Electric Power Calculator
Solve electric power with P = V × I (or I²R, V²/R), then convert that power to energy over time and estimate what it costs to run.
Reviewed by the ToolNestr Editorial Team — July 2026
Power, visualized
Low power vs. high power bulb
A dim 40 W bulb (left) next to a bright 100 W bulb (right) — more watts means more light energy radiated per second.
Typical appliance power draw
Bar heights compare a few common devices — heat-producing appliances like kettles draw far more power than electronics.
Power graphs
How it works
The core idea in one line: power is how fast energy moves (P = VI, in watts); energy is power accumulated over time (kWh) — and energy, not power, is what you pay for.
P = V × I
power from voltage & current
P = I²R = V²/R
equivalent forms via Ohm’s law
E (kWh) = P (kW) × t (h)
energy over time
Cost = E × price/kWh
what the utility actually bills
Once you know power in watts, convert to kilowatts (÷1000) and multiply by hours run to get energy: E = P × t. Multiply that energy by your utility's price per kWh to get the cost — the same two-step chain behind every electricity bill line item.
Worked example 1 — electric kettle
Given: A 1500 W kettle runs for 15 minutes (0.25 h). Electricity costs $0.15 per kWh. Find the energy used and the cost.
Worked example 2 — LED bulb, a full day
Given: A 100 W bulb runs 5 hours per day. Electricity costs $0.12 per kWh. Find the daily energy and cost.
Over a 30-day month that is 15 kWh and about $1.80 — a useful way to compare running costs across appliances.
Typical appliance power draw
Approximate, real devices vary by model — use these as reasonable reference points.
| Appliance | Typical power | Energy / hour |
|---|---|---|
| LED bulb | 10 W | 0.010 kWh |
| Laptop | 65 W | 0.065 kWh |
| Refrigerator | 150 W | 0.150 kWh |
| Microwave | 1000 W | 1.000 kWh |
| Electric kettle | 1500 W | 1.500 kWh |
| Space heater | 1500 W | 1.500 kWh |
Multiply energy/hour by your utility’s price per kWh to estimate running cost.
Where electric power & energy cost actually matter
💰 Household energy bill estimation
Multiplying each appliance’s power by its daily hours of use and summing the kWh gives a realistic estimate of a monthly electricity bill — before the bill arrives.
🛒 Appliance efficiency shopping
Comparing wattage and expected annual kWh between two similar appliances (e.g. two refrigerator models) shows which one actually costs less to run over its lifetime, not just to buy.
☀️ Solar panel sizing
Sizing a solar array starts from total daily energy demand in kWh, built up from the power and run-time of every device the system needs to support.
🔌 EV charging cost
An EV charger’s power rating (kW) times charging hours gives the kWh added to the battery, which multiplied by the electricity price gives the cost of a full charge.
Common misconceptions
“Watts and watt-hours are the same unit.”
They measure different things. Watts (W) measure instantaneous power — the rate of energy use right now. Watt-hours (or kWh) measure energy — power accumulated over time. Your utility bills you for kWh, not W.
“A higher-voltage appliance always uses more power.”
Power depends on both voltage and current: P = V × I. A high-voltage device drawing little current can use less power than a low-voltage device drawing a lot of current. Voltage alone does not determine power.
“Leaving a device plugged in but off costs nothing.”
Many electronics draw small standby power even when “off.” It is usually just a few watts, but over months that adds measurable kWh — which is why it is called “phantom load.”
“Doubling the voltage doubles the power.”
Only true if current stays fixed. For a fixed resistance, P = V²/R — doubling voltage actually quadruples power, because current rises too (Ohm’s law).
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, University Physics Volume 2 — electric power in circuits (free, peer-reviewed). openstax.org
- • Halliday, Resnick & Walker, Fundamentals of Physics — chapter on current, resistance, and power in circuits.
- • U.S. Energy Information Administration — electricity use and kWh billing basics.
Power = voltage × current (P = VI); energy = power × time; cost = energy × price per kWh. Results are rounded for display.
How to use this calculator
Pick the mode
“Power” solves P = V×I for any missing value; “Energy cost” converts watts and run-time into kWh and dollars.
Enter your values
Fill in the known fields — the calculator solves the rest live as you type.
Compare visually
The 3D diagrams show a dim vs. bright bulb and a bar comparison of typical appliance power draw.
Related tools
Frequently asked questions
What is electric power?
Electric power (P) is the rate at which electrical energy is transferred or converted, measured in watts (W). One watt is one joule per second. It tells you how fast a device consumes energy at any instant — not how much energy it uses in total.
How does P = VI relate to the other Ohm’s-law power forms?
P = VI is the base definition. Substituting Ohm’s law V = IR gives P = I²R (power from current and resistance), and substituting I = V/R gives P = V²/R (power from voltage and resistance). All three are algebraically identical and give the same answer for a resistive load.
How do I convert watts to kilowatt-hours?
Watts measure instantaneous power; kilowatt-hours (kWh) measure energy used over time. Divide power in watts by 1000 to get kilowatts, then multiply by the running time in hours: E (kWh) = P (kW) × t (h). A 1000 W device run for 1 hour uses exactly 1 kWh.
How is electricity actually billed?
Utilities bill for energy (kWh), not power (W). Your bill is Energy (kWh) × price per kWh. A high-power device used briefly can cost less than a low-power device left running for days, because cost tracks total energy, not peak wattage.
Why do some appliances draw far more power than others?
Devices that generate heat directly — kettles, heaters, toasters — convert electrical energy to heat with resistive elements and typically draw 1000–2000 W. Devices that just run electronics or motors (LED bulbs, laptops, refrigerators) draw far less, often under 100 W, because they are not fighting resistance to produce heat.