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Free calculator

Use this calculator to

  • Convert an appliance's wattage and running hours into kWh per day and per period
  • Build the annual consumption figure a supplier or broker needs to price your contract
  • Compare loads in like-for-like kWh before deciding which equipment to upgrade
  • Express large totals in MWh for multi-site or half-hourly metered reporting

Energy Consumption Calculator

Calculate how much energy an appliance uses over time.

W
hr
days
Result
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Formulas

  • E (kWh/day) = P(W) × t(hr/day) / 1000
  • E (kWh total) = E(kWh/day) × days

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Annual kWh by asset is the unit of work for every meaningful energy review: ESOS Phase 4 submissions, SECR baselines, refurbishment business cases, supplier tenders. The calculation is simple in isolation and brittle at scale, because diversity factors, operating schedules and seasonality all sit between the nameplate watt rating and the actual half-hourly profile. A spreadsheet that gets the headline numbers wrong by 10% can quietly invalidate the whole report.

For a multi-site mid-market business, the asset audit is usually the longest line item in the energy strategy. Five hundred items across thirty sites at fifteen minutes each is forty days of work, and at the end of it the team has a baseline that the FD has to defend in a budget meeting. Building the baseline against measured HH data, not just asset-level estimates, is the way to make it stick.

Purely Energy uses Purely Insights to reconcile asset-level estimates against measured half-hourly consumption per site, so the variance between bottom-up and top-down models is visible from day one and traceable to specific meters. For ESOS, SECR and B-Corp recertification work that lands on the same finance team in the same quarter, the saved hours, the consistent methodology and the audit-trail confidence are usually worth more than the bill savings on their own, even though both compound.

Common questions

How do I calculate energy consumption in kWh?

Multiply the power in watts by the hours of use, then divide by 1,000: E (kWh per day) = W x hours / 1000. Multiply by the number of days for the period total. A 1,500 W load running 4 hours a day uses 6 kWh daily, or 180 kWh over a 30 day month. This is the same arithmetic your meter performs continuously, summed across every load on the supply.

What is the difference between kW and kWh?

Kilowatts measure power, the instantaneous rate of consumption, while kilowatt-hours measure energy, which is power multiplied by time. A 2 kW machine running for 3 hours consumes 6 kWh. Suppliers bill in kWh, while connection agreements and equipment ratings are set in kW or kVA. Keeping the two straight matters when you compare equipment: a higher-kW machine used briefly can consume less than a low-kW one left on.

How do I handle appliances that cycle on and off?

Enter the effective on time, not the plugged-in time. Refrigeration, heating and compressors run under thermostatic control, so a cold room compressor rated 1,200 W might only run a fraction of each hour once it is at temperature. If you know the duty cycle, multiply the hours by it; if not, use the average consumption from the energy label or a plug-in meter reading over a representative day.

How do I convert kWh to MWh?

Divide by 1,000: 1 MWh = 1,000 kWh. The calculator shows the MWh figure alongside the total because larger business supplies, half-hourly settled meters and wholesale market prices are quoted per MWh. For example, a load consuming 10,512 kWh a year is 10.5 MWh. Working in MWh also keeps multi-site totals readable when you aggregate consumption across a portfolio.

Why does annual consumption matter when getting an energy quote?

Suppliers price a business contract against the estimated annual consumption and the shape of usage across the day. The EAC on your bill drives which tariff bands you qualify for, and an inaccurate figure leads to mispriced contracts or volume tolerance disputes. Building the figure bottom-up from your main loads, as this calculator does, lets you sanity-check the supplier's estimate before you sign and gives a baseline for measuring efficiency projects.

Related calculators

The conversions that procurement and engineering use alongside this one.

Energy Consumption Calculator | kWh per Year | Purely Energy