How to read a solar panel datasheet
You are entitled to the datasheet for the exact modules in your quote — MCS’s installation standard requires it before the contract is awarded. Six numbers on it decide what you are buying, and the sales conversation usually mentions two.
Six numbers, in the order they matter.
- Rated power at STC, in watts — what the panel is called, and what kWp is computed from.
- Efficiency, as a percentage — how much of that fits in a square metre. Matters when the roof is the constraint.
- Temperature coefficient of Pmax, in % per °C — how much output is lost as the module heats up.
- Degradation and warranty terms — the first-year drop, the annual rate, the year and percentage guaranteed, and the separate product warranty.
- Dimensions and weight — what fits on the roof and what the roof carries.
- Standards — BS EN IEC 61215-1 for design qualification, BS EN IEC 61730-1 for safety.
The sales conversation will mention the first and, if it is good, the fourth.
You are owed this document
MIS 3002 sets out the minimum technical information that must be communicated in writing at or before the point the contract is awarded. It includes the performance estimate, the manufacturer’s datasheet for the proposed solar modules, the datasheet for the proposed inverter, datasheets for any module-level power electronics and any storage, a drawing with the proposed module layout, and the sunpath diagram where shading was modelled.
If a quote arrives without them, the request is not a favour. Ask.
Rated power, and what STC actually means
The headline — 430 W, 440 W — is measured at Standard Test Conditions: 1,000 W/m² of irradiance, a cell temperature of 25°C, and a defined air mass.
Two consequences worth internalising:
Your roof is not STC. Real irradiance is usually lower and real module temperatures on a sunny day are far above 25°C. That gap is one reason our modelling applies 20% system losses rather than assuming plate ratings.
kWp is a sum of plate values. MIS 3002 defines the kWp used in the performance estimate as the sum of the data plate values at STC of all modules installed. So “4.4 kWp” means ten 440 W modules, not a prediction about output — see kWp vs kWh.
Efficiency: when it matters and when it does not
Efficiency is watts per square metre expressed as a percentage. Approved Document L’s solar calculation assumes 0.22 kWp per m², which it describes as an efficiency of 1 kWp per 4.5 m² — a useful benchmark for what “normal” looks like in 2026.
Two panels rated at 440 W generate the same electricity, whatever their efficiency. A watt is a watt.
Efficiency tells you how much area those watts take. So it matters in exactly one circumstance: when your roof runs out before your budget does. On a small or awkward roof, higher efficiency buys real capacity. On a large clear roof, it mostly buys a higher price per watt.
If a salesperson leads with efficiency and your roof is half empty, they are selling you the wrong number.
Temperature coefficient
Expressed as % per °C of Pmax, always negative, and applied above 25°C. A module at −0.29%/°C loses less on a hot roof than one at −0.35%/°C.
In UK conditions this is a modest effect — our climate is the one advantage we have here — but it is one of the few places where datasheets differ in a way that shows up in generation rather than in marketing.
Degradation and the two warranties
Datasheets normally carry a performance warranty: a first-year decline, then an annual rate, then a guaranteed percentage of nameplate at year 25 or 30.
They also carry a separate product warranty covering defects, usually shorter. These are different documents with different durations and the sales material tends to quote whichever is longer.
Three questions worth asking:
- What is the guaranteed output at year 25, as a percentage?
- What is the product warranty, separately?
- Who honours it — the manufacturer, an importer, or the installer? A twenty-five year promise is worth what the company behind it is worth in twenty-five years.
We have no independent UK data on real degradation rates, and we will not pretend otherwise.
Dimensions, weight and the roof
The numbers that decide whether the array fits and whether the roof takes it:
- Dimensions — with roughly 2 m² per module as a working figure, and the roof area calculator for the arithmetic
- Weight per module, which the structural assessment works from
- Frame and mounting compatibility, particularly for in-roof systems
Standards on the datasheet
The government’s own plug-in solar specification names the module standards, and they are the same ones a rooftop module should carry:
- BS EN IEC 61215-1:2021 — terrestrial PV modules, design qualification and type approval
- BS EN IEC 61730-1:2018 — PV module safety qualification
A datasheet that does not mention testing standards at all is a datasheet worth asking about.
What to do with two datasheets in front of you
- Check the wattage and count — that is the kWp, and it is what you divide the price by for the benchmark.
- Check whether the area fits your roof plane with the clearances.
- Compare temperature coefficients — a real difference, if a small one.
- Compare warranty at year 25, not headline warranty length.
- Ignore the brand argument until the numbers are equal. Then it is about who will still be there.
Sources
- MIS 3002: The Solar PV Standard, issue 6.0 Read locally. Section 4.2 on the minimum technical information, and the kWp definition. MCS restricts reproduction, so we cite clauses and quote sparingly.
- Plug-in Solar Device Interim Product Specification, version 2 Names the module standards — BS EN IEC 61215-1:2021 and BS EN IEC 61730-1:2018 — and the panel efficiency assumption we quote.
- Approved Document L, Volume 1: Dwellings, 2026 edition 0.22 kWp per square metre, described as an efficiency of 1 kWp per 4.5 square metres.
Contains public sector information licensed under the Open Government Licence v3.0.
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