The short answer
Use a microwave’s electrical input or power-consumption rating for a watts-and-time estimate, rather than its advertised cooking-output watts. Multiply input kW by compatible operating hours and your tariff. Output power describes the cooking task and can be lower than the appliance’s electrical consumption.
Cooking watts describe a different quantity
The number used in recipe instructions or advertised as microwave cooking power can differ from the electricity consumed by the appliance. For a power-and-time electricity estimate, use electrical input or power consumption, with a compatible operating-time assumption.
For example, Panasonic’s official NN-SD765S specification lists microwave power consumption of 1,460 W and cooking power of 1,200 W. This is one model’s specification, not a typical ratio to apply to every microwave.
Find the source for your exact model
| Description accompanying the number | Suitable use |
|---|---|
| Electrical input or power consumption | A constant-input electricity estimate for the stated mode |
| Cooking power or microwave output | Cooking-performance information; not a substitute for input |
| Grill or combination-mode consumption | Check which mode and components the figure covers |
| Measured accumulated kWh | The energy used during the measured complete task |
Use the rating information or manual for the appliance you own. An electrical connection or circuit limit is not proof of the draw throughout a cooking programme.
Do not infer input by adding an arbitrary percentage to cooking output. The manufacturer example only demonstrates why the descriptions need checking.
Calculate a short session
Using 1,460 W as a constant-input scenario for five minutes:
1.460 kW × 5 ÷ 60 hours = about 0.121667 kWh.
At an illustrative £0.28/kWh, that energy costs about £0.0341, or 3.41 pence. A currency display rounded to two decimal places shows £0.03.
For 30 identical sessions, the unrounded total is 3.65 kWh and £1.022, displayed as £1.02. Multiplying the rounded £0.03 by 30 would give a different answer.
In the electricity calculator, enter input watts, five minutes per use day and 30 use days to model one such session on each of those days. The model and timing example above is not an energy measurement.
Measure the programme when input varies
Do not assume a selected cooking-output percentage is the same percentage of electrical input. A completed-task energy reading gives a different basis from holding the rating constant.
Retain the portion, programme and time with the reading. If you use the calculator’s complete-day measured mode, collect complete-day energy for the routine; do not describe a five-minute reading as a whole-day measurement.
Idle use outside the cooking session is excluded from the power-and-time session estimate. A complete-day measurement includes whatever idle use falls inside its boundary.
Compare equivalent cooking tasks
Energy Saving Trust discusses microwaves as useful for reheating and quick cooking. A fair cost comparison still needs the same amount of food prepared to the intended result; appliance wattage alone cannot determine the cheapest meal.
Read the air-fryer and oven guide for portion, batching and preheating boundaries, and watts-to-cost arithmetic to check the units.