Solar generation
Panel size multiplied by annual yield for the roof direction, then reduced by the shading factor.
Calculator assumptions
Every number the calculator uses.
The calculator is a screening tool, not a quote. These assumptions are deliberately visible so you can see what moves the result.
Constants
Founder-tunable values
| Assumption | Value | Why it is used |
|---|---|---|
| South-facing annual yield | 950 kWh/kWp | Initial SE England planning assumption for calculator screening. |
| South-east to south-west yield | 900 kWh/kWp | Used for roofs close to south but not perfectly aligned. |
| East-west annual yield | 800 kWh/kWp | Used where generation is spread across morning and afternoon. |
| North-facing annual yield | 550 kWh/kWp | Included so the calculator can show weak-fit cases plainly. |
| Some shading factor | 85% | Simple screening reduction before a survey checks shade properly. |
| Heavy shading factor | 65% | Conservative reduction for roofs likely to need a no-go conversation. |
| Import electricity price | 26p/kWh | Typical single-rate import assumption for the first calculator release. |
| Export electricity price | 15p/kWh | Representative Smart Export Guarantee assumption. |
| Self-use without battery | 35% | Used to compare panels alone with a battery-backed system. |
| Self-use with battery | 75% | Used for solar-and-battery year-1 bill saving estimates. |
| Battery round-trip efficiency | 90% | Allows for losses when power is charged and discharged. |
| Off-peak spread | 12p/kWh | Difference between cheap-rate charging and peak-rate use in the battery-only model. |
| Arbitrage cycles per year | 250 cycles | Battery-only economics driver for a suitable smart tariff. |
| Battery earnings model | £120-250 | Initial flexibility-market earnings range to be checked against the partner programme before launch. |
| VAT note | 0% VAT until 31 Mar 2027 | Current domestic solar and battery VAT treatment deadline. |
| Panel degradation | 1% | Used in the 10-year ledger projection. |
| Electricity inflation | 3% | Used in the 10-year ledger projection, not in year-1 headline savings. |
| EV usage uplift | 1800 kWh/year | Screening uplift when the household has or plans an EV. |
| Solar-and-battery price bands | To be confirmed before launch | Founder quote ranges are needed before payback can be shown. |
| Battery-only price bands | To be confirmed before launch | Founder quote ranges are needed before payback can be shown. |
Formulas
How the headline results are calculated
Solar-and-battery bill saving
Generation split between self-use at the import price and export at the export price, using the battery self-use assumption.
Battery-only bill saving
Battery size multiplied by annual cycles, round-trip efficiency and the off-peak spread.
Battery earnings
The modelled flexibility-market range is added separately to the bill saving, so the two sources of benefit stay visible.
10-year ledger
Bill saving is projected with panel degradation and electricity inflation. Battery earnings are held flat in the current model.
Payback
Simple payback needs real price bands. It will be enabled once founder-approved quote ranges are added.