Running cost calculator: methodology

The one rule

Running cost is the energy used times the unit rate. The standing charge is left out on purpose: it is a daily fee you pay whatever you switch on, so it is not part of what using something costs, and using less saves none of it. Prices start at the price cap rates and you can put in your own. Everything is worked out in your browser.

Showers heated by gas

A shower’s cost is the heat it puts into the water. Warming a litre of water by one degree takes 4.186 kJ, so:

heat (kWh) = litres × 4.186 × (shower °C − cold °C) ÷ 3,600
litres     = flow (litres a minute) × minutes
gas (kWh)  = heat ÷ boiler efficiency

We start at 12 litres a minute and cold water at 12°C, the figures Triton Showers uses for UK homes, and a 40°C shower. Mains water is colder in winter and warmer in summer, so a winter shower costs more than the average. Boiler efficiency starts at 85%: our assumption for a modern condensing combi heating water, which you can change. An older boiler, a long pipe run, or water left to go cold in the pipes all lower it. A cylinder heated by the boiler also loses heat while it stands, which this doesn’t count.

Electric showers

An electric shower heats water as it flows, at its full rating, and almost all of that becomes hot water. So it costs its rating in kW times the hours it runs. We start at 8.5 kW, the unit Triton uses as its reference; most are 7 to 10.5 kW (National Energy Action). The flow shown is what that rating can heat to your shower temperature, which is why electric showers run slower in winter.

Appliances

Each appliance’s range is from National Energy Action’s guide to appliance running costs. Most are given as a power rating in watts, and cost watts ÷ 1,000 × hours × unit rate. The washing machine and tumble dryer are given as a cost per full-load cycle at 34p a kWh, which we turn back into kWh per cycle.

Each starts at the middle of its range with a guess at minutes per use and uses a week. Both are there to be changed: an oven at 200°C for an hour doesn’t draw its full rating the whole time, and a heater on a thermostat cycles on and off, so their real cost is often lower than rating × time. A fridge-freezer cycles too, which is why it isn’t in the list: its energy label’s yearly kWh is the figure to use.

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