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Solar Panel Payback

Why solar payback is not a single number, how to work yours out, and the one thing that matters more than the price of the system.

Ask five sources how long solar takes to pay for itself in the UK and you will get five answers between about six and twelve years. They are not disagreeing so much as measuring different things.

We are not going to give you a single number, because a single number would be misleading and we could not stand behind it. What follows is the method, which is more useful, and the one variable that matters more than all the others.

Why one number cannot be right

Payback depends on:

  • What the system cost
  • How much electricity it produces, which depends on your roof, its direction, its pitch, shading and where in the country you live
  • How much of that electricity you use in the house rather than export
  • What you pay for electricity
  • What you are paid for exports
  • Whether you have a battery
  • What tariff you are on

Two households with identical systems on identical roofs can have paybacks years apart. That is not an evasion. It is the actual situation.

The variable that matters most

Self-consumption The share of your generation that you use at home instead of exporting.

At the current price cap you pay about 26.32p per unit of electricity. Flat export rates pay roughly half that.

So a unit you use is worth about twice a unit you export. Everything else in the calculation is secondary to this.

Practically:

  • Someone at home during the day gets far more from the same panels than someone out from eight until six.
  • Running the washing machine, dishwasher and immersion heater during daylight moves units from the export column to the used column.
  • A battery moves a lot of them, which is its whole purpose.
  • A heat pump or an electric car moves a great many of them, because they use much more electricity than anything else in a house.

This is also why the standard advice to “fill the roof” is not always right for somebody with low daytime use and no battery.

Electricity price from the Ofgem price cap for 1 Oct 2026 to 31 Dec 2026. Checked 4 Nov 2026.

Working yours out

Ask your installer for these four numbers and do the arithmetic yourself.

  1. Total installed cost. The cash price, not a monthly figure.
  2. Estimated annual generation in kWh, worked out for your roof. If it looks like a number from a table, ask how it was produced.
  3. Estimated self-consumption, as a percentage. A typical figure without a battery is well under half. With a battery it rises substantially. Ask what they assumed and why.
  4. The export rate you expect to be on.

Then:

Units used at home  = annual generation x self-consumption %
Units exported      = annual generation - units used at home

Annual saving       = (units used at home x your import price)
                    + (units exported x your export rate)

Payback in years    = total installed cost / annual saving

Do it twice. Once with the installer’s self-consumption figure, and once with a figure ten percentage points lower. If the answer only works on the optimistic version, treat the quote with caution.

What the calculation leaves out

Being straight about both sides of it.

Things that improve the real picture:

  • Electricity prices have generally risen over time. A saving made against future prices is worth more than a saving made against today’s.
  • Panels last a long time. Payback is not the end of the benefit, it is the point where the benefit starts being profit.
  • If you later add a heat pump or an electric car, your self-consumption rises and the system gets better without you spending anything more.

Things that make it worse:

  • The inverter will probably need replacing before the panels do.
  • Output falls slightly every year. See panel degradation.
  • Money spent on solar is money not invested elsewhere.
  • There is currently no VAT on electricity bills from 1 Oct 2026 to 31 Mar 2027, which makes the electricity you are avoiding buying temporarily cheaper, and therefore makes your saving temporarily smaller.

That last point is the kind of thing most solar content leaves out. It is genuine and it is temporary.

The figure we are willing to quote

Government’s own estimate is that households could save around £500 a year by installing rooftop solar. That comes from the Solar Roadmap, it is the government’s figure rather than ours, and like any average it will be wrong for any particular house.

Source: Solar Roadmap. Checked 4 Nov 2026.

Payback is not the only reason people install

Worth saying, because framing solar purely as an investment sets up a comparison it does not always win:

  • Predictability. Generating some of your own electricity makes your bills less sensitive to the price cap.
  • Carbon. See carbon reduction.
  • It pairs with other things. Solar plus a battery plus a smart tariff plus an electric car is a system, and each part makes the others work harder.
  • Property. An installed system is an asset on the house.

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