How much can solar panels save?

There is no single answer, and anyone offering one without seeing your roof and your consumption is guessing. A solar saving is the sum of two streams — the electricity you no longer buy, and the export you are paid for — and both depend on variables specific to your property and your household. What we can do usefully is set out exactly which variables drive the result, and what assumptions any credible estimate has to state. Read a quotation with those in hand and you can tell a serious estimate from an optimistic one in about two minutes.

The two streams

Avoided import. Every unit your house consumes from the array is a unit you do not buy. It is worth your import rate, including the tax and levies on that rate. This is almost always the larger stream.

Export income. Every unit you do not use flows to the grid. Under the Smart Export Guarantee, larger licensed suppliers must offer an export tariff, but each supplier sets its own rate, contract length and terms — the only requirement is that the rate is above zero. Ofgem does not set the rate.

Because export rates are generally lower than import rates, the saving is dominated by how much you use yourself.

The variables that actually move the number

Generation. Determined by array size, orientation, pitch, shading and location. South-facing gives the highest annual total; east–west spreads output across morning and evening. Shading from a chimney or a neighbour’s tree can cost more than the panel count suggests.

Self-consumption share. The proportion of generation used on site. A household at home during the day, or one with storage, an EV or a heat pump, uses far more of its own generation than an empty house on a flat tariff.

Your import tariff. A high unit rate makes avoided import more valuable. A low one makes it less so. Standing charges do not change with solar.

Your export tariff. Rate structure matters as much as headline rate — a time-varying export tariff pays differently by hour.

System cost. Including VAT treatment. Installation of qualifying energy-saving materials in residential accommodation in Great Britain is zero-rated from 1 May 2023 to 31 March 2027, reverting to the 5% reduced rate from 1 April 2027.

Future energy prices. Nobody knows these. Any payback figure embeds an assumption about them, and that assumption should be visible.

Ongoing costs. Inverters have a service life and will need replacing at some point. Systems benefit from occasional cleaning and checks. A payback calculation that assumes zero lifetime cost is incomplete.

What a credible estimate states

Ask for these in writing:

• The estimated annual generation and the method used to produce it
• The assumed self-consumption percentage, and why
• The import tariff and export rate used
• Any assumed energy price inflation
• Whether the figures are annual or cumulative
• What is excluded — inverter replacement, maintenance, cleaning

If a document gives you a number and none of the above, it is marketing, not analysis.

Where published figures fit

Independent bodies such as the Energy Saving Trust publish illustrative savings for typical households. These are useful for orientation and for sanity-checking a quotation that looks wildly out of line. They are industry figures for an illustrative property, not Solar Fx figures for yours, and they carry their own stated assumptions which may not resemble your situation at all.

Use them as a reasonableness check, never as a forecast.

Levers you control after installation

The saving is not fixed at commissioning. Households that pay attention typically do better than those that do not:

• Shifting flexible loads — laundry, dishwasher, EV charging, immersion — into generation hours
• Configuring battery charge and discharge windows to suit the tariff
• Reviewing your import and export tariffs periodically rather than once
• Watching monitoring data and investigating anything that looks wrong early

How we handle it

We produce a performance estimate from the survey data, using the standard method, and we print the assumptions next to the figures. We would rather have an awkward conversation about a modest number at quotation stage than a much worse one two years later.

Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.