Solar Panel Payback: How It Is Calculated

In its simplest form, payback in years is the installed cost of the system divided by the annual financial benefit it produces. The installed cost is the fixed written quotation for the actual scope of work, including scaffolding and any electrical work the property needs. The annual benefit is made of two quite different things, which should never be combined without being shown separately.

Avoided import. Every unit generated and used inside the property is a unit you do not buy. It is worth your import unit rate. The number of such units is annual generation multiplied by your self-consumption rate — the share of generation your household actually uses at the moment it is produced, or stores for later if you have a battery.

Export income. Every unit not used in the property goes to the grid and is paid for at your Smart Export Guarantee rate. SEG licensees set their own rates, contract lengths and terms; Ofgem does not set the rate, and rates change. Because export rates are normally lower than import rates, exported units are typically worth less than units used at home.

So the annual benefit is: generation multiplied by self-consumption rate, valued at your import rate, plus generation multiplied by the remaining share, valued at your export rate. Anything more sophisticated — price inflation, discounting future money to present value, inverter replacement, degradation of the array over its life — is a refinement layered on top, and each refinement is another assumption that ought to be visible.

Two conventions are worth knowing about. Some calculations apply an assumed annual increase in electricity prices, which shortens payback; others hold prices flat, which lengthens it. Some subtract nothing for the gradual decline in array output; others apply the manufacturer's published degradation rate. Neither approach is wrong, and a figure produced under one is not comparable with a figure produced under the other. This is the single most common reason two payback figures for the same house disagree.

This distinction does more work than any other in a payback calculation, and it is the one most often blurred.

A saving is avoided import. It has the value of the electricity you did not buy, which is your import unit rate — the rate on your bill, including any standing structure your supplier applies to units. Savings happen only when generation and demand coincide, or when a battery moves generation to a moment when demand exists.

Export income is payment for units sent to the grid under the Smart Export Guarantee. It is real income, it arrives from your SEG supplier rather than as a reduction on your import bill, and it is normally paid at a lower rate than you pay to import. Eligibility depends on the installation being certified appropriately; suppliers will usually ask to see the certificate.

The practical consequence: raising self-consumption converts lower-value exported units into higher-value avoided import. That is the mechanism behind most sensible advice about running appliances in the middle of the day, and it is the main reason storage can improve the financial picture. It is also why the same array on the same roof performs differently in financial terms for two different households.

Our advice articles on how much solar panels can save and the Smart Export Guarantee go through both sides in more detail.

These are two different investments and they should be assessed as two cases on the same set of assumptions, then compared. Presenting the combined system as though it were simply a better version of the array obscures which part of the benefit comes from which part of the spend.

Solar only

The array generates, the household uses what it can at the moment of generation, and the rest is exported. The self-consumption rate is set by the occupancy pattern and by how loads are timed. The financial result is strongest where someone is at home during the day, or where large loads can be moved into daylight hours. Adding capacity beyond what the household can use raises export volume, which is worth less per unit than avoided import.

Solar plus battery

The battery stores surplus that would have been exported and releases it in the evening, so more generation is valued at the import rate instead of the export rate. On a time-of-use tariff it can also charge from the grid when electricity is cheap and discharge when it is expensive, which produces value all year rather than only in the months when the array has surplus. Against that, the battery adds to the installed cost, carries its own warranty limits in throughput or cycles, and loses a small proportion of the energy that passes through it to round-trip efficiency.

The battery case therefore depends heavily on two things the array case does not: how often the battery actually cycles, and how wide the price spread on your tariff is. Both are covered on our battery storage cost page. A comparison worth having puts the two cases side by side, on the same generation estimate, the same tariff assumptions and the same treatment of future prices, and shows the working for each.

The cost half of a payback sum is the installed price, and VAT treatment is part of it. Installation of qualifying energy-saving materials — including solar panels, and electrical storage batteries since 1 February 2024 — in residential accommodation in Great Britain is zero-rated for VAT from 1 May 2023 to 31 March 2027, reverting to the 5% reduced rate from 1 April 2027. A calculation carried out for work completed after that date has a different cost input from one carried out before it, and any comparison across that boundary should say so. HMRC VAT Notice 708/6 is cited in the sources below.