No, solar panels will not work effectively through a window. The Energy Saving Trust states that solar panels require direct sunlight to generate electricity at their rated output, and glass significantly reduces the light intensity (Energy Saving Trust, 2026). A panel placed behind a window typically produces only 10–30% of its normal output.
The key variable is the glass itself. Standard window glass blocks or reflects a large portion of the ultraviolet and infrared light that solar cells need. Even a small amount of dirt, tinting, or double glazing reduces performance further. This setup is only viable for low-power devices like phone chargers, not for powering a home. It also fails the eligibility criteria for the Smart Export Guarantee (SEG), which requires panels to be installed on a building or structure in a permanent, unobstructed location (Ofgem, 2026).
Glass reduces sunlight by 50 to 90 percent
Most modern windows are double-glazed, which can cut light transmission by up to 40% compared to open air. Low-emissivity coatings, common in newer windows, reflect infrared light and reduce solar panel output by a further 20–30%. The Energy Saving Trust notes that even a clean, single-pane window reduces a panel’s efficiency by at least 50% (Energy Saving Trust, 2026). Tinted or frosted glass can push losses above 80%. In real-world conditions, a 300W panel behind a double-glazed window might output just 30–60W on a sunny day.
Heat build-up damages panels indoors
Solar panels generate less electricity as their temperature rises. Behind glass, panels can reach 60–70°C on a sunny day, which reduces their efficiency by 10–25% compared to an outdoor installation. The Microgeneration Certification Scheme (MCS) requires panels to be installed in a ventilated location to avoid overheating and fire risk (MCS, 2026). Indoor placement also voids most manufacturer warranties. The trapped heat accelerates degradation, potentially halving the panel’s lifespan from 25 years to 10–12 years.
Grants and payments require outdoor installation
The Smart Export Guarantee (SEG) pays households for electricity exported to the grid, but only for panels installed on a roof or ground mount with no shading from windows or walls (Ofgem, 2026). Similarly, the Great British Insulation Scheme and other government grants for solar PV require MCS-certified outdoor installation. A window-mounted panel is not eligible for any UK grant or export tariff. The typical payback period for a 3.5kWp outdoor system is 10–15 years, but a window setup will never pay back its cost.
A worked example
A UK homeowner placing a 100W solar panel behind a south-facing double-glazed window in a 1930s semi-detached house would generate roughly 30W of usable power. The Energy Saving Trust confirms that even a clean single-pane window halves output, and double glazing with low-e coatings cuts it by 70% or more. This setup would produce about 60kWh per year, enough to charge a phone or run a laptop, but not to power a kettle or fridge. The upfront cost for a small panel and battery is around £200, but the yearly electricity saving is only £10. The payback period stretches beyond 20 years. That makes it far worse than a roof-mounted installation. Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust advises that indoor panels are not a viable home energy solution.
| Item | Figure |
|---|---|
| Upfront cost after grants | £200 |
| Yearly savings | £10 |
| Payback period | 20 years |
| 25-year lifetime savings | £50 |
What homeowners often get wrong
The most common mistake is assuming any window will pass enough sunlight for meaningful power generation. Here are three key errors that waste money and time.
- Believing direct sunlight through glass is the same as outdoors Many people think a south-facing windowsill is as good as a roof. In reality, double glazing and low-e coatings block 40–60% of usable light, and the panel can overheat behind glass, further reducing output by 20%.
- Thinking the setup qualifies for government support Homeowners expect the Smart Export Guarantee or the ECO4 scheme to apply. The SEG requires panels in a permanent outdoor location, and ECO4 only funds whole-home measures like insulation and heat pumps, indoor panels are excluded entirely.
- Forgetting that shading and dirt have a bigger effect indoors A clean outdoor panel loses 5% to shading, but an indoor panel behind a dusty, tinted, or frosted window can lose 90% of its potential. This mistake can void a panel warranty if the manufacturer specifies outdoor use only.
Quick reference
- A solar panel behind double glazing produces only 10–30% of its rated output, according to the Energy Saving Trust.
- Low-emissivity coatings on modern windows reflect infrared light, cutting solar panel efficiency by a further 20–30%.
- The Smart Export Guarantee requires panels to be installed on a building or structure in a permanent, unobstructed outdoor location.
- An indoor 100W panel saves about £10 per year, whereas a roof-mounted equivalent saves £50 or more annually.
- Even a clean single-pane window reduces panel output by at least 50%, making indoor setups uneconomical for home power.
Frequently Asked Questions
No, solar panels cannot work effectively through a window. The Energy Saving Trust states that even a clean single-pane window reduces output by at least 50%.
Window glass reduces solar panel efficiency by 50 to 90 percent depending on glazing type. Double glazing with low-emissivity coatings can cut light transmission by up to 40% and further reduce output by 20–30%.
No, panels behind a window are not eligible for the Smart Export Guarantee. Ofgem requires panels to be installed in a permanent, unobstructed location.
