kWh, MWh, GWh — the energy units on every utility invoice
A kilowatt-hour is a watt sustained for a thousand hours — or, equivalently, a thousand watts sustained for one hour. Bills come in three factors of a thousand between homes and power stations.
The kilowatt-hour is the most successful metric-imperial hybrid ever created. It combines a metric power (kW) with a customary time unit (hour) and is the billing basis for every electric utility in the world.
The exact factors
1 kWh = 3,600,000 J = 3.6 MJ (exact) > 1 MWh = 1,000 kWh > 1 GWh = 1,000,000 kWh > 1 TWh = 10⁹ kWh (annual national scale) > 1 kWh = 3,412.14 BTU
Sanity-check anchors
US home average: 900 kWh/month = 30 kWh/day = 1.25 kW average draw. LED bulb: 10 W = 0.24 kWh/day if on all day. Fridge: 1.5 kWh/day. Electric water heater: 12 kWh/day. Central AC: 3 kW × 8 hrs = 24 kWh/day in summer. Tesla Model 3 charge: 60 kWh for 350 km range.
Rate structures
US average residential rate: $0.16/kWh (varies from $0.10 in Idaho to $0.44 in Hawaii, 2025). Time-of-use rates can spread peak/off-peak by 3–5×. Industrial customers billed on MWh with demand charges: a factory paying $0.08/kWh may also pay $15/kW-month for its peak 15-minute draw — often more than the energy cost itself.
Worked example — a 5 kW solar array
A 5 kW rooftop PV system in Los Angeles produces ~5 × 5.5 sun-hours/day = 27.5 kWh/day = 825 kWh/month = 9,900 kWh/year. At $0.30/kWh (California residential), that offsets $2,970/year. Payback on a $15,000 installed cost: ~5 years. Northern climates get about half that.
Grid vs plant scale
A modern gas turbine: 500 MW output, run 8,000 hours/year = 4 TWh/year. A nuclear reactor: 1,000 MW × 8,500 hrs = 8.5 TWh/year. Total US electricity consumption 2024: ~4,100 TWh. So a single nuclear reactor supplies about 0.2% of US demand — which is why decarbonising the grid takes hundreds of large plants.
The five-second recap
kWh = homes. MWh = factories. GWh = towns. TWh = nations. Divide watts by 1,000, multiply by hours, that's kWh.
Try the converters mentioned in this article
FAQ
Why not just use joules?+
1 kWh = 3.6 MJ. Utility bills would show 90 GJ per household per month — awkward and unfamiliar. kWh survived because the numbers are the right size.
What is a demand charge?+
A fee based on your peak instantaneous power draw, typically averaged over 15 minutes. Common on commercial bills; separate from energy (kWh) charges.
How much CO₂ per kWh?+
US grid average: ~380 g CO₂/kWh. Coal: 900 g/kWh. Natural gas: 400 g/kWh. Wind/solar/nuclear: <50 g/kWh lifecycle.
This article was written by The Turbo Unit Converter engineering desk (Energy & utilities) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.