What it uses
Solar Panel Savings Calculator
Change anything. Results update as you type. Defaults are a 4 kW south-facing array in Whiteley, a medium-use household that is part of the day at home, and a 5 kWh battery.
Change the prices and assumptions
Month by month (kWh)
| Month | Generated | Used directly | From battery | Exported | Bought |
|---|
How the Calculator Works, Step by Step
There is no black box. For each month of the year the calculator does the following, for a bright day and a dull day (half the days each) and averages them:
- Takes the month’s PVGIS yield for your roof direction and angle, scaled for your area, and multiplies by the size of your array.
- Works out how much of your household’s daylight use the panels cover directly — your use, times the share of it that falls in daylight.
- Treats whatever the panels make beyond that as surplus. A battery, if you have one, stores as much of the surplus as it can hold.
- Uses the stored energy for your evening and night use, losing 10% on the way through the battery.
- Exports what is left, and buys the rest from the grid.
- Adds up the year: the electricity you avoid buying at the import price, plus the export income, less any cheap-rate charging cost.
- Divides the installed cost by the yearly benefit for a simple payback.
It is deliberately simple, so you can check it. It is also why it carries a health warning: a model that treats every day of a month as one of two types is kinder to a battery than real weather is.
What the Calculator Assumes — and What It Leaves Out
| Assumption | What it is | Which way it errs |
|---|---|---|
| Sunshine | PVGIS-SARAH3 monthly yields at 14 places, 14% system losses, scaled from Hayling Island by roof direction | Neutral for an unshaded roof; optimistic if you are shaded |
| Bright and dull days | Half of each month at 160% and half at 40% of its average | Still kinder to a battery than real weather |
| Demand | Flat across the year, with the share in daylight set by you | Real winter use is higher; flattering to solar in winter |
| Battery | 90% round-trip efficiency; priced on a line through Octopus’s £3,947 and £5,336 | Neutral |
| Prices | Cap 26.32p; Outgoing Octopus 12p; DESNZ South East cost | Rates change; a lower export rate makes a battery look better, a higher one worse |
| Left out | Degradation, price changes, standing charges, finance, shading | Mixed |
Sources: EU JRC PVGIS 5.3, PVGIS-SARAH3 2005-2023, 14% system loss; Ofgem energy price cap, 1 October to 31 December 2026; Octopus Outgoing, region H (OUTGOING-VAR-24-10-26, from 1 March 2026); DESNZ Solar PV cost data 2025/26 (published 28 May 2026), South East mean; Octopus Energy, battery-only installation (April 2026 prices, read 7 October 2026).
Four Worked Examples From the Same Model
These are computed by the same model as the calculator, for the default Whiteley roof, so you can check the tool against them:
| Household and system | Generated (kWh a year) | Your electricity covered | Yearly benefit | Installed cost | Payback |
|---|---|---|---|---|---|
| Medium use, part of the day at home, 4 kW, no battery | 4,526 | 43% | £699 | £7,436 | 11 years |
| Medium use, part of the day at home, 4 kW, 5 kWh battery | 4,526 | 85% | £832 | £11,383 | 14 years |
| Low use, out all day, 3 kW, no battery | 3,395 | 30% | £476 | £5,577 | 12 years |
| High use, home most of the day, 5 kW, 10 kWh battery | 5,658 | 79% | £1,095 | £13,943 | 13 years |
Medium, low and high use are Ofgem’s typical 2,500, 1,600 and 3,800 kWh. Daylight share: 45% / 30% / 60% as stated. Costs: DESNZ solar plus Octopus battery line.
Notice what the examples show. The panels do most of the earning; a battery is worth the least per pound and is worth more the less you are home. The cost page, solar panels and battery cost, sets out the published prices behind the cost column, and our solar and battery storage page works through when the battery pays.
Solar Panel Calculator: Questions Answered
How to read the numbers, and what they leave out.
It is a model, not a quote, and it says so. It uses PVGIS sunshine data for your area and roof, your own figures for use and time at home, the current price cap and export rate, and published average installation costs. It cannot see your shading, your roof’s real condition or the price of your actual quote. Treat the output as a sense-check, and a survey as the real answer.
Monthly sunshine from PVGIS (EU Joint Research Centre), with each month split into bright and dull days; flat demand across the year; a 90% battery round-trip efficiency; the 26.32p price cap and the 12p Outgoing Octopus export rate; DESNZ South East average installed cost for solar and a straight line through Octopus’s published battery prices. No degradation, price changes, standing charges or finance.
Because it decides how much of what your panels make you use yourself, which is worth the price cap, instead of exporting, which pays 12p. It is the biggest single variable and only you know it. Try all three settings.
Yes: choose a size, or none. It shows the panels alone, the whole system, and what the battery adds by itself — and in most homes the battery adds the least per pound.
The Ofgem price cap for 1 October to 31 December 2026, 26.32p per kWh (Direct Debit), and Outgoing Octopus for the South of England, 12.0p per kWh flat since 1 March 2026. You can change both under “Change the prices and assumptions”, which is worth doing if you are on a different tariff.
Panel degradation, future price changes, standing charges, finance costs, shading, and the difference between a sunny year and a dull one. It also treats every household’s daytime use as a single percentage. Each of these can move the answer.
Where these figures come from
Every figure above comes from a published source or from arithmetic on one. Read 7 October 2026. Where a source updates, the page should be re-read before a figure is reused.
- EU JRC PVGIS 5.3, PVGIS-SARAH3 2005-2023, 14% system loss — modelled yields
- Ofgem energy price cap, 1 October to 31 December 2026 — electricity 26.32p/kWh (Direct Debit)
- Octopus Outgoing, region H (OUTGOING-VAR-24-10-26, from 1 March 2026) — export 12.0p/kWh
- DESNZ Solar PV cost data 2025/26 (published 28 May 2026), South East mean — £1,859/kW up to 4 kW; £1,721/kW for 4 to 10 kW
- Octopus Energy, battery-only installation (April 2026 prices, read 7 October 2026) — 5 kWh £3,947; 10 kWh £5,336; Tesla Powerwall 3 £7,999; £500 non-refundable advance payment
- Ofgem typical domestic consumption values, from 1 July 2026 — 1,600 / 2,500 / 3,800 kWh a year (low / medium / high)
Get a free, no-pressure quote
Tell us where you are and what you are considering — you will get a straight answer and a fixed written quote after a survey.
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