MCS Performance Estimate: How to Read Your Solar Quote

What the MCS performance estimate actually is

MCS, the Microgeneration Certification Scheme, is the quality mark behind almost every domestic solar job in the UK. Install to its standards (MIS 3002 is the one covering solar PV design and installation) and you keep the door open to Smart Export Guarantee payments and most grant routes. Skip it and your system still works, but you can’t claim SEG payments unless it’s covered by an equivalent scheme such as Flexi-Orb.

Part of working to the standard is giving you an estimate of your system’s annual output before you sign. It isn’t a bespoke simulation with a hidden spreadsheet behind it. MCS sets out a standard method:

Kk is a yield factor in kWh per kWp, looked up from MCS tables using your postcode region, your roof’s orientation (degrees from south) and its pitch. An assumed performance ratio of 0.8 is already built into it, so heat, inverter and cable losses are covered. The shade factor runs from 1.00 (no shading) downwards, and comes from a shading survey. That’s all the estimate is, and it’s why it’s so easy to check.

Here’s what the estimate page looks like, using illustrative numbers for a south-facing Leeds semi:

Two things the estimate is not. It isn’t a guarantee, because the weather doesn’t sign contracts, and it isn’t a measurement of any particular year. It’s a normalised forecast for an average one. An honest installer says so out loud. The MCS website publishes the standards installers work to, and I’ve written separately about what the MCS badge on a van should and shouldn’t reassure you about.

The five things to find on the estimate

1. System size, in kWp. Kilowatt-peak: panel count times panel wattage. It’s a capacity, not an energy amount. A 4.0 kWp array tells you nothing about your year until it meets your roof and your sky.

2. Postcode region, orientation and pitch. These three inputs choose your Kk. If any of them is wrong or missing, the estimate is decoration. Check them against your own roof.

3. Kk, in kWh per kWp. The yield factor itself. Because it already includes the 0.8 performance ratio, you shouldn’t see a separate “loss stack” that quietly takes another 20% off, and you shouldn’t see a yield far above the regional bands below.

4. The shade factor. 1.00 means no shading. Anything below should come with a shading survey (MCS uses a sunpath diagram). A chimney, a tree or a neighbour’s extension on your survey photos with a shade factor of 1.00 is a red flag.

5. Estimated annual generation and your implied yield. The headline kWh is what the salesperson quotes on the phone. Divide it by the kWp and you get kWh per kWp, which equals Kk × shade factor. That’s the number that lets you compare any two quotes on any roof in the country.

The 60-second sanity check

One division turns any quote into a comparable number, and that number has a ceiling set by British weather. Here’s how it plays out on four worked examples (illustrative, not real customers):

What a realistic UK yield looks like in 2026

For an unshaded, south-facing roof at 30–40° pitch, in year one and before degradation, these are the bands I expect a quote to land in. They match the regional yields in our payback period guide:

Bands, not table values: MCS splits the UK into more zones than this, so your exact Kk will differ a little. The east–west columns apply the 80–85% (pitched) and 86–90% (flat roof) orientation indices to the south-facing range.

Postcode matters more than people think, though less than the sunshine-hours league tables suggest. Scottish yields run roughly 15–20% below the south coast, which is enough to change your payback by a couple of years. I keep a note of the UK’s sunniest and least sunny postcodes handy when a quote lands.

My rule: if a quote’s implied yield sits more than about 5% above the top of your regional band, ask why. Above 1,100 kWh/kWp almost anywhere in the UK, I want to see the inputs behind it, not a summary of them.

Orientation, tilt and the shading tax

South at 30–40° is the benchmark everything else is measured against. Once your roof points somewhere else, the yield slides, and a quote that ignores that slide is inflated by omission, even if nobody meant it. These indices come from MCS-style irradiance data and PVGIS modelling, and they’re the same ones we use in our guide to east–west solar arrays:

North-east and north-west faces sit between the east–west and north figures, so ask for the Kk for your exact angle. If you’re weighing up a north slope, our north-facing roof guide goes through when it still makes sense.

East–west roofs aren’t the disaster folklore makes them. They lose peak output but gain a long, flat generation curve that suits people home in the mornings and evenings, and a split roof can still beat a small south slope on total kWh because you can fit more panels.

Shading is the silent tax, and it costs more than the shaded area suggests. On a basic string system, a shadow over one panel can drag the whole string down; our underperforming panels guide gives an example where shading just 10% of a panel cuts a string’s output by 30–40%. Optimisers or microinverters contain that damage to the shaded panel alone, which is exactly what our micro inverter vs string inverter comparison digs into. If your survey shows shading but the design has no mitigation, ask how the shade factor was worked out and what it assumes about the inverter set-up.

How yields get inflated (usually by accident)

Outright invention is rarer than carelessness. Because the MCS method is just kWp × Kk × shade factor, there are only a few places for an error to creep in. Here’s the usual list, roughly in the order I meet it:

• The wrong roof direction. The orientation stays at south because nobody changed the dropdown, so an east–west roof gets a south-facing Kk.
• The template postcode region. The Kk row never gets updated from head office’s home patch, so a roof in Essex quietly borrows a sunnier region’s number.
• A shade factor of 1.00 over a chimney or tree. The survey photos show the problem, the estimate page doesn’t.
• A different number on the front page. Sales software, brochure maths (panel wattage multiplied by sunshine hours in somebody’s head) or a simulation tool gives a rosier figure than the MCS estimate page. The MCS number is the one that should match the contract.
• “Up to” and “first year” weasel words. A ceiling or a best case standing in for an average.
• Payback pressure. A bigger yield means a shorter payback means a signature today. Sometimes that one is deliberate.

None of this changes what your roof will actually produce. It only changes what you believed on the night you signed. Several of these tricks overlap with the wider patterns in our seven red flags of UK solar scams.

Cross-check the number yourself

1. Check the inputs first. Open the estimate page and confirm the postcode region, orientation, pitch and shade factor describe your roof. A compass app and a look at your survey photos take two minutes.
2. Run PVGIS yourself. The European Commission’s free PVGIS tool lets you drop a pin on your roof, set the slope and azimuth, and model 1 kWp. Set the system losses to 20% so it mirrors MCS’s 0.8 performance ratio. Expect a sensible quote to land within roughly 5–10% of what PVGIS gives you, because the two use different weather datasets.
3. Compare three quotes on implied yield, not headline kWh. Different firms quote different sizes. Normalise everything to kWh/kWp and the outlier walks out of the page on its own.
4. After install, hold them to it. Your paperwork should carry the same estimate, and a decent monitoring setup lets you check year one against it. Compare against the same week last year, never last month. A sustained drop of 5% or more over two to three weeks of normal weather is worth investigating, and a consistent shortfall of 15% or more on clear days points to a fault, not a mood. Allow for a dull year, but a persistent gap against a normal-weather year is a conversation to have with your installer, and my underperforming panels guide is where I’d start.

Ten minutes, free, and it turns a twelve-page PDF into one comparable number. There’s no good reason to skip it.

If the number doesn’t survive contact

1. Email, don’t phone. Ask for the inputs in writing: postcode region, orientation, pitch, Kk, shade factor and the shading survey. Written questions get written answers, and written answers are evidence.
2. Ask for the design pack. A site survey record and sunpath diagram show whether somebody actually looked at your chimneys or guessed from a satellite thumbnail.
3. Get competing quotes from certified firms. My guide to choosing a solar installer covers how to shortlist, and our Find an Installer page lists MCS-certified companies by region.
4. Check the certification is current, not just claimed on a van door. Search the company on the official MCS finder.
5. Know your exit rights. If you agreed the contract at home, by phone or online, you can usually cancel for any reason within 14 days (day one is the day after you sign), and the window stretches if you were never given proper cancellation details. Our guide on how to cancel a solar panel contract has the counting rules and a letter you can copy. Past that, an inflated promise can amount to mis-selling, and unwinding the contract within 90 days may still be possible, so check whether you were mis-sold before those rights expire.

Here’s the tell: a good installer enjoys this email. It’s their language, and answering it properly is half an hour of work they’d rather do than lose a customer to a cowboy. A bad one goes quiet, or gets offended. Silence is your answer.

Turning the estimate into money

The estimate is only half the money story. What you keep depends on how much of that generation lands while you’re home using it. A typical household without a battery uses around half of what its roof makes and exports the rest. Add a battery and the self-use share rises to around 80%.

A unit you use yourself saves roughly 26p at the October 2026 price cap (see our price cap guide). Exported units earn Smart Export Guarantee rates, roughly 4p to 15p per kWh on most open tariffs, with a few tied or time-of-use deals paying more. That’s why I keep a live comparison of the best SEG tariffs, and why I’d run a quote’s export figure through our SEG income estimator rather than trusting the savings column of a sales PDF. The GOV.UK SEG guidance spells out what suppliers will ask to see.

Here’s what an inflated yield does to that sum. Take Examples A and B above, with 50% self-use, 26p for self-used units and 7p for exports:

Illustrative sums using the same assumptions as our homepage figures. Your self-use share, tariffs and system price will differ.

That’s roughly £215 a year of savings that exist only on paper, and three years of payback that vanish when the roof does what roofs do. Keep the policy clock in mind too. The 0% VAT rate on domestic solar and battery installation runs to 31 March 2027, and what happens to solar VAT in April 2027 changes the maths on waiting for a better price.

And sanity-check the package price while you’re at it. I’ve written about what MCS data says a 4 kWp system should actually cost, because an inflated yield and an inflated price usually travel in the same van. Payback maths only works when both numbers are honest.

None of this needs an engineering degree. It needs a calculator, ten minutes and the willingness to send one slightly awkward email. Do it anyway. A yield estimate is a promise about maths, not weather, and if the maths holds up, the rest of the quote usually does too.