A typical 20m² room with electric underfloor heating draws roughly 3–4kW while it’s running, which would need around 10–13 standard 350W solar panels generating at full output simultaneously to cover directly – and that’s the catch, because underfloor heating is usually needed in the morning and evening, exactly when solar generation is lowest or non-existent. In practice, solar works best for underfloor heating alongside a battery that stores daytime generation for use when the heating actually needs to run.
Why Direct Solar Doesn’t Suit Underfloor Heating Well
Underfloor heating is typically scheduled for when you’re home and want warmth – early morning and evening in most households – which is a poor match for solar generation, which peaks in the middle of the day. Without storage, you’d be drawing from the grid for most underfloor heating use regardless of how many panels you have, since the sun simply isn’t up, or isn’t strong enough, at the times you need the heat most.
Working Out the Panels Needed, by Room Size
| Room size | Typical heating draw | Panels needed to match instantaneous draw (350W panels) |
|---|---|---|
| 10m² | 1.5–2kW | 5–6 panels |
| 20m² | 3–4kW | 10–13 panels |
| 40m² (open-plan kitchen/diner) | 6–8kW | 18–24 panels |
These figures assume electric underfloor heating (roughly 150–200W per square metre) drawing its full load at once, with panels performing at their rated output – realistically higher than what most homes would fit or need, given the heating isn’t running at full power constantly. Wet (water-based) underfloor heating run from a heat pump uses considerably less electricity for the same warmth, since the heat pump multiplies the electricity input several times over as heat output.
Electric vs Water-Based (Wet) Underfloor Heating With Solar
Electric underfloor heating mats convert electricity to heat directly, at roughly a 1:1 ratio, which makes them expensive to run from grid electricity and a heavier draw on any solar-plus-battery setup. Wet underfloor heating fed by a heat pump is far more efficient, typically producing three to four units of heat for every unit of electricity used, which means a smaller solar array paired with a battery can cover a meaningfully larger share of your heating than the same system could manage with electric mats. If you’re planning underfloor heating from scratch alongside solar, wet systems paired with a heat pump are usually the better long-term combination.
Why a Battery Changes the Calculation
A battery lets you store the electricity your panels generate in the middle of the day and release it in the morning and evening, exactly when underfloor heating typically runs. This is the single biggest lever for making solar genuinely useful for underfloor heating, more so than simply adding more panels – our guide to what size solar battery you need covers sizing a battery around a load like this.
A More Realistic Way to Size Your System
Rather than sizing for instantaneous draw, work from your underfloor heating’s actual daily energy use in kWh (check a smart thermostat or previous bills if it’s already installed), then size your solar array and battery to cover a meaningful share of that daily total rather than the full peak load. A typical 3.5–4kW domestic solar array with a modest battery will usually offset a solid portion of a well-insulated home’s underfloor heating use across spring, summer and autumn, with winter naturally relying more on the grid given shorter days and colder weather.
Frequently Asked Questions About Solar and Underfloor Heating
Can solar panels fully power underfloor heating?
Rarely on their own, mainly because heating demand peaks at times solar generation doesn’t. A solar-plus-battery system can cover a substantial share, especially outside winter, but expect to still draw some grid electricity, particularly on short winter days.
Is wet or electric underfloor heating better with solar?
Wet (water-based) underfloor heating paired with a heat pump generally works better with solar, since a heat pump produces several units of heat per unit of electricity, stretching your solar generation and battery storage much further than electric mats can.
Do I need a battery to make solar worthwhile for underfloor heating?
Not strictly, but it makes a significant difference, since underfloor heating typically runs when solar generation is low or zero. Without a battery, you’re mostly limited to whatever heating happens to run during the middle of a sunny day.
How many kWh does underfloor heating use per day?
It depends heavily on room size, insulation and how long it runs, but as a rough guide, electric underfloor heating in a well-insulated 20m² room might use 5–10kWh on a cold day, while a heat-pump-fed wet system covering the same area could use considerably less for the same warmth.
