Every assumption behind our payback figures.
Our calculator uses fixed figures so results are consistent, and shows its answer as a range. This page lists them. They are our own planning assumptions, not guarantees, and a survey replaces them with figures for your property. Rates were last checked in September and October 2026.
How much your panels make.
Annual generation is system size multiplied by 850 kWh per kWp, adjusted for roof direction and shading, using 465 W panels. Output falls 0.4% a year.
Roof direction
South 100%, south-east or south-west 95%, east or west 85%, north 65%.
Shading
None 100%, some 90%, heavy 75%.
Panel ageing
Output falls 0.4% a year over 25 years.
Monthly split
Approximate for central Scotland: about three-quarters between April and September. Within each day, output follows the sun from sunrise to sunset.
How savings are worked out.
We simulate a typical day in every month, hour by hour, and add up the year. Savings come from electricity you use instead of buying, from the battery shifting power to when it is worth more, and from electricity you sell back. Each unit generated is counted once: used directly, stored then used, or sold. Several inputs are our own planning assumptions rather than verified figures (how much of a home's use falls in the day, the battery limits, the tariff presets and the 5% price rise), so the result is shown as a range.
Who is home
Your home's electricity use follows a daily pattern, which changes with whether anyone is in the daytime. This is a planning assumption.
- Out all day, part of the day, or mostly home
- More use in winter than summer
- Whatever the panels make beyond what the home is using at that moment goes into the battery, or is sold back
- A larger array on a smaller home therefore sells more back
Tariffs
The tariff you choose sets the price of each hour. These are illustrative presets, not your own tariff.
- Standard: 26.3p a unit all day (Ofgem cap, October 2026), export 15p
- Cheap night: 13.89p from midnight to 5am, 26.3p otherwise, export 15p
- Peak export (Octopus Flux, South Scotland, September 2026): import 15.0p, 25.0p, then 34.9p at 4pm to 7pm; export 4.4p, 9.6p, then 27.2p at 4pm to 7pm
- Tariff rates change often, so these are illustrative planning figures and may be out of date
Price rises
Electricity prices are assumed to rise each year. This is a planning assumption, not a forecast.
- 5% a year, applied to every rate
- Payback shown with and without price rises
- Figures cover 25 years
Battery and EV assumptions.
Extra items in the model. All are planning assumptions.
Battery
95% of its size is usable and 92% comes back out after losses. It charges and discharges at up to 5 kW. These are our planning assumptions, based on the Fox ESS EVO range, and are not verified for every installation.
Battery life
Capacity fades 2.5% a year and its benefit is counted for 12 years. Both are our own assumptions.
How it is used
It stores spare solar for the evening. On a cheap night tariff it tops up from the grid only as much as the next day needs. On a peak-export tariff it fills overnight and sells at the peak.
EV charging
8,000 miles a year at 3.5 miles per kWh, about 2,290 kWh. It charges overnight on a smart tariff, in the evening on a standard one, and more in the daytime if someone is home.
Prices.
Prices in the calculator are EcoGrid's set prices by panel count and battery size, with a discount when solar and a battery are bought together, and a fixed price for the EV charger. Solar and battery installs at a home are 0% VAT until 31 March 2027. A survey confirms your actual price.
- Set prices by panel count and battery size
- Discount when solar and battery are combined
- Fixed price for a fitted EV charger
- 0% VAT on solar and batteries at a home to 31 March 2027
- Confirmed after a free survey
What these figures do not cover.
Real results vary. Please read these limits alongside any estimate.
Export income
Payments for electricity sold back are made through the Smart Export Guarantee, which needs an MCS-certified installation. Export rates depend on the supplier and tariff.
Your home is different
Real savings depend on your actual usage pattern, tariff, roof, shading and future energy prices. A typical day is a model, not a meter reading.
Electricity VAT
The 26.3p is the Ofgem cap, which includes the temporary 0% VAT on electricity to 31 March 2027. VAT returns after that, which would nudge prices up.
Questions about the calculator.
Can I change the assumptions?
No. They are fixed so every estimate uses the same method. You can choose whether anyone is home in the daytime and which tariff you are on. A survey produces figures specific to your property.
Why is payback shown as a range?
Several inputs are planning assumptions rather than verified figures, and real prices and usage are uncertain, so a single number would suggest more precision than we have.
Why is payback shown two ways?
One assumes electricity prices stay the same and one assumes they rise 5% a year. Real prices are uncertain.
Why does the battery sometimes add very little?
A battery only earns money by moving electricity to a time when it is worth more. On a standard tariff the gain is small. A cheap night tariff helps, and a peak-export tariff helps most, but every tariff has limits and the rates change.
Are the prices final?
No. They are set prices for planning. Your quote is confirmed after a survey.
How often are the figures updated?
We review them when prices, tariffs or our pricing change. Tariff rates were last checked in September and October 2026.
Want figures for your own home?
Use the calculator for an estimate, or call to book a free survey and get a quote based on your property.