costs ranging from £1,500 to £10,000. Use your installed quote and be cautious about how much extra annual benefit the battery creates.

A sensible order of work

Start with a solar-only case using the default assumptions. Then replace one input at a time with real data from your bill, quote, export tariff and roof estimate.

Once you have a believable middle case, run a careful case with lower generation and lower export income. If both cases still look acceptable, you have a much more useful basis for a buying decision.

• Enter the full installed solar-only quote.
• Use annual kWh from PVGIS or installer modelling.
• Replace the unit rate with your own tariff.
• Use an export rate you can actually apply for.
• Run no battery first, then add storage as a separate scenario.