Electric Underfloor Heating: Running Costs and the Wiring Side

Electric underfloor heating is a thin heating cable laid under your floor finish, either fixed to a mesh mat, laid loose or bonded into a foil sheet, and switched by a thermostat that reads a probe in the floor. Most systems are rated at 100 to 200 watts per square metre, so a 4m² bathroom draws about 2.6 amps and costs roughly 16p an hour at full output on the current price cap.

The cable is the easy bit. Whether you get years of warm feet or a cold floor the week after the grout goes on comes down to the electrical side, and that's the part most guides skip.

Mats, loose cable or foil: which system goes where

All three do the same job. The difference is how the cable arrives and which floor it suits. Warmup, one of the bigger UK brands, makes all three and publishes the outputs.

• Heating mat (Warmup StickyMat): 150 or 200 W/m², cable pre-spaced on a 0.5m wide self-adhesive mesh. Best in square or rectangular rooms, under tiles or inside a levelling compound.
• Loose cable (Warmup Loose Wire): averages 150 W/m², with the spacing set by the installer. Made for awkward rooms full of fixtures, and Warmup gives the bathroom as its example.
• Foil heater (Warmup Foil Heater): 140 W/m², cable encased in aluminium foil and laid straight under floating laminate or engineered wood, no levelling compound needed. Carpet and vinyl need Warmup's overlay board on top.

Source: Warmup product pages for StickyMat, Loose Wire and the Foil Heater.

With loose cable, the installer chooses the output. Fastwarm's cable gives off 10 watts per metre, so the closer the runs, the more heat per square metre:

• 50mm spacing: 200 W/m²
• 65mm spacing: 150 W/m²
• 75mm spacing: 130 W/m²
• 100mm spacing: 100 W/m²

Source: Fastwarm cable coverage guide.

Warmup's guidance is 150 W/m² for main heating in a well-insulated room, and 200 W/m² for rooms that lose heat fast, like conservatories. Don't skip the insulation boards. They stop heat sinking into the subfloor, and Warmup says good floor insulation can cut running costs by as much as 50%.

What it costs to run, worked through

Running cost is simple arithmetic once you know the wattage. We've used Ofgem's price cap for 1 October to 31 December 2026: 26.32p per kWh, the average for Direct Debit customers across England, Scotland and Wales. Your own rate depends on where you live and how you pay.

Example 1: a 4m² bathroom on a 150 W/m² mat

• Rated load: 4m² x 150W = 600W, or 0.6kW
• Current drawn: 600W ÷ 230V = 2.6A
• Cost per hour at full output: 0.6kW x 26.32p = 15.8p
• Two hours a day at full output: 31.6p
• A 30-day month: £9.48

Source: Ofgem energy price cap unit rates, 1 October to 31 December 2026.

Example 2: a 15m² kitchen-diner as the main heat, 150 W/m²

• Rated load: 15m² x 150W = 2,250W, or 2.25kW
• Current drawn: 2,250W ÷ 230V = 9.8A
• Cost per hour at full output: 2.25kW x 26.32p = 59.2p
• Six hours a day at full output: £3.55
• A 30-day month: £106.60

Source: Ofgem price cap, 26.32p per kWh, 1 October to 31 December 2026.

Those are ceilings. Once the floor is warm, the thermostat cycles the cable on and off. Warmup's own running-cost table puts the average after the first heat-up at 0.7836p per m² per hour, worked out at 11.61p per kWh over 10mm insulation boards. Rescaled to the current cap:

• Energy behind Warmup's figure: 0.7836p ÷ 11.61p = 0.0675kWh per m² per hour
• At 26.32p per kWh: 0.0675 x 26.32p = 1.78p per m² per hour
• The 15m² kitchen-diner, six hours a day: 15 x 1.78p x 6 = £1.60 a day
• A 30-day month: about £48

Source: Warmup underfloor heating cost guide, rescaled with Ofgem's cap. It's the manufacturer's estimate on an insulated floor, so treat it as your best case and full output as your worst.

For a bathroom, those numbers are modest. As the only heating in an all-electric flat, compare carefully. If you're swapping out old night storage units, our guide to replacing storage heaters sets out the other options.

What it costs to buy and fit

Installation cost comes in two halves, the kit and the labour. Warmup publishes example figures that make a sensible rough guide.

• 4m² bathroom kit (loose cable, insulation and thermostat): around £470 + VAT RRP
• 10m² heating mat: £511.66 + VAT
• Insulation boards for that 10m²: £268.24 + VAT
• Smart Wi-Fi thermostat: £141.66 + VAT
• Fitting labour: £200 to £300 a day, most rooms done in 1 to 1.5 days, about £400 in all

Source: Warmup underfloor heating cost guide. These are manufacturer list prices and suggestions, not our prices.

That labour figure leaves out the electrics: a spur or new circuit, RCD protection, the thermostat back box, testing and certification. A board with no spare way or no RCD adds a consumer unit upgrade on top. If the kitchen's being refitted anyway, plan the heating circuit alongside the oven and hob, the same visit as any appliance installation. Get a fixed quote that lists the electrical side separately before anyone orders tiles.

The electrical side most guides skip

Start with the load in amps

Every decision about the supply follows one sum: watts divided by 230 gives amps.

• Up to 13A (about 3kW, roughly 20m² at 150 W/m²): can often run from an existing circuit through a fused spur, if that circuit has spare capacity and 30mA RCD protection.
• 13A to 16A (up to 3,680W, roughly 24m² at 150 W/m²): needs its own circuit. Warmup rates its thermostats at 16A maximum, which is 3,680W at 230V.
• Over 16A: the thermostat switches a contactor, with the load split across 16A protected supplies and no more than 7.5kW of heating on each 30mA RCD or RCBO.

Sources: Warmup StickyMat installation manual, and Electrical Safety First on plugs and fuses, which puts a 13A fuse on loads up to 3,000 watts, "the maximum rating of a wall socket".

When a fused spur is enough

For a small bathroom or a few square metres of kitchen, a fused spur off an existing circuit is often the neat answer. The catch is the circuit you're borrowing from. Warmup's manual says someone has to calculate whether it can handle the extra load, and a kitchen ring already running the kettle and washing machine may not have much spare on a cold morning. Bigger areas, or heating that's the main heat for a room, get a dedicated circuit. If your board is full or has no RCD, that's when a consumer unit upgrade joins the job.

30mA RCD protection, every time

Warmup's manual says the thermostat's supply must be protected by a 30mA RCD or RCBO at all times, and time-delay RCDs must not be used. A warning label from the packaging goes on the consumer unit, directly beneath the heating breaker, so whoever opens the board next knows there's cable under that floor.

If the RCD trips whenever the heating starts, reset it once. If it trips again, leave it off and don't open the thermostat. That's insulation testing with proper instruments, and it's a job for our fault finding and repairs team.

Thermostats and floor probes

The heater must be controlled by a thermostat with a floor sensor. The probe sits centrally between two neighbouring runs of cable, away from hot water pipes, with its tip left untaped. Get that wrong and the thermostat reads the wrong temperature for as long as the tiles are down.

In bathrooms, Warmup's manual notes that thermostats, contactors, fused spurs and junction boxes can't go in zones 0 or 1, and mains-powered kit in zone 2 needs at least IPX4. The usual fix is a thermostat just outside the room, running on the floor probe alone. That controls floor temperature rather than air temperature, which suits most bathrooms fine. Our guide to bathroom electrical zones shows where those zones fall.

Bathrooms are a special location, so Part P applies

Under Approved Document P, a new circuit and a replacement consumer unit are always notifiable, and so is any addition or alteration to existing circuits in a special location. In a room with a bath or shower, that means the space up to 2.25 metres from the floor and 0.6 metres out from the edge of the bath or shower tray.

A bathroom heating mat usually runs right up to the bath panel or shower tray, so it sits inside that space. The job is notifiable even if it's only a spur. A registered competent person, such as an NICEIC-registered electrician, self-certifies it: you get a BS 7671 electrical installation certificate, and the building regulations compliance certificate follows within 30 days.

A spur feeding heating in a kitchen isn't notifiable, but Approved Document P still expects it to be inspected, tested and certificated to BS 7671 (the IET Wiring Regulations). Source: Approved Document P, gov.uk.

Who does what: the floor fitter or the electrician

Most underfloor heating problems start in the handover between trades. This order works:

1. Electrician, first fix: before the floor goes down, run the supply with RCD protection, set a 35mm deep back box for the thermostat and give the cold tail and probe a conduit route to floor level.
2. Floor fitter or tiler: lay the insulation and heater, place the probe, then test and record the heater's resistance before fitting, after fixing it down and again after tiling.
3. Electrician, second fix: make the final connections at the thermostat, test, label the consumer unit, certify and notify where needed.

Warmup's manual puts the final mains connection with a qualified electrician. The resistance readings are the step that gets skipped. If they aren't written down at each stage and the floor stays cold, nobody can say when the cable was damaged, and finding out means lifting tiles you paid good money for.

Frequently asked questions

How much does electric underfloor heating cost to run per hour?

Multiply area by output, then by your unit rate. A 4m² bathroom at 150 W/m² draws 0.6kW, about 15.8p an hour at full output on Ofgem's 26.32p cap rate for October to December 2026. Once the floor is warm the thermostat cycles, so the real figure is lower.

Does electric underfloor heating need its own circuit?

Not always. Up to about 13A, or 3kW, it can often run from an existing circuit through a fused spur if there's spare capacity and RCD protection. Anything bigger, or main heating for a room, should have a dedicated circuit.

Can a tiler install electric underfloor heating?

A tiler can lay the heater and tile over it, and plenty do it well. The final mains connection needs a qualified electrician, and in a bathroom the work is notifiable, so someone registered has to certify it.

Do I need building regulations approval for underfloor heating in a bathroom?

Usually, yes. A new circuit is notifiable anywhere, and adding to a circuit in the space around a bath or shower is notifiable under Approved Document P. A registered electrician self-certifies the work and handles the notification.

Can you put electric underfloor heating under laminate or carpet?

Yes, with the right system. Foil heaters suit floating laminate and engineered wood, and Warmup says carpet and vinyl need its overlay board on top. Mats and loose cable go under tiles or inside a levelling compound.

Planning underfloor heating anywhere in London and want the electrics right first time? See our consumer unit upgrade service or call us on 020 3653 2600 for a straight answer and a fixed quote.