How to choose solar panels: the spec-sheet numbers that matter

Solar panel spec sheets are full of abbreviations. This guide explains the figures that matter and shows how they spread across the 1,000 panels currently in our solar panel database.

First, where our panel data comes from

All our panel data comes from the California Energy Commission (CEC) Solar Equipment List ¹, one of the few public, machine-readable lists with electrical data for tens of thousands of modules. Two honest caveats:

• The figures are declared by the manufacturer; the CEC checks them administratively but does not re-test them.
• It is a US list. Not every panel is sold in the UK, and our current selection is mostly older models: the most powerful panel in our database is 385 W and the highest efficiency is 20.0%. According to Fraunhofer ISE, commercial wafer-based modules reached almost 23% in 2025 ².

So use our figures mainly to understand how the specs relate to each other, and always check the current datasheet for the exact panel in your quote.

Wattage (Wp): power under test conditions

Panel power in watts-peak is measured under standard test conditions (STC): 1,000 W/m² of sunlight and a cell temperature of 25 °C. A UK roof rarely sees that. Wattage is mainly useful for sizing: the Energy Saving Trust says the average home system is around 4.5 kWp, typically around 12 panels on 20–30 m² of roof ³.

In our data, 72-cell panels have a median of 310 Wp and 60-cell panels 253 Wp – mostly because they are bigger, not because the cells are better.

Efficiency: what matters on a small roof

Efficiency is power per square metre. Higher efficiency puts more kWp on the same roof, which matters on terraced houses, dormers and roofs with chimneys or skylights. If you have plenty of space, a slightly less efficient panel at a lower price per watt can be just as sensible. In our database, 12% of monocrystalline panels reach 18% or more, against under 1% of polycrystalline ones. See the most efficient solar panels.

Temperature coefficient: how much power heat costs you

Solar cells lose output as they warm up. The temperature coefficient (Pmax) tells you how many percent you lose per degree above 25 °C. Worked example with our median for crystalline panels (−0.448%/°C): at a cell temperature of 60 °C on a bright summer day that is 35 × 0.448 ≈ 16% less power. A panel at −0.30%/°C would lose about 10.5%.

Our data shows a clear pattern ¹:

• Crystalline silicon (909 panels): median −0.448%/°C, best −0.36%/°C.
• Thin film (CdTe/CIGS) (91 panels): median −0.306%/°C, but lower efficiency (CdTe median 14.6%).

See the panels that cope best with heat.

PTC and NOCT: closer to real life

Alongside the STC rating, the CEC publishes a PTC rating, calculated under more realistic conditions with a higher cell temperature from ambient air and wind ¹. In our data, PTC for crystalline panels is a median 90% of nameplate power, and 93% for thin film. The median NOCT (nominal operating cell temperature) is 46 °C; lower is better.

Cell types: mono, poly, TOPCon, HJT and IBC

Our current database is 50% polycrystalline, 41% monocrystalline and 9% thin film. New panels on sale in the UK are almost always monocrystalline, using one of these cell designs:

• PERC – the classic mono cell with a reflective rear layer.
• TOPCon (tunnel oxide passivated contact) – a development of PERC with lower losses at the rear.
• HJT (heterojunction) – crystalline silicon with thin amorphous layers; usually a relatively good temperature coefficient.
• IBC (interdigitated back contact) – all contacts on the back, so no silver lines on the front; high efficiency and a clean look.

The CEC list doesn’t always record the cell design, and none of the panels in our current selection is explicitly labelled TOPCon, HJT or IBC. When in doubt, compare measured values rather than marketing names.

Bifacial and all-black

• Bifacial panels also collect light from the back. That pays off mainly with a light surface underneath and space behind the panel (flat-roof frames, carports, ground mounts), hardly at all on panels fixed close to a pitched tiled roof. 13 panels in our data are flagged as bifacial.
• All-black panels have a black backsheet and frame. They look neater on dark roofs – which can matter in conservation areas – but can run slightly warmer. Their median efficiency in our data is 17.5%. See all-black solar panels.

Checklist for your quote

1. Ask for the datasheet for the exact panel and compare wattage, efficiency and temperature coefficient.
2. Use an MCS-certified installer: MCS requires the roof structure to be checked and pitched-roof mounting kits to be certified to MCS 012 ⁴. MCS certification is also needed for the Smart Export Guarantee.
3. Ask for certificates to IEC 61215 and IEC 61730.
4. Check the warranties – product and performance warranty separately.
5. Match the inverter to the array; the Energy Saving Trust notes inverters typically need replacing after around 12 years ³. See our inverters.