Electrical Power Calculator
Power, current, voltage and resistance are linked by two formulas. Enter any two and this calculator returns the rest — useful for sizing fuses, cables and power supplies.
Formula
P = V × I
- P in watts, V in volts, I in amps
- With resistance: P = I²R = V² ÷ R
230 V drawing 13 A: 230 × 13 = 2,990 W, just under a UK 3 kW limit
The Power Formulas
Power combines with Ohm's law to give three equivalent expressions:
- P = V × I — watts from volts and amps
- P = I² × R — from current and resistance
- P = V² ÷ R — from voltage and resistance
Rearranged, I = P ÷ V is the one you need most often: a 3,000 W kettle on a 230 V supply draws 13.04 amps.
Voltage Changes Everything
The same appliance draws very different current depending on supply voltage:
- UK (230 V): a 1,200 W toaster draws 5.2 A
- US (120 V): the same toaster draws 10 A
This is why UK ring mains and US branch circuits use different cable and breaker ratings for equivalent appliances.
Checking a Circuit Load
Add the wattage of everything on a circuit and divide by the supply voltage to get total current. Compare that against the breaker or fuse rating.
A UK 13 A plug fuse tops out around 3,000 W, which is why kettles and heaters sit near the limit and why running two from one extension lead is a genuine fire risk. Leave headroom rather than working to the exact rating, and treat anything involving fixed wiring as a job for a qualified electrician.
Quick Reference
| Appliance | Power | Current | Cost per hour at 25p/kWh |
|---|---|---|---|
| LED bulb | 9 W | 0.04 A | 0.2p |
| Laptop | 60 W | 0.26 A | 1.5p |
| Fridge-freezer | 150 W | 0.65 A | 3.8p |
| Washing machine | 2,100 W | 9.1 A | 52.5p |
| Kettle | 3,000 W | 13.0 A | 75.0p |
Frequently Asked Questions
- How do I convert watts to amps?
- Divide watts by volts. A 3,000 W kettle on a 230 V supply draws about 13 amps.
- How many watts can a 13 A plug take?
- About 3,000 W at 230 V. Appliances near that figure should not share an extension lead.
- Why does the same appliance draw different current abroad?
- Current is power divided by voltage. At 120 V an appliance draws roughly twice the current it would at 230 V for the same wattage.
- What size fuse do I need?
- Above the appliance's normal current draw but below the cable's rating. UK plugs commonly use 3 A for low-power items and 13 A for heating appliances. Check the manufacturer's specification.