How to check a solar panel savings forecast

The five inputs behind every forecast

A payback claim is a sum, not a fact. It multiplies how much power your panels make by what each unit is worth. Then it divides the price by the yearly saving. Because every step uses an assumption, the result can swing a lot.

You can check each part, although five inputs matter most.

1. Annual generation in kWh.
2. The share you use at home, called self-consumption.
3. The price of the electricity you no longer buy.
4. The export rate for what you send to the grid.
5. How output falls as the panels age.

Ask the installer for the figure and the source behind each one.

1. Generation: the MCS method and PVGIS

Under the MCS solar standard (MIS 3002 Issue 6.0, dated 18 March 2026), an installer must work out annual output with a set method. First they find your kWp, postcode region, roof pitch and direction. Next they look up a value called Kk in a table. Then they apply a shading factor.

The formula is annual AC output in kWh = kWp x Kk x SF. The installer must give you the result before the contract is awarded, in a set table.

The estimate must carry a warning. MCS says the performance of solar PV is impossible to predict with certainty. Therefore the figure is guidance for the first year only.

You can sense-check the result with PVGIS, the free tool from the European Commission’s Joint Research Centre. We ran PVGIS v5.2 for 1 kWp facing south at a 35 degree pitch with 14% losses. It uses the PVGIS-SARAH2 data for 2005 to 2020.

Your roof will differ, so your MCS figure may not match. However, a quote far above these numbers for a similar roof needs an explanation. Roof direction and system size matter too. Our article on east-west versus south-facing panels explains direction, and our guide to how many panels you need covers size.

2. Self-consumption

Self-consumption is the part of your solar power you use at home. MCS says the estimate must follow its MGD 003 method. The table must name the occupancy pattern it assumed: home all day, home half the day or out all day.

Check that this matches your life. A household out all day uses less solar power directly. Therefore a forecast built for someone at home all day will look better than your real result.

Batteries change this share. If the quote includes storage, MCS says the table must state the usable capacity it assumed. Yet a bigger share is not free, because the battery adds cost. Read our guide to solar battery costs and the article on whether a battery is worth it before you accept that gain.

3. Import price

Each unit you use from your panels saves you the price of a unit from the grid. Since rates change, use a current, dated rate. Ofgem’s price cap for 1 October to 31 December 2026 is 26.32p per kWh for electricity. That is the average for Direct Debit customers in England, Scotland and Wales.

Ofgem sets the cap every three months, and it applies only to tariffs that vary with the market. Ofgem also notes there is no VAT on electricity from 1 October 2026 to 31 March 2027, so this unit rate carries none. Later periods may differ, because Ofgem resets the cap each quarter.

Use your own bill if you are on a fixed tariff. Be wary of any forecast that assumes prices will rise every year without saying by how much.

4. Export rate

Units you do not use go to the grid. Under the Smart Export Guarantee you are paid for them, although the rate depends on the supplier. Our Smart Export Guarantee guide lists dated examples.

For this page, we use the Octopus SEG rate of 4.1p per kWh (its own page, read 10 October 2026). Some suppliers pay more to their own customers, which would shorten the payback.

5. Degradation

Panels lose a little output each year. A review by Jordan and Kurtz for the US National Renewable Energy Laboratory (June 2012) gathered nearly 2,000 measured rates. The median was 0.5% per year.

That review is older, so treat it as a guide. Still, after 10 years, 0.5% a year leaves about 95% of the first-year output. Ask what the panel warranty guarantees. Our article on degradation and performance warranties covers this in more detail.

A worked example

This is arithmetic, not a quote. Every input is dated above, except the self-use share, which only your own estimate can supply. We show three cases because that share matters so much.

• System: 4 kWp in Manchester, so generation is 4 x 886.24 = 3,544.96 kWh a year.
• Cost: 4 x £1,595.06 = £6,380.24. That is the DESNZ 2025/26 median per kW for 0-4 kW installs (total installed capacity basis), multiplied by 4. It is not a price anyone offered.
• Import value: 26.32p per kWh. Export rate: 4.1p per kWh.

Now add degradation. At 50% self-use, losing 0.5% of output each year, with prices held flat, payback rises from 11.8 to about 12.2 years. If the export rate were 12p per kWh instead, the 50% case saves about £679 a year and pays back in 9.4 years.

For comparison, Energy Saving Trust shows 10 years for Manchester when a household is home half the day. Its figures use fuel prices from July 2026 and include export payments. Our result is a little longer. However, the Energy Saving Trust page does not list every input, so we cannot trace the gap.

Questions to put to your installer

Use these on any savings claim.

• Where is the MCS estimate table, and does kWp x Kk x SF match?
• Which occupancy pattern did you assume, and why?
• What unit price and export rate did you use, and when were they dated?
• Do your savings include yearly price rises, and at what rate?
• Did you allow for degradation?
• Is the payback simple, or does it include finance costs?

Finally, compare your quotes side by side using the same inputs. The checks work wherever you live. Homes in the north can read about solar quotes in the North West, and households in the south can see what South East quotes look like.