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CALCULATOR · Finance

Water Heating Cost Calculator

What one shower, bath or tank of hot water costs — and what it adds to the monthly bill.

Show formula
water energy = litres × ΔT × 4.186 ÷ 3600 · purchased energy = water energy ÷ heater efficiency · cost = purchased × price per kWh · month = session cost × sessions

Inputs

How much hot water one use draws — a shower runs 50–70 L, a bath 150–200 L, a dishwasher cycle about 12 L.

10

The temperature of the incoming water main — around 10 °C in a temperate climate, colder in winter.

40
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Gas boilers sit around 70–90%, electric elements are effectively 100%, and heat pumps exceed 100% — enter COP × 100 (a COP of 3 is 300%).

What one kWh of delivered energy costs on your bill. A gas meter reading in m³ or therms converts first: 1 m³ ≈ 10.5 kWh, 1 therm = 29.3 kWh.

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How many times a month you heat this much water — 30 is once a day.

Result

Your result appears here

Enter Water per session and the result fills in as you type.

How it works

  1. Enter the hot water one use draws — a shower is 50–70 liters, a bath 150–200, a dishwasher cycle about 12 — and the two temperatures: the cold supply and the hot delivery.
  2. Add the heater efficiency — around 85% for a gas boiler, effectively 100% for an electric element, COP × 100 for a heat pump — and your price per kWh from the bill.
  3. Enter how many times a month you heat that water to get the cost per session plus the monthly and yearly figures the bill will show.

Frequently asked questions

Why is the energy on my meter larger than the energy the water gained?

Heating is never loss-free: a gas boiler sends part of its heat up the flue, and a storage tank loses warmth to the room around the clock. The calculator divides by the heater efficiency, so the cost column is the one the energy bill actually sees — not the smaller textbook figure that stops at the water.

My gas bill charges per cubic meter — how do I use it?

Convert the price first: one cubic meter of natural gas holds roughly 10.5 kWh, and one therm is 29.3 kWh. A tariff of 0.90 per m³ works out near 0.086 per kWh — enter that figure and the rest of the maths is identical.

Is a heat pump water heater really cheaper to run?

Yes, and the efficiency field shows it directly: a heat pump with a COP of 3 delivers three units of heat per unit of electricity, entered as 300%. The same shower then bills about a third of what a resistive element does — the yearly figure is where the difference adds up.

Examples

A 60 L shower heated from 10 °C to 40 °C with an 85%-efficient boiler at 0.12 per kWh, 90 showers a month
2.09 kWh into the water, 2.46 kWh from the meter — 0.30 per shower, 26.59 a month.
A 150 L tank heated from 10 °C to 60 °C on a 100%-efficient electric element at 0.30 per kWh, 30 fills a month
8.72 kWh into the water — 2.62 per fill, 78.49 a month and 941.85 a year.

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