Calculate Energy

Think of electricity like water through a pipe. Wattage is how wide the pipe is — how fast power flows when the device is on. Hours of use is how long you leave the tap open. The amount of water that actually flows through is the energy consumed, measured in kilowatt-hours. Your utility charges you for that flow, not for the size of the pipe.

The calculation is deliberately simple: divide watts by 1,000 to get kilowatts, multiply by hours used to get kWh, then multiply by your rate. What makes this useful is applying it consistently across devices. A 1,500-watt space heater running 4 hours costs the same as a 6,000-watt electric dryer running 1 hour — the math does not care which device it is.

Where people go wrong is assuming that high-wattage devices are always expensive, or that low-wattage devices are always cheap. A 50-watt aquarium pump running 24 hours a day consumes 438 kWh per year — more than running a 1,500-watt hair dryer for 8 minutes every morning. Duration matters at least as much as wattage, and this tool makes that tradeoff visible immediately.

Use this tool when evaluating whether to replace an old appliance with a more efficient model. Calculate the annual cost of the current unit, then calculate the projected cost of the replacement using its rated wattage. The difference tells you how many months it takes to recover the purchase price through energy savings.

This is also the right tool for auditing devices you suspect are driving up your electricity bill. Run the calculation for every major device — refrigerator, HVAC, water heater, washer, dryer — and rank them by annual cost. The results are often counterintuitive: the always-on devices like refrigerators and pool pumps frequently outrank the high-wattage but infrequently used ones like electric ovens.

This tool is not appropriate for estimating your total household electricity bill, because it does not account for tiered pricing, time-of-use rates, demand charges, or fixed fees. It also does not model devices with variable power draw — such as inverter air conditioners, EV chargers with dynamic load balancing, or induction cooktops that cycle between power levels. For those, you need a smart plug with energy monitoring to capture real consumption rather than rated wattage.

The most common mistake is entering the circuit breaker rating instead of the device wattage. A 20-amp circuit does not mean your device draws 2,400 watts — that is the maximum the circuit can handle. Look at the device label or manual for actual rated wattage.

A second mistake is forgetting standby power. Televisions, game consoles, microwaves, and smart speakers draw power continuously even when you think they are off. Standby draw is typically 1 to 25 watts per device. Across 10 devices in standby, that adds up to a meaningful portion of your bill — and none of it appears if you only calculate active use.

The third mistake is assuming the monthly estimate applies to every month equally. Seasonal devices like air conditioners or electric heaters run far more in peak months. If you use this tool to estimate an annual heating cost, account for the fact that the device may only run for 4 to 5 months, not 12. Run the calculation at your peak usage level and multiply by the months it actually runs.

The core formula is: Energy (kWh) = Power (W) / 1,000 x Hours Per Day x Days Per Year. Cost = Energy (kWh) x Rate ($/kWh).

For monthly figures, the tool uses 365/12 days per month (approximately 30.42 days), adjusted by your days-per-week input. Weekly energy is calculated first — daily kWh multiplied by days per week — then scaled to a monthly figure using the factor 365 / 12 / 7, which converts a weekly rate into a monthly average. This gives a more accurate result than simply multiplying daily cost by 30.

The yearly figure uses 52 weeks, keeping it consistent with the weekly calculation rather than mixing 365-day and 30-day approximations. For most planning purposes the difference is under 1%, but it matters if you are comparing this estimate against an annual utility bill.

Rated wattage is a nameplate maximum, not an operating average. Resistive loads like space heaters and incandescent bulbs draw very close to their rated wattage. Motor loads like refrigerators and washing machines draw significantly more at startup than at steady state, and significantly less when lightly loaded. For a refrigerator compressor cycling 40% of the day, the actual daily kWh may be 40 to 60% of what you would calculate from nameplate wattage times 24 hours. The only way to know true consumption for variable-load devices is a smart plug with energy monitoring — the calculated figure here is a useful upper bound, not a precise measurement.