How are the numbers calculated?
The calculator combines postcode-area solar yield with an hourly model of electricity use. Production comes from the European Commission Joint Research Centre’s PVGIS-SARAH3 database through API 5.3, with 14% system losses, monthly postcode-area totals and regional hourly profiles. The calculator’s data notes do not specify its historical years.
The hourly model accounts for your home’s base load, the inverter output limit and battery settings. The UK limit is 800 W maximum rated AC output, not panel DC power. Default base load is 164 W, taken from NRDC’s 2015 study of northern California homes rather than a UK household survey; default battery round-trip efficiency is 0.9 when a specification gives none.
The default electricity rate is Ofgem’s GB-average Direct Debit rate of 26.32p/kWh for 1 October 2026 to 31 December 2026. Review the named assumptions and change the calculator inputs to match your installation and tariff. In Northern Ireland, enter your own unit rate because Ofgem’s cap does not apply.
What does the result mean?
Electricity used at home saves money; production alone is not your saving. The calculator separates energy produced, used at home, sent to the grid and lost to the inverter limit. Energy clipped by that limit is unavailable for either home use or export.
For London SW postcodes, the calculator gives 596 kWh a year for a south-facing, vertical 800 W kit. With a 164 W base load, 374 kWh, or 63%, is used at home, worth £99 a year at the default price-cap rate. Export is valued at zero unless you enter a rate; do not assume a Smart Export Guarantee payment without checking eligibility and your export tariff.
Payback compares your entered cost with the calculated annual benefit. The ten-year figure holds the tariff steady and does not model panel ageing, so it is a scenario rather than a forecast. Payback above 25 years is shown as “more than 25 years”; the regional table provides typical-year comparisons, not guaranteed production.
What changes your savings?
Mounting position, household demand, battery settings and electricity rate change the result. Use the calculator to compare each change against the same starting setup, so you can see what actually improves savings.
The calculator examples shown here quantify tilt and regional differences, but do not provide separate direction, base-load, battery or tariff comparisons. A battery cannot be judged from production alone: compare its calculated benefit with its added cost.
- Direction: the London reference faces south and produces 596 kWh a year; do not apply that result unchanged to another direction.
- Tilt: the calculator’s London result rises from 596 kWh vertically to 812 kWh at 30° tilt, with annual savings rising from £99 to £119.
- Base load: the London reference uses 374 kWh at home with a 164 W base load; replace that assumption to reflect your home.
- Battery: compare battery settings against the London reference’s 374 kWh used at home; no separate battery saving is shown here.
- Rate: the London reference saves £99 a year at 26.32p/kWh; enter your actual unit rate rather than assuming that saving.