Estimating solar panel output helps you understand how much electricity your roof could generate, how much you could save on energy bills and how long it might take to recover the installation cost.
This guide explains these calculations step by step, using the approach followed in MCS-certified quotes. The examples provide a starting point, but a full survey is needed for an estimate tailored to your property, taking account of your roof’s orientation, pitch and shading, alongside your household’s electricity use.
The Formula: How Solar Output Is Calculated
To estimate how much electricity your solar panels could generate in a year, MCS-certified installers in the UK use a standardised calculation set out in MCS 032, the solar PV performance-estimate standard. It combines three inputs: your system’s size, a generation value based on your location and roof position, and an adjustment for shading:
Annual output (kWh) = System size (kWp) × Kk value × Shade factor (SF)
The three inputs are:
System size in kWp - the total peak power of your panels. A typical UK home installs between 3 and 5 kWp.
Kk value - an MCS estimate of annual electricity generation per kWp, selected for your postcode region, roof orientation and panel pitch. The MCS Kk tables use PVGIS solar-resource data and already include the standard 0.8 performance adjustment for typical system losses, so no further 0.8 adjustment is applied.
Shade factor (SF) - for the general examples below, we use SF = 1.00. This assumes an obvious clear horizon with no meaningful near or far shading. Where trees, chimneys, nearby buildings or other obstructions shade the array, an MCS installer will calculate a lower site-specific factor. A predicted 10% annual shading loss gives SF = 0.90.
Step One — Find Your Kk Value
MCS assigns every UK postcode area to a Kk region, and publishes Kk tables for each region at every roof pitch and orientation. The higher the Kk, the more electricity each kWp of panels is expected to generate.
The table below gives example Kk values for a south-facing roof at 35° pitch in a range of UK locations. It is a guide, not a postcode lookup - some regions cover several postcode areas, and not every region is shown.
| Region | Example location | Kk (kWh/kWp/year) |
|---|---|---|
| Zone 2 | Brighton | 938 |
| Zone 4 | Plymouth | 907 |
| Zone 3 | Southampton | 857 |
| Zone 1 | London | 828 |
| Zone 5E | Bristol | 820 |
| Zone 12 | Norwich | 805 |
| Zone 5W | Cardiff | 803 |
| Zone 6 | Birmingham | 789 |
| Zone 13 | Aberystwyth | 789 |
| Zone 7W | Chester | 779 |
| Zone 11 | Sheffield | 750 |
| Zone 10 | Middlesbrough | 750 |
| Zone 9E | Newcastle | 742 |
| Zone 9S | Edinburgh | 738 |
| Zone 7E | Manchester | 735 |
| Zone 8E | Carlisle | 731 |
| Zone 8S | Dumfries | 722 |
| Zone 16 | Aberdeen | 712 |
| Zone 21 | Belfast | 711 |
| Zone 14 | Glasgow | 701 |
| Zone 17 | Inverness | 691 |
| Zone 18 | Stornoway | 678 |
| Zone 19 | Kirkwall | 649 |
| Zone 20 | Lerwick | 630 |
Source: MCS Kk datasets, south-facing (0°), 35° pitch. Kk values already include the MCS 0.8 performance adjustment.
Use the location closest to you as a rough guide. For a precise figure, MCS-certified installers use the Kk table for your postcode region and your roof's measured orientation and pitch - your Switch Together quote will do this automatically.
Step Two — Calculate Your Annual Output
Multiply your system size by your Kk value, then by your shade factor (1.00 for an unshaded roof). Here are worked examples for three common home types across three UK locations:
| Home Type | System Size | Brighton (Kk 938) | London (Kk 828) | Manchester (Kk 735) |
|---|---|---|---|---|
| Small terraced | 3 kWp | 2,814 kWh/yr | 2,484 kWh/yr | 2,205 kWh/yr |
| Semi-detached | 4 kWp | 3,752 kWh/yr | 3,312 kWh/yr | 2,940 kWh/yr |
| Detached | 5 kWp | 4,690 kWh/yr | 4,140 kWh/yr | 3,675 kWh/yr |
Calculated as kWp × Kk × 1.00 (no shading), south-facing, 35° pitch.
To put those numbers in context, DESNZ statistics for 2024 show mean annual domestic electricity consumption of 3,323 kWh per meter across Great Britain (the median is lower, at 2,471 kWh). A 4 kWp system in London would generate roughly the same amount over a year. But generation and demand happen at different times: without storage, households still import electricity at night and during lower-generation periods, so matching annual demand does not mean meeting all of it from solar.
Step Three — Calculate Your Savings
To estimate your direct bill savings, you need to know how much of the electricity your panels generate is used in your home. This is called self-consumption. Each unit of solar electricity you use replaces a unit you would otherwise buy from the grid, so annual generation alone does not tell you how much you will save.
Your self-consumption rate depends on how much electricity your household uses and how closely that use matches the times your panels generate power. If you have a battery, its usable storage capacity and the way it is controlled also affect this rate. The examples below assume that you use 40% of your solar generation without a battery and 70% with a battery. These are illustrative assumptions, not predictions for your home; an MCS quote should estimate self-consumption using your household’s electricity demand and the times people are at home.
Solar panel savings calculation formula
Direct bill saving (£) = Annual output (kWh) × Self-consumption rate × Import unit rate (p/kWh) ÷ 100
We use an illustrative electricity import rate of 26.32p/kWh - Ofgem's average price cap unit rate for customers paying by Direct Debit from 1 October to 31 December 2026. Your actual saving depends on your tariff, region, payment method and future energy prices.
| Scenario | System | Location | Output | Self-consumption | Direct Bill Saving |
|---|---|---|---|---|---|
| No battery | 4 kWp | Brighton | 3,752 kWh | 40% | ~£395 |
| With battery | 4 kWp | Brighton | 3,752 kWh | 70% | ~£691 |
| No battery | 4 kWp | London | 3,312 kWh | 40% | ~£349 |
| With battery | 4 kWp | London | 3,312 kWh | 70% | ~£610 |
| No battery | 4 kWp | Manchester | 2,940 kWh | 40% | ~£310 |
| With battery | 4 kWp | Manchester | 2,940 kWh | 70% | ~£542 |
Note: these figures cover direct savings from self-consumed solar only. Electricity you export can also earn Smart Export Guarantee (SEG) payments, which add to your total financial benefit. A battery raises self-consumption, but it also adds to the upfront cost.
For full savings figures including battery storage combinations and home types, see our guide: How Much Can Solar Panels Save on Energy Bills?
Step Four — Calculate Your Payback Period
Once you know your annual financial benefit, a simple payback period is:
Simple payback (years) = Installation cost (£) ÷ Annual financial benefit (£)
Your annual financial benefit is your direct bill saving plus any export income. A lifecycle view would also allow for ongoing costs such as maintenance and inverter replacement.
Worked examples (panels only, no battery):
These examples use an installation cost of £6,630 for a 4 kWp panels-only system, based on the 2025 average of £1,657.47 per kWp for domestic solar PV installations on the MCS Data Dashboard (January to December 2025). They also use an illustrative export rate of 12p/kWh on the 60% of generation not used in the home. Your own quote and export tariff will differ, so use those figures for your calculation.
| System | Location | Install Cost | Direct Saving Only | Payback (direct saving only) | Direct Saving + Export | Payback (incl. export) |
|---|---|---|---|---|---|---|
| 4 kWp | Brighton | £6,630 | ~£395 | ~16.8 years | ~£665 | ~10.0 years |
| 4 kWp | London | £6,630 | ~£349 | ~19.0 years | ~£587 | ~11.3 years |
| 4 kWp | Manchester | £6,630 | ~£310 | ~21.4 years | ~£521 | ~12.7 years |
A few important caveats on these figures. If unit rates rise, savings increase and payback shortens; if they fall, payback lengthens. Export tariffs vary widely between suppliers and change often, so check current rates before relying on export income. Adding a battery or switching to a time-of-use tariff can change the numbers in either direction, depending on the extra cost and how the system is used.
Get your personalised solar estimate
The figures above are illustrative examples - your actual output depends on your exact roof angle, orientation, any shading, and your household's energy habits. Register with Switch Together, and we'll calculate it properly for your property, for free, with no obligation.
How Roof Orientation Affects Your Calculation
The worked examples above assume a south-facing roof at 35° pitch. Output varies with both orientation and roof pitch. A south-facing array near the optimal tilt generally maximises annual generation, but east–west arrays can still be worthwhile and may spread production more evenly across the day. Even north-facing arrays can be viable at low pitches.
There is no single percentage adjustment for orientation: the MCS Kk tables cover every pitch from 0° to 90° in 1° steps and orientations in 5° steps, and your installer should use the Kk value for your array's measured orientation and inclination.
For a full breakdown of how orientation, shading, and panel efficiency interact, see our guide: Solar Panel Efficiency Explained
Frequently Asked Questions
How do I calculate how many solar panels I need? As a rough check, compare your annual electricity use (from your energy bill) with the expected output per panel. In London, a 400W panel on a south-facing 35° roof generates around 331 kWh per year (0.4 kWp × 828 × 1.00), so around ten panels (a 4 kWp system) would generate about 3,323 kWh a year - the GB mean annual consumption per meter. But matching annual demand isn't the same as the right system size. Usable roof area, layout, shading, future loads such as an EV or heat pump, network (DNO) limits, export arrangements and budget all matter, and you'll still use the grid at night unless you add storage.
What is a kWp and how does it affect my calculation? kWp stands for kilowatt-peak — the maximum power your system produces under ideal test conditions. It's the standard unit used to size solar systems. A 400W panel has a rating of 0.4 kWp. Ten of those panels gives you a 4 kWp system. The higher your kWp, the more you generate — but you're limited by roof space and budget.
How accurate are solar power calculations? MCS estimates its shade-assessment method will give results within 10% of actual annual generation for most systems, but no estimate is a performance guarantee. Real-world output varies with weather from year to year, shading not captured in the survey, and how well the system is maintained. MCS installers must make clear that their estimate is guidance, not a guarantee. Even so, an estimate based on your postcode's Kk table and your actual roof orientation, pitch and shading will be more reliable than one based on national averages.
What is the Smart Export Guarantee and should I include it in my calculations? The Smart Export Guarantee requires electricity suppliers with more than 150,000 domestic customers to offer at least one export tariff. Rates and tariff terms vary by supplier, and you need a compatible export meter and an eligible installation to be paid. Using the 40% self-consumption assumption above, a home without a battery would export around 60% of what it generates, so export income can add meaningfully to your returns. Ask your installer to include export projections in their estimate, and check current tariffs before you choose a supplier.
Does the calculation change if I have an electric vehicle or heat pump? Yes — both significantly increase your household consumption, which changes the optimal system size and self-consumption rate. An EV charged during the day can absorb a large share of your solar generation directly. A heat pump running in winter draws more power when solar output is lowest. If you have either, or plan to, tell your installer — it changes the sizing recommendation considerably.