Warmer Bills publishes the arithmetic behind its running-cost figures so you can see exactly how a number was produced. Every cost claim carries a number, a unit and a date, with the wattage, usage time and energy rate stated rather than hidden behind phrases such as “cheap to run” or “uses a lot of electricity”.
The basic electricity running-cost formula
For an electrical appliance, the basic calculation is:
wattage ÷ 1,000 × hours used × electricity unit rate
Appliance power is usually shown in watts, while electricity is billed in kilowatt-hours, or kWh. Dividing the wattage by 1,000 converts watts into kilowatts. The electricity unit rate is normally expressed in pence per kWh.
- Find the appliance wattage. Check the manufacturer’s rating plate, manual or product specification for its stated power consumption.
- Convert watts to kilowatts. Divide the wattage by 1,000. A 2,000 W appliance is therefore rated at 2 kW.
- Multiply by the time used. A 2 kW appliance running for 1.5 hours would use 3 kWh if it drew that power continuously for the whole period.
- Apply the electricity unit rate. Multiply the kWh figure by the applicable price per kWh. When the rate is given in pence, the result is also in pence.
- Total the cost for the required period. A single-use figure can be multiplied by daily, weekly or annual usage where that pattern is a reasonable assumption.
A worked example
Suppose an appliance is rated at 1,500 W, is used for 2 hours, and electricity costs an illustrative 25p per kWh. The 25p figure here is an example rate only; it is not presented as today’s price-cap rate.
First, convert the wattage: 1,500 ÷ 1,000 = 1.5 kW.
Next, calculate the energy used over two hours: 1.5 kW × 2 hours = 3 kWh.
Then apply the example unit rate: 3 kWh × 25p = 75p.
On those assumptions, the appliance would cost 75p for two hours if it actually drew 1.5 kW throughout that period. That final condition matters because many appliances do not operate continuously at their maximum rated power.
Where our unit rates come from
Warmer Bills normally calculates household electricity and gas costs using the relevant Ofgem price-cap unit rate unless an article explicitly states that another tariff or rate is being used. The price cap changes quarterly, so a calculation is only meaningful when the rate and its date are shown.
Every cost figure on the site therefore states the rate used and the date at which the calculation applies. Figures are reviewed when the relevant price-cap rate changes rather than being presented indefinitely as though energy prices had remained fixed.
Why standing charge is separate
The standing charge is different from the unit rate. It is a daily charge for having an energy supply and is paid independently of how many kWh a particular appliance consumes.
For that reason, we do not add a share of the household standing charge to every kettle, washing machine, heater or other individual appliance calculation. Doing so would repeatedly allocate the same fixed charge and make appliance comparisons less useful. Where standing charges matter to a wider bill calculation, they are shown separately.
Rated wattage and real-world consumption
A rating plate or specification gives us a useful starting point, but stated maximum wattage and real consumption are not always identical. A thermostatically controlled heater may switch its heating element on and off. A fridge compressor cycles rather than running continuously. Some appliances enter standby, while others vary their power according to programme, temperature, load or operating conditions.
We therefore avoid implying that every appliance continuously draws its full rated wattage. Where a duty cycle, measured consumption figure or typical operating pattern is more appropriate, the article should explain which assumption has been used.
How we handle usage time
Usage varies from one household to another, so typical hours are assumptions rather than universal facts. If we calculate a daily, monthly or annual cost from an assumed number of hours, that assumption is stated in the article.
A washing programme used three times a week, for example, should not silently be converted into an annual figure without showing that frequency. The same principle applies to heating hours, standby periods and EV charging sessions.
Redoing the calculation with your own figures
You can reproduce most Warmer Bills running-cost calculations by replacing our assumptions with figures from your own home. Before doing so, check:
- the appliance wattage or a more representative measured consumption figure;
- the number of hours or operating cycles you actually use;
- your own tariff’s electricity or gas unit rate;
- whether the appliance cycles on and off or varies its power;
- the date on which the energy rate applies.
These calculations are estimates, not meter readings. Actual consumption can differ because of thermostats, duty cycles, standby behaviour, operating conditions and variable-power use. An older, damaged or poorly maintained appliance may also consume differently from the assumed figure.
For the most useful result, take the same formula, enter your own tariff rate and your own usage hours, and reproduce the calculation for the way the appliance is actually used in your home.