Electricity use is straightforward to estimate when you know an appliance's power, operating time, and the price you pay per kWh. This guide explains watts, kilowatts, kilowatt-hours, the core formulas, worked appliance examples, and direct cost calculations when your kWh usage is already known.
Power and energy use are different quantities
Watts (W) and kilowatts (kW) describe power: the rate at which a device uses energy at a given moment. Kilowatt-hours (kWh) describe energy used over time.
Energy, kWh = Power, kW × Time, hours
A 1500 W appliance has a power rating of 1.5 kW. If it operates for two hours, the corresponding energy use is 3 kWh. kWh is the useful quantity for estimating an electricity charge.
The basic electricity cost formula
Once energy use is known in kWh, cost is calculated by multiplying it by the electricity rate.
Use the rate that actually applies to your plan. LUKIK does not insert automatic country or supplier tariffs, so the calculation does not depend on a built-in price table that may become outdated.
Worked example: a 1500 W appliance
Suppose a device is rated at 1500 W, runs for 2 hours per day, is used 30 days per month, and the entered electricity rate is 4.32 UAH per kWh.
| Step | Calculation | Result |
|---|---|---|
| Power in kW | 1500 ÷ 1000 | 1.5 kW |
| Daily energy | 1.5 × 2 | 3 kWh |
| Monthly energy | 3 × 30 | 90 kWh |
| Monthly cost | 90 × 4.32 | 388.80 UAH |
With the same usage pattern, the annual estimate is 1080 kWh and 4665.60 UAH.
Want to test your own numbers? Open the LUKIK Electricity Cost Calculator and enter power, runtime, and your own electricity rate.
How to include multiple identical devices
If several identical devices operate at the same time, multiply the power of one device by the quantity. Four 100 W lamps, for example, have a combined power of 400 W, or 0.4 kW.
If those four devices operate for 5 hours per day, daily energy use is 0.4 × 5 = 2 kWh. Over 30 days, that becomes 60 kWh.
Average load and duty cycle
Nameplate power does not always mean a device continuously draws that amount. Thermostatically controlled heaters, refrigerators, water heaters, air conditioners, and inverter-driven equipment may cycle or vary their power.
The calculator therefore includes an optional average-load setting. A value of 100% means continuous operation at the entered power. A value of 50% means the average effective power over the selected period is approximately half the entered power.
This is a mathematical estimate, not an appliance preset. For a better real-world estimate, use measured consumption, manufacturer data, or an energy monitor rather than guessing a duty-cycle percentage.
Daily, monthly, and annual estimates
When hours per day and days per month are known, the same physical formula can be extended across useful time periods.
- Daily: power × hours per day.
- Monthly: daily energy × days used.
- Annual: monthly estimate × 12.
- Cost: each kWh result × the entered rate.
An annual number should be treated as an estimate when usage is seasonal. Heating and cooling equipment, for example, often does not follow the same monthly pattern throughout the year.
When you already know the kWh
Sometimes there is no need to work from watts and runtime. If consumption is already known from a bill, meter, smart plug, or another calculation, simply multiply the known kWh by the rate.
This is the purpose of the calculator's “Known kWh / Bill” mode.
Flat rate and two-rate pricing
With a flat rate, all energy use is multiplied by one price per kWh. With two-rate pricing, two portions of consumption are priced separately and then added together.
The calculator does not assume that rate 1 must be a daytime rate or rate 2 must be a night rate. They are two generic user-entered rate components, so you can enter the actual values from your own plan.
For a two-rate calculation, the blended energy rate is also useful: total energy cost divided by total kWh. It gives a simple effective price for the combined consumption.
Optional fixed daily charge
Some electricity plans may include a separate fixed daily amount. If that applies to your bill, it can be added in Known kWh mode.
If your plan has no such charge, leave the field blank. The calculator never adds a fixed charge automatically.
Common calculation mistakes
- Multiplying watts without first converting them to kilowatts.
- Confusing kW, which is power, with kWh, which is energy.
- Assuming rated power is always the same as average real consumption.
- Using someone else's or an outdated electricity rate.
- Counting hours or days twice when moving between periods.
- Treating an annual estimate as exact for seasonal equipment.
What this calculator does not model
The tool is intended for household and general electricity-use estimates. It does not calculate reactive power, power factor, industrial three-phase load design, taxes, complex demand charges, solar export, net metering, EV-specific charging logic, or automatically maintained supplier tariffs.
Those are separate engineering or tariff problems. Keeping them outside this calculator makes the basic power × time × price workflow easier to understand and harder to misuse.
A quick calculation checklist
- Find the appliance power in W or kW.
- If necessary, convert W to kW by dividing by 1000.
- Multiply kW by operating hours.
- Include the quantity of identical devices and average load if needed.
- Extend the energy result across the days used.
- Multiply kWh by your electricity rate to estimate cost.
If kWh is already known, start directly with the rate calculation; there is no need to reconstruct watts and runtime.
Frequently asked questions
How much energy does a 1 kW device use in one hour?
If it runs continuously at 1 kW for one hour, it uses 1 kWh.
Can the electricity rate be zero?
Yes. A zero rate is mathematically valid and produces zero monetary cost while the energy-use result remains valid.
Why can the real bill differ from the estimate?
Actual duty cycle, seasonal behavior, additional tariff components, and differences between estimated and measured operating time can all change the final bill.
Can I calculate two electricity rates?
Yes. Enter the kWh and price for each of the two rate portions. The calculator adds both cost components and shows a blended rate.
Conclusion
For a basic electricity estimate you need only power, time, and price. Convert power to kW, multiply by hours to obtain kWh, then multiply kWh by your own electricity rate. If your energy use is already known, use the direct flat-rate or two-rate calculation instead.