Electricity cost calculator

Use the measurement you actually have

If a device draws roughly steady power, average watts and daily running time are enough. If it cycles, sleeps, heats, cools, or changes workload, a measured kWh total over several representative days is usually better. The calculator annualizes either input; it does not pretend a momentary watt reading is a full-year measurement.

A plug-in energy meter, compatible smart plug, utility submeter, or Home Assistant energy reading can provide the kWh option. Use a long enough period to include the behavior that matters. Seven ordinary days tell me more about a cycling device than one unusually busy afternoon.

One watt left on all year

One continuous watt uses 8.76 kWh in a 365-day year. At $0.20/kWh, that is about $1.75 per year. Ten watts is therefore about $17.52 per year at the same rate. This shortcut is useful for deciding which small standby loads deserve attention and which are barely visible on the bill.

The formulas

From power: daily kWh = watts × hours per day ÷ 1,000
From a meter: daily kWh = measured kWh ÷ measurement days
Annual cost = daily kWh × price per kWh × 365

A worked example

A hypothetical device averaging 40 W around the clock uses 0.96 kWh a day. At an illustrative $0.20/kWh, that is $0.192 a day, $5.84 per average month, and $70.08 a year. A meter recording 6.72 kWh over seven days produces the same estimate. These are example inputs, not a claim about a specific product or the rate where you live.

Where I get the inputs

I use average wall power for the complete setup, not the number printed on a power supply. A 300 W computer power supply describes capacity; it does not mean the computer continually consumes 300 W. If a computer also needs external storage, a network switch, or another powered box to do the same job, I include those loads in the comparison.

For electricity price, I enter the variable per-kWh amount that changes with usage, including applicable per-kWh charges. Enter 0.20 for 20 cents, not 20. With time-of-use pricing, I calculate each rate period separately. The currency selector changes the displayed symbol; it does not fetch rates or convert currencies.

What the result cannot decide

Electricity cost is only one part of the decision. A router, security base, recording device, or backup system may be doing useful work while nobody is looking at it. This calculator excludes fixed account charges, demand charges, maintenance, purchase price, and the cost of losing whatever service the device provides.

Before buying hardware to reduce the result, use the upgrade payback calculator. Before putting a small load behind another always-on device, check whether a smart plug can actually repay itself.

Source and method

The watts-to-kWh method follows the US Energy Information Administration’s guide to measuring electricity. The measured-kWh path simply annualizes the energy used over the entered period. Examples are my arithmetic. Methods and corrections.