Power Consumption Calculator

Power consumption in kilowatt-hours equals watts times hours of use divided by 1,000. A 100 W device running 24 hours a day uses 100 × 24 ÷ 1,000 = 2.4 kWh per day. Over a 30-day month that is 72 kWh, and over a year about 876 kWh. At a $0.17/kWh rate, that one device costs roughly $149 a year. The calculator above computes daily, monthly, and yearly energy plus annual cost from three inputs.

The reason this matters: your electric bill is billed in kilowatt-hours, not watts. Watts measure the rate of power draw; kilowatt-hours measure energy used over time. A 1,500 W space heater sounds alarming, but run only 2 hours it uses 3 kWh — less than a 100 W device left on all day. Time on is often more important than wattage.

The core equation and its extensions:

Dividing by 1,000 converts watt-hours into kilowatt-hours, the unit on your utility bill. Multiply by your rate — the price per kWh from your bill, typically $0.10–$0.30 in the United States — to get cost. Remember to use the appliance's real running watts, not its maximum nameplate: many devices (fridges, laptops, HVAC) draw far less on average than their peak rating because they cycle or throttle.

If you only know amps, convert to watts first (watts = volts × amps × power factor for AC), then use this tool. For inductive loads where volt-amps differ from watts, see the discussion of VA vs watts and power factor.

Example 1 — 100 W device, 24 h/day, $0.17/kWh: Daily = 100 × 24 ÷ 1,000 = 2.4 kWh. Monthly = 72 kWh, yearly = 876 kWh, yearly cost ≈ $149.

Example 2 — 1,500 W space heater, 3 h/day, $0.15/kWh: Daily = 1,500 × 3 ÷ 1,000 = 4.5 kWh. Yearly (winter-adjusted, say 120 days) ≈ 540 kWh, about $81 for the season. Run it 8 hours a day and cost triples.

Example 3 — 60 W ceiling fan, 8 h/day, $0.20/kWh: Daily = 60 × 8 ÷ 1,000 = 0.48 kWh, roughly $35 a year — cheap comfort compared with air conditioning.

Example 4 — old vs new refrigerator: A 20-year-old fridge averaging 150 W over the day uses 150 × 24 ÷ 1,000 = 3.6 kWh/day = 1,314 kWh/year ≈ $223 at $0.17. A modern unit averaging 50 W uses 438 kWh/year ≈ $74 — a $149 annual saving that can pay back the new appliance in a few years.

The most common confusion in energy is treating watts and kilowatt-hours as the same thing. They are not:

• Watts (W) are power — the instantaneous rate of energy use. A 60 W bulb draws 60 W the moment it is on.
• Watt-hours (Wh) and kilowatt-hours (kWh) are energy — power multiplied by time. That 60 W bulb on for 10 hours uses 600 Wh = 0.6 kWh.

Your utility bills energy (kWh), so run time drives cost as much as wattage. This is why a high-wattage appliance used briefly (toaster, kettle, hair dryer) can cost less than a modest always-on load (aquarium pump, DVR, old fridge, standby electronics). It is also why hunting phantom loads — devices drawing standby power around the clock — often saves more than it seems: even 10 W of standby is 88 kWh a year. For a whole-home picture, add up each load's daily kWh with this tool, or estimate a device's average watts with a plug-in meter first.

An appliance's label lists its maximum (nameplate) wattage, which is often much higher than its average draw. Using nameplate values overstates energy use for cycling and throttling devices.

• Refrigerators list 150–250 W but the compressor runs only part of each hour, so daily average is closer to 40–80 W. Enter effective run-hours (e.g., 8 h at full watts) to approximate the average.
• Air conditioners and heat pumps cycle with temperature; a "1,000 W" unit may average far less in mild weather.
• Computers and TVs vary with brightness and workload; idle draw is a fraction of peak.
• Motors surge at start-up but settle to running watts within a second — use running watts for energy, surge only for inverter sizing.

For accuracy, measure real consumption with an inexpensive plug-in energy monitor (it reads kWh directly over days), or estimate an effective run-time that captures the duty cycle. When sizing an inverter or generator instead of estimating cost, use the peak running watts and account for surge with the inverter sizing calculator and the generator sizing calculator.

Many appliance labels list amps and volts rather than watts. Convert to watts before using this calculator, because energy billing and this tool both work in watts and kilowatt-hours.

For simple resistive devices — heaters, toasters, incandescent bulbs — watts is just volts times amps. A 120V appliance drawing 10 A uses 120 × 10 = 1,200 W. For motor-driven or electronic loads with a power factor below 1, multiply by the power factor: a 120V, 5 A motor at 0.8 power factor draws 120 × 5 × 0.8 = 480 real watts even though it pulls 600 volt-amps from the outlet. Your meter bills the real watts.

This distinction between apparent power (volt-amps) and real power (watts) is central to sizing backup equipment; it is explained in depth in the UPS sizing calculator. For everyday cost estimates, using the nameplate watts (or volts × amps) is close enough, since residential meters charge for real energy and most household devices run near unity power factor or already list true watts.

Estimates from nameplate figures are useful, but the most accurate way to find a device's consumption is to measure it. An inexpensive plug-in energy monitor sits between the outlet and the appliance and reads accumulated kilowatt-hours over hours or days, automatically capturing the real duty cycle and power factor.

Measuring is especially valuable for three categories where estimates go wrong. First, cycling appliances — refrigerators, freezers, HVAC — whose compressors run intermittently, so their daily average is far below nameplate. Second, variable electronics — computers, TVs, game consoles — whose draw swings with workload and brightness. Third, phantom loads — chargers, DVRs, and smart devices that sip power around the clock; a whole home can hide 50–100 W of standby draw, which is 440–880 kWh a year.

Once you have a measured daily kWh, this calculator's monthly, yearly, and cost projections become far more reliable. For whole-home analysis, either measure each major load or read your utility's interval data (many providers offer hourly usage online), then compare to the sum of your device estimates to find unexplained consumption. Those measured daily figures also feed directly into off-grid storage planning through the battery runtime calculator and inverter runtime calculator.

1. Enter appliance power in watts. Use average running watts for cost estimates, not the maximum nameplate, when the device cycles.
2. Enter hours used per day. For cycling appliances, estimate effective full-power hours (e.g., a fridge compressor running about 8 hours total per day).
3. Enter your electricity rate. Find $/kWh on your utility bill; the U.S. average is around $0.15–$0.18.
4. Read daily, monthly, and yearly kWh and the annual cost.
5. Compare devices by running each one to find which loads dominate your bill.

Once you know each device's yearly kWh, savings become obvious. The biggest wins usually come from the largest energy users, not the highest-wattage ones — always-on and long-run loads.

• Heating and cooling dominate most bills. Every degree of setback and better insulation cuts hours of run time.
• Water heating is often second; timers and lower setpoints help.
• Old refrigerators and freezers run 24/7 — replacing an inefficient one can save $100–$200 a year.
• Phantom loads (chargers, DVRs, game consoles in standby) add up; smart power strips eliminate them.
• Lighting switched to LED cuts per-bulb draw by 80–90%.

For off-grid and backup planning, the same daily-kWh figures feed battery and solar sizing — layer them into the battery runtime calculator to size storage for a target number of days.

• Using nameplate watts for cycling devices. Fridges and AC average far below their peak; you will overestimate cost.
• Confusing watts with kWh. A high-wattage device used briefly can cost less than a small always-on load.
• Forgetting phantom loads. Standby power runs 24/7 and quietly adds up.
• Ignoring the real rate. Tiered, time-of-use, and demand charges can make your effective rate higher than the base number.
• Using 31-day months inconsistently. This tool uses 30 days per month and 365 per year for clean comparisons.

The most useful way to act on these numbers is to rank your loads by yearly kWh, not by wattage, and attack the top of that list. A single always-on device that uses 3 kWh a day costs more over a year than a dozen high-wattage gadgets you run for a few minutes. Build the ranked list once — heating and cooling, water heating, refrigeration, laundry, entertainment, and standby — and you will usually find that two or three loads account for more than half the bill. That is where efficiency upgrades, behavior changes, and smart scheduling (running heavy loads during off-peak time-of-use windows) pay back fastest. Re-run the calculator with your real rate and revised run-hours after each change to confirm the saving, and keep the seasonal loads (space heaters, air conditioners) separate so a summer or winter spike does not distort your annual planning.