What This Calculator Does
Enter the wattage of any appliance, how many hours per day you run it, and your local electricity tariff rate. The calculator converts wattage and time into kilowatt-hours (kWh), then multiplies by your rate to produce a cost estimate for any period — daily, weekly, monthly, or annual.
This is a cost and energy consumption calculator. It does not solve for voltage, current, or resistance. For circuit power calculations (P = V × I), see the Ohm's Law Calculator instead.
How the Formula Works
Step 1 — Convert watts to kilowatts:
kW = Watts ÷ 1,000
Step 2 — Multiply by daily hours to get daily kWh:
Daily kWh = kW × hours per day
Step 3 — Multiply by your tariff rate to get cost:
Daily cost = Daily kWh × rate per kWh
Scale up for monthly (× 30) or annual (× 365) estimates. Your electricity bill always charges in kWh — this is the universal billing unit regardless of currency or country.
Worked Examples
Example 1 — Refrigerator running continuously:
Wattage: 150 W | Hours/day: 24 | Rate: $0.15/kWh
Daily kWh = 0.15 kW × 24 h = 3.6 kWh
Daily cost = 3.6 × $0.15 = $0.54 / day ≈ $16.20 / month
Most refrigerators cycle on and off, so actual usage is 30–50% lower than their rated wattage running flat-out. Enter 50–70 W for a more realistic estimate.
Example 2 — 65-inch LED TV:
Wattage: 100 W | Hours/day: 5 | Rate: £0.29/kWh
Daily kWh = 0.1 × 5 = 0.5 kWh
Daily cost = 0.5 × £0.29 = £0.145 / day ≈ £4.35 / month
Standby power (typically 0.5–2 W) is not captured here. For precise annual cost, include standby hours separately.
Example 3 — EV home charger (7.4 kW wallbox):
Wattage: 7,400 W | Hours/day: 2 | Rate: $0.13/kWh
Daily kWh = 7.4 × 2 = 14.8 kWh
Daily cost = 14.8 × $0.13 = $1.92 / charge session ≈ $57.72 / month
Most EV owners charge every 2–3 days, not daily. Adjust hours per day accordingly.
How to Reduce Electricity Cost
If your calculated cost is higher than expected, focus on the factors that have the biggest impact:
- Reduce daily usage time — this has the strongest effect on total cost across most appliances
- Replace high-wattage devices with energy-efficient alternatives (e.g. LED bulbs, heat-pump dryers, inverter-type ACs)
- Eliminate standby power from devices left plugged in overnight or unused
- Shift usage to lower-tariff periods if your provider uses time-of-use or off-peak pricing
- Target the biggest consumers first — heaters, air conditioners, and EV chargers typically dominate household bills
Even small reductions in daily usage produce noticeable savings when compounded over a month or year.
What Affects Electricity Cost Most
Electricity cost is determined by three factors:
- Power (watts) — how much energy the device draws when running
- Usage time (hours/day) — how long it runs on an average day
- Tariff rate (per kWh) — the price your electricity provider charges per unit
Usage time and high-wattage appliances usually have the greatest impact on total cost. Tariff rate matters most when comparing providers or evaluating off-peak switching.
Common Mistakes When Estimating Electricity Cost
- Entering kW instead of W: if the label says “1.5 kW”, enter 1500. Entering 1.5 produces a result 1,000× too low.
- Ignoring standby and idle power: devices left plugged in — routers, set-top boxes, chargers — draw small but continuous loads that accumulate over a year.
- Treating cycling appliances as constant: refrigerators, ACs, and washing machines do not run continuously. Use 40–70% of rated wattage, or read the annual kWh from the energy label.
- Using a default or average tariff rate: actual rates differ by country, provider, and time-of-day plan. Always use the unit rate from your most recent bill.
To measure energy use in kWh without a cost figure — useful when comparing appliance efficiency rather than bill impact — use the Energy Consumption Calculator.
When to Use This Calculator
- Identify high-cost appliances: run each major appliance through the calculator to see which is driving your bill
- Compare old vs new appliances: enter old and new wattage side-by-side to see annual savings from an upgrade
- Budget for new equipment: estimate ongoing running cost before buying an air conditioner, heater, or EV charger
- Verify electricity bills: calculate expected usage and compare to billed kWh to spot metering anomalies
If you are working from an appliance type rather than a wattage value, the Appliance Electricity Cost Calculator auto-fills typical wattage values for common devices and completes the full kWh and cost calculation in one step.
Electricity Cost vs Circuit Power Calculations
These are two distinct types of calculation:
- This calculator: estimates the cost of running an appliance over time. Inputs are wattage, hours, and tariff rate. Output is money spent.
- Ohm's Law Calculator: solves the relationship between voltage, current, and resistance (V = I × R). Used for circuit design and component selection, not for billing estimates.
- Voltage Drop Calculator: calculates voltage lost across a cable or conductor run using AWG size, run length, and NEC compliance limits. For wiring design, not consumption.
Important Limitations
- Assumes constant, steady-state usage: the calculator uses flat wattage × hours. Cycling appliances, variable-speed motors, and devices with start-up surges consume unevenly.
- No time-of-use tariff support: if your utility uses peak/off-peak pricing, multiply peak-hour usage and off-peak usage separately using the respective rates.
- Excludes inefficiency losses: EV chargers, inverters, and transformers lose 5–15% to heat. The kWh drawn from the grid is higher than the energy delivered to the load.
- Currency conversion not included: results use the rate you enter, in the currency you enter it in. Currency conversion must be done separately.
