A solar quote can look deceptively simple — a system size, a price, and a payback period. But the assumptions behind those numbers make an enormous difference to whether the installation actually delivers. Here's how to read one properly.
System size (kWp)
The system size is quoted in kilowatt-peak (kWp) — the rated output of the panels under standard test conditions. A 4kWp system doesn't generate 4kW continuously; it's the peak power under ideal conditions. Real-world annual output in the UK is roughly 850–950 kWh per kWp installed, so a 4kWp system should generate 3,400–3,800 kWh per year.
Check that the proposed system size is appropriate for your roof and your electricity usage. A system significantly larger than your annual consumption will generate more than you can use or export cheaply. A good installer will show you their sizing rationale.
Yield estimate and the tool behind it
Any reputable quote will include an estimated annual yield in kWh. Ask what software was used to generate it. Industry-standard tools include PVGIS (the EU's free tool), Solargis, or OpenSolar — these use satellite irradiance data, your postcode, roof orientation and pitch to generate location-specific estimates.
Be sceptical of round-number estimates produced without a clear methodology. Also check whether the estimate accounts for shading — if you have any nearby trees, chimneys, or dormer windows, shading losses should be modelled and reflected in the output figure.
Self-consumption and export assumptions
The savings calculation depends heavily on how much of your generated electricity you actually use on-site (self-consumption) versus export. A typical household without a battery self-consumes around 30–50% of what it generates. With a battery, self-consumption can reach 70–90%.
Check what self-consumption rate the quote assumes. If it uses 60% or higher without including a battery, challenge it — that's optimistic for most households. The difference between 35% and 55% self-consumption can be several hundred pounds per year in projected savings.
Electricity price assumptions
Most quotes will project savings using a current electricity price — typically 24–28p/kWh in 2025. Some will also apply an annual price escalation assumption (e.g. 3–5% per year) to show a more favourable long-term saving. This escalation assumption significantly affects the 25-year total saving figure.
Be cautious of quotes using very high escalation rates. Energy prices are genuinely unpredictable. A conservative quote will show savings at today's prices without any escalation, alongside a sensitivity scenario. That's a more honest basis for comparison.
Panels, inverter, and warranty terms
The quote should specify the panel brand, model, and wattage, and the inverter brand and model. These matter because panel and inverter quality varies significantly. Check the product warranty (the manufacturer's guarantee on the physical panel — typically 10–25 years) and the performance warranty (guaranteeing a minimum output percentage over time — typically 80–92% at 25 years).
For inverters, look for a minimum 5-year warranty, ideally 10 or 12 years. String inverters are the standard choice; microinverters cost more but can improve performance on shaded or complex roofs. Ask the installer to justify their inverter choice for your specific installation.
What to compare across multiple quotes
When comparing quotes, normalise for system size first — compare cost per kWp rather than total price. Then check whether the yield estimates use the same methodology and assumptions. A cheaper quote that assumes 60% self-consumption looks worse value if a pricier quote assumes 40% for the same system.
Check whether the quote includes scaffolding, DNO notification, grid connection paperwork, and commissioning. Some quotes list these as extras. Finally, verify the installer's MCS certificate number on the MCS installer database at mcscertified.com — never rely on the installer's own claim.