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Why most underfloor heating advice gets it wrong
Underfloor heating gets sold on feel, warm floors, even heat, a touch of luxury, and the running-cost question gets a vague "it can be efficient". That hides the single most important split: electric in-slab and hydronic are completely different animals on cost, and electric's cost depends almost entirely on the tariff it runs on.
Lump them together and you get people installing electric in-slab on a standard tariff and getting a nasty bill, or assuming all underfloor is expensive and skipping a heat-pump hydronic system that's actually cheap and superb. The useful advice starts by separating the two and putting the tariff front and centre.
How underfloor heating costs work
The heat source sets the efficiency; the tariff and the slab set whether electric is affordable.
Electric in-slab is resistive
Electric underfloor warms cables or mats in the floor by resistance, so it's about 100% efficient: one unit of heat per unit of power, the same ceiling every plug-in heater hits. On a flat or peak rate that makes it one of the dearest ways to heat. The saving grace is the slab itself: concrete stores heat for hours, so you can charge it on cheap off-peak or controlled-load power overnight (or solar by day) and coast warm through the expensive peak.
Hydronic underfloor is a heat-pump job
Hydronic underfloor circulates warm water through pipes in the slab, heated by a boiler or, far better, an air-to-water heat pump. Because the heat pump moves heat (around three units per unit of power) rather than making it, hydronic underfloor on a heat pump is genuinely cheap to run as well as beautifully comfortable. It's the efficient end of the underfloor world.
Underfloor running cost: tariff is everything
This calculator costs electric in-slab heating on a peak rate, the same on off-peak, and a heat-pump hydronic system, for the same warm floor. The gap between peak and off-peak electric is the whole story. Every figure also appears in the worked example below.
Underfloor heating running cost
Same warm floor, per winter. Edit any input.
Floor element ~0.15 kW per m². Electric in-slab draws that directly (resistive); heat-pump hydronic delivers the same heat at the chosen COP. Off-peak assumes the slab stores heat charged in the cheap window. Rates are editable defaults.
Worked example (the calculator's default). 20 m² of heated floor, 8 h/day for 120 winter days, peak 48 c, off-peak 22 c, heat-pump COP 3.5:
| Setup | Electricity used | Cost for the winter |
|---|---|---|
| Electric in-slab, peak rate | 2,880 kWh | ~$1,382 |
| Electric in-slab, off-peak | 2,880 kWh | ~$634 |
| Heat-pump hydronic, off-peak | ~823 kWh | ~$181 |
Same warm floor. Just moving electric in-slab from peak to off-peak saves about $748 a winter, and a heat-pump hydronic system saves more again. The tariff, not the floor, is doing the heavy lifting.
How Aussies overpay without realising it
Running resistive floor heating at peak rates is one of the dearest heating bills there is. Off-peak or controlled load is non-negotiable.
Heating the floor on demand in the evening wastes the slab's biggest asset: its ability to hold heat charged in cheap hours.
Electric and heat-pump hydronic differ by a factor of several on running cost. Choosing blind can lock in the dear one.
Underfloor in a guest bathroom run daily is comfort nobody's enjoying, on the priciest heat type.
The insider insight: the slab is a thermal battery
The detail that rescues electric underfloor is one most installers mention only in passing: the concrete slab is a thermal battery. It absorbs heat slowly and releases it for hours. That changes the whole economics, because it means you don't have to heat when you want warmth. You can pour cheap energy in overnight on an off-peak or controlled-load rate, or in the middle of the day on solar, and the floor stays warm through the expensive evening peak on stored heat.
Used that way, electric in-slab stops competing at peak rates and starts running almost entirely on the cheapest power available. It's the same trick that makes home batteries and hot-water timers work: shift the buying to the cheap window, store the result. The household that treats the slab as storage, and gets onto the right tariff to feed it, turns the dearest heating into something genuinely affordable. The one who runs it on demand at peak pays for the luxury several times over.
What you should actually do
| Do this | Why it works |
|---|---|
| For new installs, choose heat-pump hydronic | Same comfort, a fraction of the running cost of electric in-slab. |
| If you have electric in-slab, get on off-peak / controlled load | Slashes the cost of resistive floor heating versus peak rates. |
| Charge the slab overnight or on solar | Uses the floor's thermal mass to stay warm through the peak on cheap power. |
| Only heat the floors you actually use | Avoids paying for warmth in rooms nobody's in. |
| In VIC, NSW, SA, QLD, ACT: pick a plan with a strong off-peak window | The cheaper the off-peak rate, the more viable electric underfloor becomes. |
In Western Australia (Synergy) and the Northern Territory (Power and Water) there's no retailer to switch to, so use the available off-peak/controlled-load options and the slab's storage. See our hydronic heating and home heating guides.
Why this matters right now
As more homes move to time-of-use tariffs and add solar, the slab-as-battery approach gets more powerful: there's more cheap daytime solar and clearer off-peak windows to soak up. Underfloor heating has never been easier to run affordably, if you set the tariff up to match.
The takeaway: underfloor heating's bill is decided by the tariff and the slab, not the floor. Choose heat-pump hydronic where you can, and run electric in-slab on stored cheap power.
Frequently asked questions
Current figures — last updated 2026-06-13
All values in AUD and Australian units. Volatile figures change; check your own plan before relying on them.
| Electric in-slab efficiency | ~100% (resistive, 1:1) |
|---|---|
| Heat-pump hydronic efficiency (COP) | ~3 units of heat per unit of power |
| Typical floor element draw | ~0.15 kW per m² |
| Typical peak rate | ~40–55 c/kWh [verify: your plan] |
| Typical off-peak / controlled-load rate | ~15–25 c/kWh [verify: your plan] |
| Reference price (NSW, SA, SE QLD / VIC) | AER DMO / ESC VDO [verify: current value] |
Sources: energy.gov.au, Australian Energy Regulator (DMO), ESC Victoria (VDO), manufacturer specifications, network controlled-load tariff schedules.