Savings and payback, in plain numbers

Plug-in solar kits: the one that pays back

A plug-in kit makes electricity you use straight away, so every unit it makes in daylight is a unit you don't buy. The government worked out that an 800 W kit facing south and tilted makes about 690 kWh a year, and a home with someone in during the day uses about 62% of it. At 26.32p a kWh that is roughly £110 a year. On a wall or balcony it is less, because upright panels catch less sun.

Saving a year and years to pay back, with someone home most of the day and little shade, at 26.32p a kWh. South tilted and east/west wall use the government's own figures; the south wall row is our estimate.

Three things change your figure most: which way the panels face, how much shade they get, and whether anyone is home in the day to use the power. Put your own answers into the plug-in solar calculator and it shows savings and payback for every kit. The worth-it guide explains the government figures.

Over 15 years, a kit that saves £100 a year puts back £1,500. Panels are usually warranted for 20 to 25 years; the small inverter that turns their power into mains power usually lasts 10 to 12, so allow for replacing it once.

Power stations: insurance, not an investment

People ask whether they can charge a power station cheaply overnight and run the house from it by day. You can, but it rarely pays. The saving per unit is the gap between the night rate and the day rate, about 17.8p, and every charge uses up one of the battery's rated cycles. Once you count that wear, the net saving is a few pounds to a few tens of pounds a year, so the station itself takes decades to pay back, as the table shows. It also assumes you really do use a full charge every single day.

One full charge a night at 8.5p a kWh (a cheap overnight tariff), used the next day instead of 26.32p electricity, with 85% of the stored power reaching your appliances. Battery wear is the price spread over the maker's rated charge cycles. Your tariff will differ: check your own rates.

What a power station does buy you is cover: a fridge, router, lamps and phones through a power cut, or mains power where there is none. Judge it on hours of power-cut cover and what it can run, not on bill savings. Charging it from a folding panel does save money, because the sunshine is free.

Folding panels: free charging, slow payback

A folding panel charges a power station from sunshine. A 200 W panel adds about 0.47 kWh on an average UK day, which is a fridge-freezer's daily use. The saving is real but small, so a panel pays back fastest when you already own the station and use the stored power every day, for example in a shed, van or home office.

Panel moved to face the sun, government UK sunshine figure, every unit stored then used in the home at 26.32p a kWh. Days you don't use the stored power don't count, so treat these as best-case figures.

Home solar batteries: ask for a quote, not a rule of thumb

A home battery is wired in by an electrician and sits outside the plug-in rules, so we list them without scores or savings figures. What it saves depends on three things we cannot know from here: whether you have rooftop solar to fill it, whether your tariff has a cheap overnight rate, and how much electricity you use in the evening. Installers quote savings of a few hundred pounds a year for a typical 5 to 10 kWh battery with solar, against a fitted cost of several thousand pounds, so paybacks of roughly 8 to 12 years are common. Get two written quotes that show the sums for your own home. Why batteries need an electrician.

Costs people forget

• Electrical safety check (EICR) if your wiring hasn't been checked recently: £100 to £250.
• New outdoor socket fitted by an electrician: £140 to £260.
• Replacing the inverter on a plug-in kit once in its life: usually £100 to £250.
• Standby power. A power station left switched on uses a little power itself; turn it off when idle.

An electrical check or new socket adds a year or two to a kit's payback. The calculator lets you add them.

How we work it out

• Electricity price: 26.32p a kWh, the Ofgem price cap unit rate (Ofgem). We update it each quarter.
• Sunshine: 862 kWh a year for every 1,000 W of panels facing south at 30° (DESNZ analytical annex).
• Self-use: 62% of a tilted kit's output and 72% of an upright wall kit's output is used in the home when someone is in by day (DESNZ). Out all day, we assume less.
• Night rate: 8.5p a kWh, our rounded figure for a cheap overnight tariff. Your own rate may be higher.
• Battery wear: price divided by the maker's rated cycles, times one cycle a day.
• Panel watts above 800 W count at half value, because the inverter caps output at 800 W in strong sun.

Full method on the how we score page. These are estimates to help you compare, not a promise of what you will save.

Not electrical advice. We compare products; we are not electricians. Before you connect a solar kit to your home, ask a qualified electrician to check the socket, the circuit and your consumer unit, and follow the maker's instructions.