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Free calculator

Use this calculator to

  • Convert an appliance's wattage and running hours into kWh on the meter
  • Estimate weekly or monthly consumption for a single piece of equipment
  • Compare the running energy of two machines before you buy
  • Build an appliance-by-appliance load inventory for an energy audit

Watts to kWh Calculator

Convert power (W) and time (hours) to energy (kWh).

W
hr
Result
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Formulas

  • kWh = W × t / 1000

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

The kWh figure is the one that shows up on every supplier bill, every benchmarking report and every SECR submission. Working backwards from equipment wattage and operating hours into kWh is the analytical move that lets a finance team or facilities lead say, with confidence, that the bill they received is consistent with the kit they own and the hours it runs. A 1.5kW server cabinet running 24/7 for a year consumes 13,140 kWh. At a unit rate of 28 p/kWh, that is £3,679 a year, in one cabinet, in one server room.

For multi-site portfolios, the watts to kWh conversion drives the asset audit. Walk a site, log the nameplate watts of every plug load, estimate the operating hours, and the resulting annual kWh figure tells operations where to look first. Always-on phantom loads (printers in sleep, vending machines that never get switched off, CCTV NVRs sitting idle) typically account for 10 to 20% of total consumption on commercial sites, and almost all of that is recoverable through simple scheduling or replacement.

Purely Energy turns this site-level work into portfolio-scale savings for the 2,000+ sites we manage. We use the Purely Insights platform to monitor consumption continuously, flagging sites where the baseload has crept up between contract years, and we feed those findings into the next contract negotiation so the volume forecast and the tariff shape track what the operation actually does. The result is a contract that is cheaper to buy and easier to manage than one specified against last year's bill.

Common questions

How do I convert watts to kWh?

Multiply the power in watts by the running time in hours, then divide by 1,000: kWh = W x h / 1000. A 3 kW unit heater running through an 8 hour working day uses 3000 x 8 / 1000, which is 24 kWh. The same formula scales to any period: multiply the daily figure by working days to get a weekly or monthly total for that appliance.

What is the difference between a watt and a kilowatt-hour?

A watt measures the rate of energy use at an instant; a kilowatt-hour measures the amount of energy used over time. The relationship is the same as speed and distance: watts are the speedometer, kWh is the mileage. Your bill charges for kWh, not watts, which is why a high-wattage appliance used briefly can cost less to run than a low-wattage one that runs all day. A 3 kW kettle used 10 minutes a day uses 0.5 kWh; a 100 W device left on for 24 hours uses 2.4 kWh.

Should I use the nameplate wattage for this calculation?

Treat the nameplate as a maximum, not an average. Thermostatically controlled equipment such as fridges, heaters and ovens cycles on and off, so average draw is well below nameplate. Variable speed drives and modern compressors modulate too. For a quick estimate, multiply nameplate watts by a duty cycle (a display fridge might run its compressor 40 to 60 percent of the time). For anything material to a bill, a plug-in meter or half-hourly data beats any estimate.

How do I turn kWh into a running cost?

Multiply the kWh figure by the unit rate on your electricity contract, quoted in pence per kWh, and add any standing or capacity charges if you want a fully loaded figure. Rates vary by contract, site and meter type, so use the rate from your own bill rather than a published average. Once you know an appliance's kWh per week, the arithmetic is a single multiplication, and it makes the business case for replacement or control changes very quick to test.

How many kWh does a 3 kW heater use per day?

Running flat-out for an 8 hour day, a 3 kW heater uses 24 kWh. In practice a thermostat cycles the element, so a heater holding a set temperature in a reasonably insulated space might run at a 50 percent duty cycle and use closer to 12 kWh. That gap between nameplate arithmetic and metered reality is exactly why controls, set points and insulation are usually the first recommendations in an energy audit.

Related calculators

The conversions that procurement and engineering use alongside this one.

Watts to kWh Calculator | Purely Energy