Why two solar quotes for the same roof differ by thousands

You had three companies look at the same roof and the prices came back thousands apart. The instinct is that someone is trying it on. Usually the real answer is that you have been given three different products and asked to compare them on price alone.

Before deciding anyone is expensive, normalise the quotes. Most of the gap disappears, and what remains is a question you can actually ask. There is also an external yardstick to hold the results against: the government publishes the per kilowatt cost of real installations, so you are not comparing three strangers only against each other.

Step one: get everything onto cost per kilowatt peak

Find the system size on each quote and divide the total by it.

This single step resolves the most common cause of confusion, which is that the quotes are for different amounts of generating capacity. A twelve panel system and a nine panel system are not two prices for one thing, and bigger systems are systematically cheaper per kilowatt, so the larger quote can be dearer in total and better value at once.

A worked illustration. Three quotes arrive for the same roof:

• Quote A: twelve panels, 5.2kW, £8,600. That is £1,654 per kilowatt.
• Quote B: nine panels, 3.9kW, £6,600. That is £1,692 per kilowatt.
• Quote C: twelve panels, 5.2kW, £9,900. That is £1,904 per kilowatt.

The £3,300 headline spread between B and C was never a like for like difference: £2,000 of it is simply that B quotes less capacity. On a per kilowatt basis A and B nearly converge, and the real question narrows to why C carries a premium, which the next step usually answers.

For scale, the DESNZ solar PV cost data puts the 2025/26 median at £1,780 per kilowatt for systems up to 4kW and £1,697 per kilowatt for 4 to 10kW systems. A normalised quote far above those medians needs a reason you can point at; one far below them deserves suspicion rather than celebration, because the medians are what real, certified, warranted installations actually cost.

Step two: identify the inverter class

Three broad options, at three price levels:

• String inverter. The baseline. Fine for a single unshaded pitch.
• String plus optimisers, or microinverters. Per panel electronics, which help with shading or split orientations. Materially more expensive.
• Hybrid inverter. Can run a battery later without replacement, so it saves a second inverter purchase if storage is likely.

An installer who surveyed properly and found shading may have specified per panel electronics for good reason. One who specified a hybrid may have listened when you mentioned a battery. Neither is padding, but both make the quote look expensive next to a bare string inverter. In the illustration above, if Quote C’s premium is a hybrid inverter plus optimisers on a pitch the surveyor found partly shaded, C is not the expensive quote: it is the only one priced for your actual roof.

Step three: read the generation estimate

This is where quotes diverge in a way that matters more than price.

Each quote should carry an estimated annual generation figure. If two quotes for similar capacity on the same roof carry noticeably different generation estimates, at least one of them is wrong, and the optimistic one makes its own savings figure look better.

Certified installers are required to produce that estimate using the MCS standard method, which works from your postcode’s irradiance zone, the roof’s pitch and orientation, and a shading assessment, rather than from whatever number closes the sale. So ask each installer three things: whether the figure follows the MCS estimation method, whether shading was measured on site or assumed, and what the estimate assumes about panel degradation. The answers separate a surveyed quote from a templated one very quickly, and an installer who cannot say which method produced their number has told you something useful about the rest of the document.

While you are checking, confirm each firm’s MCS registration on the register itself. Certification is not decoration: export tariffs under the Smart Export Guarantee require MCS or equivalent certification, so the cheapest quote in the pile is worthless if it comes from a firm whose work cannot be registered for export payments.

Step four: put the exclusions back

Add the likely cost of anything excluded onto each quote. Scaffolding, consumer unit work, bird protection, long cable runs. A quote that included all of them may be cheaper than one that excluded all of them and looked lower. The full checklist of what belongs in a quote covers which exclusions are legitimate and which are padding in disguise, and the same page covers the VAT treatment a domestic quote should show.

What is left after normalising

Once capacity, inverter class, generation method and exclusions are equalised, a real difference usually comes down to one of these:

Equipment tier. Different panel and inverter brands with different warranties and different failure histories. Worth something, though less than the sales conversation implies: the datasheet warranty terms are the checkable part, so compare those rather than adjectives.

Business model. A national firm with a sales force, a call centre and a marketing budget has costs a two person local outfit does not. You may be buying a stronger warranty covenant, or you may be buying advertising. Check which consumer code each belongs to, such as RECC, and what its deposit protection covers: that is the part of the “big company reassurance” you can verify.

Workload. A busy installer prices high because they are busy. This is ordinary and not a scandal.

Margin. Sometimes it is simply higher.

The question that cuts through

Ask each installer the same thing:

“Here are the other quotes with the names removed. Mine is the same roof in all three. Can you tell me what your quote includes that these do not?”

An installer with a genuine specification difference will explain it immediately and specifically. One whose price is mostly margin will talk about quality in general terms without naming anything concrete.

Do not let price alone decide

The cheapest quote is the right choice only if the specification, the generation estimate and the exclusions genuinely match. Quite often the middle quote is the cheapest once adjusted, and occasionally the dearest is, because it was the only one that surveyed the shading properly and specified for it.

Once you have a normalised cost and an honest generation estimate, put them into the ROI calculator with your own consumption and tariff. That converts three confusing documents into one comparable number: the year each system pays itself back, in today’s money, under identical assumptions. It is the comparison the three quotes were never going to give you, because each was produced by someone with a price to defend.