How to compare three solar quotes properly

Step 1: get the same information from each

Before comparing anything, make sure you have the same fields from all three. For an MCS installer this is not a favour — MIS 3002 requires the estimate in a prescribed format before the contract is awarded, as set out on what a good quote looks like.

From each quote, extract:

• capacity in kWp, and the number and rating of modules
• orientation in degrees from south, and pitch
• the kWh/kWp figure used, and the shade factor
• estimated annual generation in kWh
• the self-consumption assumption and occupancy archetype
• assumed import rate and export rate
• the total price, and what is included

If a quote will not yield these, that is your first finding.

Step 2: check each against an independent model

Do not compare the three generation figures against each other. Compare each against the same external reference.

Divide estimated annual output by kWp, then by the shade factor if one was applied. That gives the implied kWh per kWp. Compare it against our published dataset for your location, pitch and orientation.

A quote whose implied yield is materially above the others, for the same roof, is not a better system. It is a more optimistic model — and that difference will not appear in your electricity bill.

Step 3: normalise the money

Rebuild all three on one set of assumptions.

Pick one self-consumption figure and apply it to all three. MGD 003 is the authoritative UK method — for a 2,500–2,999 kWh household with a 3,600–3,899 kWh array it gives 23% home all day, 19% in half the day, 14% out all day. Our self-consumption page explains it.

Pick one import rate and one export rate. Use your actual import rate. For export, use a rate you can actually get — our SEG table lists what is available, and the spread between the best untied rate and the common ones is large.

Then compute the annual value for each quote, using its own modelled generation corrected as above.

Our rooftop calculator will do this arithmetic if you would rather not — it takes generation, self-consumption as a range, and both rates.

Step 4: compare price properly

Price per kWp is the right headline comparison. Divide total price by capacity.

Then check whether you are comparing like with like:

A cheaper quote that excludes scaffolding on a three-storey house is not cheaper.

Step 5: what actually differs

Once price is normalised, these are the real choices:

Panel specification. Rating, efficiency, and the performance warranty — the number of years and the guaranteed output at the end of them. Warranties differ between products and the degradation terms are worth reading rather than assuming.

Inverter. Usually the first component to need replacing, so its specification and warranty matter more than buyers expect. Also check the DC:AC ratio — a smaller inverter than the array is normal and deliberate.

Module-level electronics. Optimisers or microinverters reduce the effect of shading on the rest of the string. On a shaded roof they can matter a great deal; on an unshaded one they are cost without much benefit.

Warranty terms — and who stands behind them. A workmanship warranty is a promise from the company. Check the insurance-backed guarantee term against the warranty term; a ten-year warranty backed by a two-year policy is two years of protection.

The company. Certifications and Companies House.

The design. Has anyone been on the roof? A quote produced from an aerial photograph carries a required note saying performance “may be significantly lower or higher”.

What not to compare on

Payback period. It compounds every uncertain assumption into a single number, and it moves by years on small changes to self-consumption or export rate. If a quote leads with payback, rebuild it from generation and rates instead.

Total savings over 25 years. Requires assumptions about future electricity prices that nobody can make credibly.

Percentage of your bill eliminated. Depends entirely on the self-consumption assumption you have just normalised away.

Panel brand alone. The design and the installation matter more than the badge.

A note on three quotes from one company

Worth checking. Solar is heavily intermediated, and three quotes obtained through a comparison service can route to brands under common ownership or to one installer.

Ask each: which legal entity will carry out the installation, and what is its company number? Then check that they are actually three. It also matters for section 75 and for knowing whose certifications you are relying on.