Lower bills
Is solar worth it for you? Run the numbers.
Put in your own figures. See payback and yearly return, then compare with leaving the money in savings or borrowing to buy.
We are not qualified or regulated financial advisers. The calculator works from the figures you enter and from assumptions that may not hold: energy prices, export rates, interest rates and system output all change. Speak to a regulated financial adviser before borrowing or moving savings.
Renting, or no roof to use? Plug-in solar kits plug into a socket and have their own calculator.
What would you like to work out?
Battery only: buy power cheap, use it later
Many tariffs sell electricity far more cheaply at set times, usually overnight. A battery fills up in those hours and runs your home through the rest of the day, so less of what you use is bought at the full price.
- Charge at the cheap rate. The battery fills from the grid while power is cheap.
- Use it when power costs more. Your home draws on the battery instead of buying at the full price.
- Your average price falls. More of your electricity is bought at the cheap rate, so each kWh costs you less on average.
About 10% of the energy is lost going in and out of the battery, so the gap between the two rates is what matters: the wider it is, the more you save. With no solar there is nothing to export, so the sums are simpler.
With the right wiring the same battery can also keep your essentials running in a power cut. See power-cut protection.
You need a tariff with a cheap period. Rates change often, so check what suppliers offer now.
How the sums work
Solar saves money in two ways. Each unit you use from your own panels is a unit you do not buy, so it saves the full price you pay: 26.32p under the current price cap. Each unit you cannot use is exported and earns your export rate, typically about 12p.
The calculator multiplies your array size by a yearly yield to get generation, splits it into self-use and export, and prices each part. It then repeats this for every year, with panel output falling 0.5% a year and import prices rising at the rate you set. Export rates are held flat.
A pound in 20 years buys less than a pound today, so every later figure is turned into today’s money using the general inflation rate you set (3% to start). That applies equally to solar, savings, investments and loan repayments, so the comparison stays fair. Year one is at today’s prices.
Battery only
Choose Battery only at the top of the calculator. It works out how much of your full-price use a battery can move to your cheap rate each day, up to its usable size. Each unit moved saves the normal price less the cheap price, with 10% allowed for losses. Battery capacity is assumed to fall 1.5% a year.
Peak and off-peak tariffs
If your price changes through the day, tick the time-of-use box and enter each band’s price and times. Solar you use as it is made replaces whatever you pay at that hour, which is usually the day or peak rate. Solar stored in a battery is used in your dearest hours. With grid charging on, the battery also fills up at your cheapest rate and is used when power costs more. The calculator allows for 10% lost in the battery.
It works from a typical day for each month and a typical household pattern of use. It does not know your actual half-hourly readings, so treat the result as a guide.
What moves the answer most
- How much you use yourself. A home that is empty all day might use a third of its solar. A battery, a hot water diverter or daytime use of appliances raises that.
- What you paid. Quotes for the same system vary by thousands. Get three.
- Your tariff. Some export tariffs pay well above the standard rate if you also buy your electricity from that supplier.
- The battery. A battery raises the saving but often lengthens the payback, because it adds a lot to the cost. Tick the battery box to see both.
What solar does not reduce
The daily standing charge, 54.83p a day at present. You pay it however little you import.
Cash, savings or a loan?
The useful comparison is between rates.
- Solar against savings. If solar's estimated return is higher than your savings rate, the panels are likely to do more for you than the bank. Savings are certain and instantly available. Solar is neither.
- Solar against a loan. Borrowing helps only when the system returns more than the loan costs. At a low APR the bill savings can cover the repayments. At a high APR you pay the lender more than the sun gives you.
- Solar against your debts. Paying off a credit card at 23% APR is a certain 23% saving. Very little beats that. Clear expensive debt first.
Rates used as starting points
| Electricity unit rate (price cap average, 1 October to 31 December 2026) | 26.32p per kWh |
| Standing charge | 54.83p a day |
| Export rate | 12p per kWh |
| Savings rate | 4.5% AER |
| Investment return assumed (before inflation) | 8% a year |
| General inflation assumed | 3% a year |
| Bank of England base rate | 4% |
| 4 kWp solar, installed, no battery | £7,350 |
| 4 kWp solar with a battery, installed | £11,500 |
Rates checked 10 November 2026. Sources: Ofgem, Bank of England, MCS installation data and published supplier tariffs.
VAT
Solar panels and batteries installed in homes are charged at 0% VAT until 31 March 2027. The relief applies to installed systems. A battery bought on its own for DIY fitting is charged at the standard rate.
Use these results to ask better questions, not to make the decision for you.