How this water heater calculator works
Water heating is the second-largest energy use in most homes after space heating, typically 15–20% of household energy. The cost depends on three things: how much heat your hot water needs, how efficiently the heater turns fuel into that heat, and how much heat a storage tank loses while it waits to be used.
The calculator starts from physics. You enter the number of people, the hot water each person uses and the temperatures involved. From these it computes the useful heat required each day, adds pipe losses, and then models six technologies side by side:
- Electric storage tank: resistance element, nearly 100% efficient, but the tank loses heat around the clock (standby loss).
- Heat-pump water heater: moves heat from the surrounding air into the tank, delivering 2.5–3.5 kWh of heat per kWh of electricity (its COP). Standby losses are covered at that COP too.
- Gas storage tank: burner efficiency around 75–80% plus continuous flue and pilot losses.
- Gas tankless / combi: heats on demand at 85–95% efficiency with no storage losses.
- Electric tankless: 99% efficient and no storage losses, but billed at full electricity price.
- Solar thermal with electric backup: the sun covers a share of the demand (the solar fraction) and an electric element tops up the rest of a larger tank.
For each one it calculates the energy bought, the running cost, the installed price spread over a typical lifetime, and CO₂.
The formula
Electric tank: E = (Q + standby) ÷ 0.98 Heat pump: E = (Q + standby) ÷ COP Gas tank: E = Q ÷ η + standby/pilot gas Tankless: E = Q ÷ η Solar thermal: E = (Q + 1.3 × standby) × (1 − solar fraction) ÷ 0.98
Annual total cost = E × price + installed cost ÷ lifetime
Worked example
The defaults describe a three-person household using 45 litres per person of 55 °C water, with 12 °C cold water and 10% pipe losses. That is 135 L a day at a temperature rise of 43 K:
135 × 4.186 × 43 ÷ 3,600 × 1.10 = 7.4 kWh of useful heat per day, or about 2,710 kWh a year.
An electric storage tank adds 1.3 kWh/day of standby losses, so it buys about 3,250 kWh a year, or €812 at €0.25/kWh, of which about €121 is pure standby loss. Each 35-litre shower costs around €0.58.
A heat-pump water heater with a COP of 2.8 needs only about 1,140 kWh, or €284 a year, a saving of €528. Over its lifetime, though, the gas tankless/combi has the lowest total cost at about €408 a year (€308 of gas plus €2,000 spread over 20 years), while the heat-pump unit comes second at about €500. The heat pump has the lowest CO₂ at roughly 0.4 t a year, against 1.1 t for the electric tank on a 350 g/kWh grid.
How to read the results
- Running cost is what your current heater costs per year in fuel or electricity.
- Cheapest to run ignores purchase price; lowest total cost includes it, spread evenly over the typical life of each technology.
- Standby losses show how much you pay to keep a tank hot when nobody is using it. That is often 10–20% of an electric tank’s bill.
- Instant heater power for a shower reveals why electric tankless heaters often need a 40–63 A supply upgrade: a single 9 L/min shower at 40 °C needs around 17–18 kW.
- The stacked bar chart separates running cost from capital cost so you can see where the money goes.
Typical values
| Technology | Efficiency / COP | Typical life | Notes |
|---|---|---|---|
| Electric storage tank | 98% + 0.8–2 kWh/day standby | 10–15 yrs | cheap to buy, expensive to run |
| Heat-pump water heater | COP 2.5–3.5 | 10–15 yrs | needs a space above ~10 °C |
| Gas storage tank | 70–80% + pilot/flue losses | 8–12 yrs | US standard UEF ≈ 0.6–0.7 |
| Gas tankless / combi | 85–95% | 15–20 yrs | condensing models reach 95% |
| Electric tankless | 99% | 15–20 yrs | 18–27 kW for whole-house |
| Solar thermal + backup | 45–80% solar fraction | 20–25 yrs | highest upfront cost |
Tips to cut water heating costs
- Insulate the tank and the first metres of pipe. Cheap and often worth 0.3–0.5 kWh a day on older tanks.
- Use a low-flow shower head. Cutting flow from 12 to 8 L/min reduces shower energy by a third.
- Heat at night if you have a time-of-use tariff. Enable the night-tariff option to see the effect.
- Pair a heat-pump heater or immersion with solar PV. Hot water is an excellent store for midday surplus; see the solar panel calculator.
- If you already have a space-heating heat pump, let it make hot water too (see the heat pump calculator).
Limitations
The model uses fixed efficiencies; real heat-pump water heater COP falls in cold rooms and rises in warm ones, and it cools the room it sits in. Standby losses depend strongly on tank size, insulation and set-point. Hot water use varies widely between households. A plug-in meter on an electric heater or two gas readings a week apart give a better figure than the defaults. Purchase prices vary by country and installation complexity, and costs ignore financing and maintenance. Gas options assume an existing gas connection with no extra standing charge.
Method & assumptions
Daily useful heat Q = V × 4.186 × (Tset − Tcold) ÷ 3600 kWh, plus pipe losses. Energy bought: electric tank (Q + standby) ÷ 0.98; heat-pump (Q + standby) ÷ COP; gas tank Q ÷ η + standby/pilot gas; tankless Q ÷ η; solar (Q + 1.3 × standby) × (1 − solar fraction). Annual total cost = running cost + installed cost ÷ lifetime.
Frequently asked questions
How much does it cost to run an electric water heater?
Is a heat pump water heater worth it?
Is a tankless water heater cheaper than a tank?
How much energy does it take to heat water?
What temperature should a water heater be set to?
How much hot water does a person use per day?
Sources & references
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