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How to Calculate Electricity Cost from Watts & kWh

Calculate appliance electricity cost from watts, runtime, kilowatt-hours, and your own tariff for daily, monthly, and annual estimates.

Navorika Team Published August 29, 2026 Updated August 29, 2026 10 min read
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Make everyday cost and shipping estimates from transparent inputs and unit conversions.

Electricity cost depends on energy, not power alone. Watts describe the rate at which an appliance uses energy; kilowatt-hours combine that power with operating time. Multiply energy by the tariff that actually applies to estimate usage cost.

Watts, kilowatts, and kWh

Convert watts to kilowatts by dividing by 1,000. Then: Energy (kWh) = power (kW) × hours Cost = energy (kWh) × electricity rate per kWh A 100 W device running for 5 hours uses 0.1 kW × 5 h = 0.5 kWh. Watt is a power unit; kWh is an energy unit, despite the common phrase “kilowatts per hour.”

Daily, monthly, and annual examples

Suppose a 1,500 W heater runs 2 hours per day. Daily energy is 1.5 × 2 = 3 kWh. At an illustrative user-entered rate of 0.20 currency units per kWh, daily usage cost is 0.60, a 30-day estimate is 18.00, and a 365-day estimate is 219.00. The tariff is only an example; enter the current rate from your bill.

Duty cycles and variable power

Rated watts may be maximum input rather than average use. Refrigerators, air conditioners, pumps, and thermostatically controlled heaters cycle on and off. Computers and variable-speed equipment change load. For better estimates, use a measured average from a suitable energy meter or multiply rated power by a defensible duty-cycle assumption.

Standby use and multiple appliances

A small continuous standby load can accumulate. A 5 W load operating 24 hours uses 0.12 kWh per day. For several appliances, calculate each device using its own watts and runtime, then add kWh and cost. This is clearer than adding wattages when their schedules differ.

Tariffs and the full bill

A simple energy-rate estimate may not match the final bill. Real tariffs can include fixed charges, taxes, slabs or tiers, time-of-use prices, demand charges, fuel adjustments, minimum charges, export credits, or subsidies. Derive an appropriate marginal or average rate from the bill according to the decision being made, and do not hard-code a universal tariff.

How to use the Electricity Cost Calculator

Enter appliance wattage, average hours per day, number of days, and your current rate per kWh. Review energy and daily, 30-day, annual, and selected-period cost. Repeat for each appliance when building a household estimate. The tool estimates usage charges and does not reproduce every utility billing rule.

Key takeaway

Convert power to kW, multiply by realistic runtime, and apply the current relevant tariff while keeping broader bill charges separate.

Frequently asked questions

How do I convert watts to kWh?

Divide watts by 1,000, then multiply by hours of operation.

Can I use the wattage on the label?

Yes for a rough upper-bound or steady-load estimate, but cycling and variable-power devices may use less on average.

Why does the result differ from my bill?

Bills may include other loads, fixed fees, taxes, tiered prices, demand charges, and changing usage.

How do I include standby power?

Calculate the standby wattage over its actual hours—often close to 24 per day—and add that energy separately.

Sources and further reading

Sources support the general explanations above. Rules, rates, standards, and professional guidance may change; verify the current source before acting.

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