If you collect three solar quotes for the same roof, they will not agree. It is common for the highest to be nearly double the lowest, and the instinct — reasonably — is to assume someone is overcharging. Sometimes that is true. More often the quotes are for genuinely different things, and the document has been written to obscure that fact.
Start with what they measured
The first question is not about price at all. Ask what the quote is sized against. If the answer is your monthly bill total, the proposal is arithmetic, not engineering. A bill tells you what you spent; it says nothing about when you spent it, and when is the entire question in solar. A house with a heavy evening peak and a house with a heavy daytime base load might have identical bills and need completely different systems — one battery-heavy, one array-heavy.
A serious proposal is sized against a load profile: either interval data from your meter, or logging installed for a week or two. If nobody has asked to look at when your energy is used, they are guessing, and you are being quoted for a guess.
The panel wattage is a distraction
Quotes love to lead with module wattage, because it is a big number that sounds like value. It is nearly irrelevant. What matters is total array capacity in kWp, the efficiency of the modules, and whether the tier-1 manufacturer named will still honour a warranty in fifteen years. A 600 W module is not better than a 450 W module; it is bigger.
Ask this: "What is the modelled annual yield in kWh, and what did you assume for shading and soiling?" A proposal that cannot answer has not been modelled.
Find the shading study
Shading is the most expensive thing nobody quotes for. A single tree, parapet or vent stack crossing the array for two hours a day can cost more yield than the difference between the cheapest and most expensive modules on your shortlist — and unlike module choice, it is invisible on a spec sheet. A quote that does not mention shading was produced without anyone studying your roof through the day and across the seasons.
Read the battery line carefully
Battery capacity is quoted in kWh, but the number that matters is usable capacity, which is smaller, and cycle life, which decides whether it is still useful in eight years. An LFP battery rated for around 6,000 cycles and a cheaper chemistry rated for 2,000 are not the same product with different prices; they are different products. Divide the price by the cycles and the "expensive" battery frequently wins.
What is missing tells you most
The cheapest quotes are usually cheapest because of what is not in them. Look for: DC and AC protection, earthing, surge protection, isolation and labelling; mounting engineered for local wind loading rather than generic rails; roof penetrations sealed and flashed; commissioning tests, specifically an insulation resistance test; and monitoring configured and handed over. Each of these is invisible on the day and expensive to retrofit.
- What load data was this sized against — a bill, or a logged profile?
- What is the modelled annual yield, and what shading and soiling losses were assumed?
- Usable battery capacity and rated cycle life, not headline kWh
- Which protection, earthing and isolation components are included
- What commissioning tests are performed, and what is documented at handover
- Who maintains it, and what happens in year three when output drifts
The uncomfortable one
Ask every supplier what they would tell you not to buy. A proposal in which every product they sell happens to be right for your site is not an assessment; it is a catalogue. The most valuable thing a good engineer says is usually "not that, not yet, and here is why."
Written by Engineering team. This article is general guidance, not a substitute for a site-specific assessment.