kWh Calculator
Your last electricity bill shows 782 kWh – but where did that number come from? Every appliance in your home runs on electricity, and each one contributes a small slice to that total. A kWh calculator is the simplest way to see exactly how much energy any device consumes, and what it costs you.
How do you calculate kilowatt-hours from watts and hours?
The formula is straightforward, but people often mix up power and energy. Power (watts) tells you how fast a device consumes electricity at a given moment; energy (kilowatt‑hours) tells you how much is used over time.
Formula: kWh = (Watts × Hours) ÷ 1,000
Example: a 1,500‑watt space heater running for 4 hours.
(1,500 × 4) ÷ 1,000 = 6 kWh
If your electricity rate is $0.15 per kWh, those 4 hours cost $0.90. That might not sound like much, but if you run the heater 8 hours a day all winter, you are looking at around $108 per month.
The kWh calculator below does this automatically for any combination of power and time.
Enter any wattage (from a tiny phone charger at 5 W to a whole‑home electric furnace at 15,000 W) and the hours of use. The calculator instantly returns the energy consumption in kWh.
You can also flip the logic: if you know the kWh on your bill and the runtime, you can estimate the average wattage of an unknown device.
What is a kilowatt-hour?
A single kilowatt‑hour equals 1,000 watt‑hours. If you turn on a 100‑watt incandescent bulb and leave it burning for 10 hours, it consumes 1 kWh. The same energy could power a 2,000‑watt hair dryer for 30 minutes or a 10‑watt LED bulb for 100 hours.
Utility meters measure only in kWh because that unit directly corresponds to the work done by the electricity. Generating plants, transmission lines, and even battery storage systems all use kWh as the standard currency of electricity.
How to use the kWh calculator to check your bill
Grab a recent bill. Find the meter reading or the total monthly kWh. Now take one appliance you suspect costs a lot to run.
- Look at its label for the wattage (often given in watts, sometimes in kilowatts – multiply kW by 1,000 to get watts).
- Estimate how many hours per day it is actually on. A refrigerator cycles on and off; use about 8 hours of active compressor time even if it is plugged in 24/7.
- Plug those numbers into the kWh calculator above.
- Multiply the resulting daily kWh by 30 to get a monthly estimate.
- Multiply that by your rate per kWh (shown on your bill).
Electricity rates vary by location, time of day, and provider. Check your latest tariff for the exact per‑kWh price.
If the calculated monthly usage is far larger than expected, check whether the device runs on a lower setting or uses a thermostat that cycles power.
Common wattages of household appliances
Precise numbers help you spot outliers. Here are typical wattages you can use when you don’t have the label handy.
| Appliance | Wattage (range, W) | Typical daily usage |
|---|---|---|
| LED TV (55″) | 60 – 150 | 4 hours |
| Gaming PC (desktop) | 200 – 500 | 3 hours |
| Laptop | 30 – 90 | 6 hours |
| Refrigerator/freezer | 100 – 800 | 8 hours (compressor running) |
| Electric oven | 2,000 – 5,000 | 1 hour |
| Clothes dryer (electric) | 1,800 – 5,000 | 1 hour |
| Central AC (3‑ton) | 3,000 – 5,000 | 8 hours (summer) |
| Space heater (1,500 W) | 1,500 | 4 – 12 hours (depending on climate) |
| LED light bulb | 5 – 20 | 5 hours |
| Phone charger | 2 – 6 | 3 hours |
Use the kWh calculator with your actual numbers to get an accurate daily figure. Then project the cost per month, season, or year.
How to convert kWh into dollars
Your bill lists a cost per kWh – typically $0.10 to $0.40 depending on where you live. Multiply the kWh from the calculator by that rate:
Cost = kWh × rate
For example, a 200‑watt gaming PC running 5 hours a day:
(200 × 5) ÷ 1,000 = 1 kWh per day
1 kWh × 30 days = 30 kWh per month
30 kWh × $0.12 = $3.60 per month
Add a few more devices that run all day – a router, smart speakers, always‑on TV – and the “base load” can easily reach $20–50 each month.
Check your electricity plan: some utilities charge different rates during peak and off‑peak hours. To calculate precisely, you need the weighted average rate.
Why a kWh calculator is smarter than a kill‑a‑watt meter alone
Plug‑in power meters give live wattage readings. They are great for measuring an appliance that runs continuously. But a kWh calculator lets you combine multiple sources of information: the appliance’s rated max wattage, your best estimate of runtime, and your actual billing rate. You can do “what if” analysis without moving a meter from room to room.
If you only have an amp rating (e.g., 8 amps at 120 volts), use the relationship Watts = Volts × Amps. In North America, standard voltage is 120 V; in many other regions, 230 V. The calculator accepts power only in watts, so convert first.
How to reduce the kWh on your next bill
The fastest savings come from targeting the highest‑consuming devices. Use the calculator to rank your appliances by daily kWh:
- Replace 60‑watt incandescents with 9‑watt LEDs. A four‑bulb fixture used 5 hours a day drops from 1.2 kWh to 0.18 kWh daily – that’s 85 % less.
- A 1,500‑watt space heater can burn 12 kWh in an 8‑hour workday. Heating the person, not the room (electric blanket, 100 W) slashes consumption by over 90 %.
- Older refrigerators often use 1,500–2,500 kWh per year; a new Energy Star model uses under 500 kWh. A kWh calculator helps weigh the payback time.
For each kWh you avoid, you save water at the cooling tower and CO₂ at the power plant. The U.S. Energy Information Administration reports that the average residential price of electricity in 2024 was about $0.16 per kWh. That means every 100 kWh you cut trims about $16 from your bill each month.