Calculate kWh usage and running cost for any device
Energy consumption measures how much electrical energy a device uses over time, expressed in kilowatt-hours (kWh). One kWh equals 1,000 watts running for one hour — or equivalently, a 100W device running for 10 hours.
Energy (kWh) = Power (W) × Time (h) ÷ 1,000
This is the unit your electricity meter counts and your utility company charges for. Understanding kWh consumption is essential for budgeting energy costs, sizing solar or battery systems, and identifying where energy is being wasted.
| Device | Power | 1 hour use | 8 hours use |
|---|---|---|---|
| LED bulb | 10 W | 0.01 kWh | 0.08 kWh |
| Laptop | 50 W | 0.05 kWh | 0.4 kWh |
| Desktop PC | 300 W | 0.3 kWh | 2.4 kWh |
| Refrigerator | 150 W | 0.15 kWh | 1.2 kWh |
| Electric heater | 1,500 W | 1.5 kWh | 12 kWh |
| Air conditioner | 1,200 W | 1.2 kWh | 9.6 kWh |
| EV charger (L2) | 7,200 W | 7.2 kWh | 57.6 kWh |
To size a solar panel system or battery backup, you need to know your total daily energy consumption in kWh. Calculate the consumption of each major load — refrigerator, lighting, water pump, computer — and sum them up. This total daily kWh figure is your starting point for determining how many solar panels and how much battery capacity you need.
For example, a remote cabin with a 150W refrigerator running 24h (3.6 kWh), 6 × 10W LED lights running 6h (0.36 kWh), and a 50W laptop running 4h (0.2 kWh) has a daily consumption of about 4.16 kWh. At 4–5 peak sun hours per day, you would need roughly 1–1.2 kW of solar panels plus adequate battery storage.
Energy consumption (kWh) and peak load (W) are different measurements. Peak load is the maximum power draw at any instant — important for sizing inverters, fuses, and wiring. Energy consumption is the total energy used over time — important for sizing batteries, panels, and budgeting costs. A device can have a high peak load but low energy consumption if it only runs briefly, or a low wattage but high consumption if it runs continuously.
Many devices consume electricity even when turned off or in standby. A TV on standby typically draws 1–5W continuously. A gaming console in rest mode can draw 70–150W. Across a whole household, standby power (also called phantom load or vampire power) typically accounts for 5–10% of total electricity consumption — often $50–150 per year that could be eliminated by using power strips or unplugging devices not in use.
A kilowatt-hour is the amount of energy used by a 1,000-watt device running for one hour. It is the standard unit of electricity measurement used by utilities for billing. One kWh is also roughly the energy in a single AA battery multiplied by about 800.
Multiply the wattage by the hours and divide by 1,000. A 500W device running for 3 hours uses 500 × 3 ÷ 1,000 = 1.5 kWh.
An average US household uses about 30 kWh per day (roughly 900 kWh per month). European households typically use less — around 10–15 kWh per day — partly due to smaller home sizes and higher energy prices driving more efficient behaviour.
Calculate the daily kWh consumption of each device you want to power. Sum all the values to get your total daily energy requirement. This figure, divided by your location's average peak sun hours, gives the minimum solar panel capacity needed. Add 20–30% margin for losses and cloudy days.
kW (kilowatts) measures power — the rate of energy use at a specific moment, like speed. kWh (kilowatt-hours) measures energy — the total amount consumed over time, like distance. A 2kW heater running for 3 hours uses 6 kWh of energy.