kWh Calculator — Free Online Calculator
Calculate kilowatt-hours (kWh) from watts and time. Free energy usage calculator for any appliance or device.
How to Use This Calculator
Enter the power consumption in watts and the duration in hours. The calculator converts to kilowatt-hours, the unit your utility company uses for billing.
The Formula Explained
kWh = Watts × Hours / 1000. One kilowatt-hour equals 1,000 watts used for one hour, or 100 watts for 10 hours.
Understanding Kilowatt-Hours
The kilowatt-hour is the fundamental unit of electrical billing worldwide. It measures a specific quantity of energy: 1,000 watts of power delivered for 1 hour, equivalent to 3,600,000 joules. Every electric appliance, every light bulb, every motor, every charger in your home consumes some number of kWh over time. The utility reads your meter, calculates the difference from last month, multiplies by the rate, and sends you a bill. Understanding this calculation puts you in control of your energy costs.
The formula is simple: kWh equals watts times hours divided by 1,000. A 100-watt bulb running 10 hours uses 1 kWh. A 1,200-watt microwave running 5 minutes uses 0.1 kWh. A 50-inch LED TV at 100 watts running 4 hours per day for a month uses 12 kWh. Multiply any kWh figure by your electricity rate (typically 10-30 cents per kWh in the US) and you get the cost. This lets you evaluate any appliance decision on economic terms: is this efficient refrigerator worth the extra 200 dollars over 10 years of ownership?
Worked Example: Refrigerator Annual Energy Cost
A modern ENERGY STAR refrigerator is rated 400 kWh per year (this figure is printed on the yellow EnergyGuide label). At 16 cents per kWh, annual operating cost is 400 × 0.16 = 64 USD per year. Over a typical 15-year refrigerator lifespan, total energy cost is 960 USD.
Compare to an older (1995) non-ENERGY STAR fridge rated about 1,200 kWh/year. Annual cost: 192 USD. Over 15 years: 2,880 USD. The newer efficient fridge saves 1,920 USD over its lifetime — probably more than the purchase price of either unit.
This is why replacing very old appliances often pays back quickly even when the old one still works. A 20-year-old fridge costing 100 USD per year extra in electricity will pay back the entire purchase price of a modern replacement in 5-7 years of use.
Worked Example: Space Heater vs Central Heat
A 1,500W portable electric space heater running 8 hours per day during a 5-month winter season. Monthly kWh: 1,500 × 8 × 30 / 1000 = 360 kWh. Monthly cost at 16 cents: 57.60 USD. Season cost: 288 USD.
Compare to heating the same room with a natural gas furnace (assuming 80% efficient, 1.10 USD per therm gas price, and the room requires about 20,000 BTU/hr). Space heater at 1,500W = 5,120 BTU/hr. The gas furnace delivers 4x more heat for a different cost structure: 20,000 BTU/hr × 8 hr × 150 days × 1.25 (for 80% efficiency) / 100,000 = 300 therms × 1.10 = 330 USD.
Surprisingly close. The space heater is more efficient per BTU delivered (100% electric efficiency at the room), but electricity costs 3-5x more per BTU than natural gas. For spot heating small areas, electric space heaters compete favorably. For whole-house heating, gas still wins in most US markets. Heat pumps change this calculation entirely — they can deliver 3-4 BTU of heat per BTU of electricity input.
Five kWh Mistakes
1. Forgetting phantom loads. TVs, cable boxes, computers, phone chargers, microwaves with digital clocks — all draw 5-20 watts continuously even when "off". Total phantom load for an average home is 60-100 watts, or about 525-875 kWh per year, worth 85-140 USD.
2. Using nameplate watts instead of running watts. A nameplate 1,200W microwave might actually draw 1,500W when running due to magnetron inefficiency. Dryer nameplate 5,400W might average 3,500W over a full cycle because the heating element cycles on and off.
3. Ignoring duty cycle. A 3-ton AC nameplated 3,500W only actually runs about 30-50% of summer hours. Nameplate × 24 hours overstates consumption by 50-70%.
4. Confusing kWh with kW. kW is power (rate of use, instantaneous). kWh is energy (cumulative, over time). A 5 kW heater running 1 hour uses 5 kWh. Running 30 minutes uses 2.5 kWh. The "h" matters.
5. Using average rates when rates are tiered. Many utilities charge more for usage above certain thresholds. Your first 500 kWh might cost 10 cents, next 500 kWh might cost 18 cents, anything above 1,000 kWh might cost 28 cents. Marginal savings (reducing high-tier usage) are much more valuable than average savings.
Typical Monthly kWh for Common Devices
Refrigerator (modern, efficient): 30-50 kWh/month. Refrigerator (old, 1990s): 100-150 kWh/month. Freezer (chest, full size): 30-60 kWh/month. Water heater (electric, 50 gal): 350-500 kWh/month. Electric dryer: 50-100 kWh/month (depends on loads). Dishwasher: 25-40 kWh/month. Central AC (3-ton, summer): 400-800 kWh/month. Electric heater (room, winter): 200-500 kWh/month each. LED lighting (whole house): 30-80 kWh/month. Incandescent lighting (whole house, old): 150-300 kWh/month. TV (50" LED, 4 hr/day): 15-25 kWh/month. Computer (desktop, 6 hr/day): 25-40 kWh/month.
Total US residential average: 886 kWh/month, or about 10,630 kWh/year. Very efficient homes can hit 400-600 kWh/month with heat pump water heater, induction cooking, and LED everything. Homes with electric heat can peak at 3,000+ kWh/month in winter.
Energy Units and Standards
The kilowatt-hour is a non-SI unit derived from the SI units of watt (power) and second (time). 1 kWh = 3,600,000 joules exactly. Other energy units and conversions: 1 kWh = 3,412 BTU = 860 kcal = 1.34 horsepower-hours. The SI unit for energy is the joule, but the utility industry uses kWh because joules are inconveniently small for billing practical electrical consumption.
FTC EnergyGuide labels are required on most major appliances sold in the US, showing estimated annual kWh consumption and cost. ENERGY STAR certification requires appliances to consume a specified fraction less energy than the federal minimum standard. Both programs help consumers compare efficiency across brands and models.
kWh: the unit your utility actually charges for
A kilowatt-hour is energy: one kilowatt of power consumed for one hour. The utility meter integrates power over time and bills you per kWh. The math to calculate kWh for an appliance is straightforward; the harder part is finding the actual wattage when it varies with mode and time.
The formula and what it does
The first form when you know wattage. The second when you measured with a clamp meter. PF (power factor) is needed for AC measurements because utility meters bill real power (kWh), not apparent (kVAh).
Worked example
Scenario: Old 21 cu-ft refrigerator runs about 35 percent duty cycle, drawing 180 W when compressor on.
Daily kWh: (180 / 1000) x 24 x 0.35 = 1.51 kWh/day. Monthly: 1.51 x 30 = 45.3 kWh. At 16 cents/kWh: $7.25/month, $87/year. A modern EnergyStar fridge of the same size runs at about 90 W average and uses 22 kWh/month = $3.52/month, $42/year. Annual savings on swap: $45 from electricity alone, more if old fridge is at end-of-life and inefficient.
Common mistakes to avoid
undefinedFrequently asked questions
How is kWh different from kW?
kW is the rate of energy use (power). kWh is the total energy used over time. A 1 kW appliance running for 1 hour uses 1 kWh. Running for 30 minutes uses 0.5 kWh. The utility meter measures kWh.
How do I find appliance wattage?
Nameplate is the easiest, usually inside the door or on the back. If it shows only amps, multiply by voltage (usually 120 V for outlets). Smart plugs with energy monitoring (Kasa, Emporia) measure actual usage including duty cycle.
My utility bill shows kVAh. What does that mean?
Commercial bills sometimes use kVAh (kilo-volt-amp-hours), which is apparent energy. Includes both real power and reactive. Utilities use this on industrial accounts to charge for low power factor. Residential bills are always kWh (real energy only).
How accurate is my utility meter?
Within 0.5-1 percent per ANSI C12. AMI smart meters report hourly or 15-minute intervals; older meters report monthly totals.
Does the meter measure during outages?
No usage means no measurement. The meter records only when current flows. Outages do not generate phantom bills.
Why does my computed kWh differ from my bill?
You probably missed some loads. Phantom loads (TVs in standby, chargers plugged in, modems, networking) total 30-100 W constantly = 22-72 kWh/month for a typical home. Add to your accounted loads.