Heat Pump Hot Water vs Gas: Which Is Cheaper to Run?
Heat pump and gas hot water use different energy units, so comparing the tariff alone can be misleading. A proper comparison converts both systems to the same useful hot-water demand and allows for COP, gas efficiency and any gas supply charge that would disappear if the property disconnected gas.
Enter your electricity and gas rates to compare the same useful hot-water heat demand.
Compare Useful Heat, Not Just Energy Prices
Electricity is normally billed in kilowatt-hours while natural gas is commonly billed in megajoules. On top of that, the two technologies convert purchased energy into hot water differently. A heat pump can deliver several units of useful heat for each unit of compressor electricity, while a gas water heater loses some energy through combustion, flue gases, storage or cycling depending on the design.
That means a gas rate that looks cheaper per energy unit does not automatically make gas hot water cheaper to run. The calculator above converts both systems back to the same daily thermal demand so the comparison is meaningful.
If hot water is the last remaining gas appliance, replacing it may allow the property to disconnect gas and avoid the daily supply charge. If gas remains for cooking or space heating, that charge should generally not be assigned entirely to hot water.
Heat Pump kWh vs Gas MJ
The calculator converts both systems into the energy required to deliver the same useful heat to the water.
Useful thermal demand divided by operating COP gives a simplified compressor-electricity requirement.
One kWh of heat equals 3.6 MJ. Dividing by gas-system efficiency estimates the purchased gas required.
Use the actual electricity c/kWh and gas c/MJ rates from the property rather than generic national averages.
A gas supply charge matters to the comparison only if changing hot water would actually remove that cost.
Heat Pump vs Gas Storage vs Continuous Flow
Gas hot water is not one technology. Storage and continuous-flow systems have different loss patterns and operating behaviour.
| Factor | Heat Pump | Gas Storage | Gas Continuous Flow |
|---|---|---|---|
| Energy conversion | Moves heat from ambient air using electricity. | Burns gas to heat stored water. | Burns gas as water flows through the unit. |
| Standing losses | Has a storage tank, so some heat is lost over time. | Also stores hot water and has standby losses. | No large storage tank, so storage losses are much lower. |
| Recovery | Depends on heat-pump output, ambient conditions and any boost heater. | Depends on burner input and tank size. | Heats on demand, limited mainly by rated flow and temperature rise. |
| Solar PV use | Can often be scheduled into daytime solar production. | Solar PV does not directly reduce gas consumption. | Solar PV does not directly reduce gas consumption. |
| Fixed network charge | No gas supply charge required. | Gas supply charge may apply. | Gas supply charge may apply. |
| Noise / airflow | Fan and compressor require suitable siting. | No refrigeration fan/compressor. | Burner/flue and installation clearances still apply. |
What Pushes Running Cost Toward Heat Pump?
The economics improve when the heat pump operates efficiently and the property can use lower-cost electricity.
A higher real operating COP lowers the electrical input required to deliver the same amount of useful hot water.
Scheduling the heat pump into surplus solar periods can reduce imported grid electricity and lower the effective cost of heating water.
If hot water is the final gas appliance, electrification may save the fixed gas network charge as well as changing the energy cost.
What Can Narrow the Gap?
Tariffs, climate, existing gas use and the exact equipment all matter.
A low gas usage rate can improve the running-cost position of an efficient gas system, particularly where the supply charge is already being paid for other appliances.
A high electricity tariff with no rooftop solar or lower-cost heating window can reduce the heat pump's dollar advantage.
If real operating COP falls substantially, compressor electricity required for the same useful heat increases.
The gas meter measures purchased fuel energy, not just heat that reaches the water. Likewise, the heat pump's electrical input is only part of the useful heat delivered because additional heat is drawn from the surrounding air.
Heat Pump + Rooftop Solar vs Gas
Rooftop solar can reduce grid imports for the heat pump, while a gas system continues to use purchased gas regardless of PV generation.
A suitably controlled heat pump can use daytime PV output that might otherwise be exported.
The tank can be heated while solar is available and the stored hot water used later in the evening or following morning.
Solar panels can power the home's electrical loads, but they do not directly reduce the gas consumed by a gas water heater.
See how tank size, recovery and heating-window timing can increase solar self-consumption with a heat pump.
Hot Water May Be the Last Reason to Keep Gas
The hot-water comparison changes if removing the gas system allows the entire property to leave the gas network.
If gas remains for cooking, heating or another appliance, the daily network charge will still exist after the hot-water system changes.
If changing hot water allows full disconnection from gas, the avoided daily supply charge can be part of the economic comparison.
Conversion work, electrical supply, installation complexity, rebates and equipment cost should be compared separately from annual energy use.
Why the Efficiency Input Matters
A gas system must purchase more fuel energy than the useful heat that ultimately reaches the water.
| Gas Efficiency | Useful Heat Needed | Purchased Gas Needed | What It Means |
|---|---|---|---|
| 65% | 8 kWh thermal/day | About 44.3 MJ/day | More purchased gas is required because a larger share does not reach the stored/useful hot water. |
| 75% | 8 kWh thermal/day | About 38.4 MJ/day | Efficiency improvement reduces required gas input. |
| 80% | 8 kWh thermal/day | 36.0 MJ/day | A common illustrative comparison point for the calculator. |
| 90% | 8 kWh thermal/day | 32.0 MJ/day | Higher efficiency narrows the difference between purchased gas and useful heat. |
Which Is Better Overall?
The cheaper energy source is not automatically the best hot-water system for every property.
Heat pumps need airflow and suitable placement, and stored recovery should be sized for the household.
Gas systems depend on the property's gas supply and the installation requirements for the selected storage or continuous-flow product.
Include equipment, conversion work, available incentives, expected running cost, remaining gas charges and product suitability.
Use Your Own Rates, Then Compare the System
Once running cost is understood, size the heat pump and compare the practical installation requirements.
Compare heat pump, conventional electric and gas using household-specific rates.
Work out the storage and recovery required before choosing a model.
Compare current HWO heat pumps once the economics and sizing are clear.
Common Running Cost Questions
Running costs depend on actual tariffs, system efficiency, climate, household demand and whether gas fixed charges remain.
