How to calculate air-conditioner running cost
Multiply electrical input in kilowatts by hours used and your tariff in pounds per kWh. Keep rated maximum input separate from an optional compressor-duty assumption, because BTU/h describes cooling output—not electricity use.
- Formula
- Power (kW) × hours × tariff (£/kWh)
- 1,000 W example
- 25p for one maximum-input hour at 25p/kWh
- Use your evidence
- Rated watts from the exact model and the unit rate from your bill
Compare the decision at a glance
| Scenario | Energy used | Electricity cost |
|---|---|---|
| One hour at rated input | 1.0 kWh | £0.25 |
| Eight hours at rated input | 8.0 kWh | £2.00 |
| Eight hours at a hypothetical 60% duty cycle | 4.8 kWh | £1.20 |
Start with the only cost formula you need
For a transparent full-input estimate, divide the appliance's electrical input in watts by 1,000, multiply by the hours used, then multiply by the electricity unit rate in pounds per kilowatt-hour. A 1,000 W appliance is 1 kW. If it draws that power continuously for four hours it uses 4 kWh; at 26 p/kWh, the energy part of that session costs £1.04. The arithmetic is simple enough to check by hand.
This calculation needs electrical input, not cooling capacity. A product advertised as 9,000 BTU/h describes a rate of cooling output. Its rated input might be around a kilowatt, but the exact verified figure must come from that model's label, fiche, manual or manufacturer data. Never use BTU/h as though it were watts, and do not infer missing input from another product with the same headline capacity.
Use the unit rate on your own tariff
Ofgem's average direct-debit electricity unit rate for 1 July to 30 September 2026 is 26.11 p/kWh, including VAT, but that is a dated benchmark rather than everybody's price. Rates vary by region, payment method, meter and contract. Fixed deals and time-of-use tariffs may differ materially. Read the electricity line on a recent bill or supplier account and record both the value and its valid dates.
The energy price cap limits rates on covered default tariffs; it does not cap a household's total bill. More consumption still costs more. Ofgem reviews cap levels every three months, so a calculator that silently keeps an old national rate can mislead. The running-cost calculator lets you enter your own p/kWh figure. Revisit a saved scenario when the tariff changes rather than overwriting its historical assumption.
Keep the standing charge out of marginal appliance cost
A standing charge is the daily amount charged for maintaining the electricity supply whether or not the air conditioner runs. A household that already has an electricity account will normally pay it anyway, so adding the full daily standing charge to one appliance's marginal cost double-counts it. Use the unit rate for the extra consumption caused by the air conditioner, while keeping standing charges in a separate whole-tariff comparison.
There is one important distinction: if you are comparing complete tariffs, include every standing charge over the comparison period as well as every relevant unit rate. A low night rate can be offset by a higher daytime rate or fixed charge. That is a tariff decision, not an air-conditioner efficiency result. State which calculation is being shown so 'cost to run this appliance' is not confused with 'total electricity bill'.
Rated maximum input is a scenario, not a meter reading
A compressor may cycle off or modulate after the room approaches its set temperature. Outdoor conditions, solar gain, insulation, target temperature, open doors, hose sealing and control design all affect that duty cycle. A calculation that assumes rated input for every minute is a useful conservative scenario, but it is not a prediction that the appliance will consume exactly that amount on every night.
Fans, pumps, controls and displays may continue to draw power when the compressor is not at full output. Conversely, some products can briefly operate differently from a single published nominal value. Portable-air-conditioner test procedures define standard conditions so models can be assessed consistently, but a standard laboratory result is not a model of your bedroom. Label any duty-cycle percentage as a user-adjustable assumption rather than a measured product fact.
Build low, central and high-use scenarios
Avoid turning one worked example into a national promise. Take a verified 1,000 W input and a 26 p/kWh tariff: two hours at continuous input costs about £0.52, eight hours about £2.08 and 40 hours about £10.40. If you model a 60% compressor duty cycle, the eight-hour energy estimate becomes 4.8 kWh and about £1.25. The 60% is hypothetical unless a meter measured it in the same conditions.
For a useful household range, vary both operating hours and duty-cycle assumption while holding the tariff and rated input visible. The overnight running-cost tool is designed for this kind of overnight scenario. Do not apply a summer-night estimate to every day of the year. UK hot-weather use can be intermittent, and annual totals should show the selected days or weeks rather than imply continuous cooling.
Compare portable and split systems fairly
For portable models, compare verified input watts, cooling output, duct arrangement and the room in which the unit will operate. A poorly sealed single-hose setup can draw warm replacement air into the property, increasing the load without changing the number printed on the rating plate. Pre-cooling, external shading and closing the room can sometimes reduce the hours needed, but no saving should be quoted without a defined baseline.
For split and reversible air-to-air heat-pump systems, seasonal efficiency measures such as SEER or SCOP provide additional standardised context. They still do not predict an exact bill because system sizing, weather, controls and use matter. Energy labels can show cooling design load, efficiency and modelled annual consumption under defined test assumptions. Compare like with like and ask an installer to explain which values in a proposal are test data and which are project estimates.
Measure safely when a closer answer matters
A plug-in energy monitor can provide a useful observation for a compatible portable appliance if its voltage, current and power rating safely exceed the load and the manufacturer permits the arrangement. Follow the monitor and appliance instructions; do not use an underrated adaptor, damaged extension or improvised wiring. For fixed systems, consumption monitoring belongs within the designed electrical installation and should be discussed with a competent electrician or installer.
Record date, room, outdoor conditions, set point, start temperature, hours and measured kWh. One mild evening is not an annual average, but repeated observations are more informative than an invented duty cycle. This guide and calculator are budgeting tools, not a bill guarantee. Tariffs, weather and behaviour change, and the site does not claim that any product will deliver a stated saving without comparable evidence.
Resolve conflicting wattage figures before calculating
A cost estimate is only as sound as the electrical input placed into it, and one product page can show several figures that are not interchangeable. Confirm the exact model and market variant, then identify whether a number describes rated cooling input, maximum input, heating input, standby demand, current, cooling output or a modelled energy-label result. Cooling output in BTU/h or kW is the heat-removal rate, not the electricity bought from the supplier. A current rating multiplied by nominal voltage may describe a circuit or safety boundary rather than typical operating power, so it should not silently replace a published input-wattage value. Start with the rating plate, exact manual, product fiche or manufacturer record and retain the label attached to the figure. If a retailer has copied one wattage across several capacities, treat that as an unresolved conflict rather than selecting whichever value produces the most attractive cost.
When sound exact-model sources disagree, show the range of consequences instead of manufacturing certainty. Use the stronger source’s clearly defined rated input for the central calculation, retain the other value as a documented high or low scenario where its meaning is plausible, and explain why neither is being treated as a meter reading. Keep cooling, dehumidification and heating modes separate because the electrical behaviour and useful outcome can differ. An energy label’s modelled annual consumption belongs to its stated test assumptions; it should not be divided into an apparently precise household hourly cost or combined with user-selected hours as though both describe the same scenario. Date the source and tariff together, and leave the result unavailable if the input cannot be verified. That conservative pause is more useful than a polished calculation built on the wrong field. Once the appliance is owned, a suitable energy monitor can refine the scenario under real conditions, but it should validate the identified mode and model rather than excuse weak source work before purchase. Never average unlike modes to fill a blank.
Sources and further reading
Questions answered
How much does a 1,000 W air conditioner cost per hour?
At a round 25p/kWh tariff, continuous 1,000 W input costs 25p for one hour. Replace both figures with the exact model's rated input and the unit rate on your bill.
Is BTU the same as electrical wattage?
No. BTU/h describes cooling output, while input watts describe the electrical power used. Use the input-watt figure for a running-cost calculation.
Should I include the electricity standing charge?
Usually not when estimating the extra cost of operating one appliance in a home that already has an electricity supply. Include standing charges when comparing complete tariffs.
