Watts Calculator
Calculate watts from volts and amps (P = V × I), or from current and resistance (P = I²R). Includes appliance wattage table and kW/MW scale guide.
Watts measure electrical power — the rate at which a circuit converts electrical energy into heat, light, motion, or other forms of energy. There are three ways to calculate watts depending on which values you already know.
How to Calculate Watts: Three Formulas
P = V × I (from Voltage and Current)
P = I² × R (from Current and Resistance)
P = V² ÷ R (from Voltage and Resistance)
Where:
- P = Power in watts (W)
- V = Voltage in volts (V)
- I = Current in amperes (A)
- R = Resistance in ohms (Ω)
Quick examples:
- 240V supply at 10A: P = 240 × 10 = 2,400W
- 5A through a 100Ω resistor: P = 5² × 100 = 2,500W
- 12V across a 6Ω load: P = 12² ÷ 6 = 24W
Common Appliance Wattages Reference Table
| Appliance | Watts | Circuit needed |
|---|---|---|
| LED bulb | 6–12W | Standard |
| Phone charger | 5–65W | Standard |
| Laptop charger | 45–140W | Standard |
| Desktop PC (mid-range) | 200–400W | Standard |
| LED TV (55") | 70–120W | Standard |
| Fridge-freezer | 100–200W | Standard |
| Washing machine | 1,000–2,000W | Standard |
| Dishwasher | 1,200–2,400W | Standard |
| Microwave | 700–1,500W | Standard |
| Kettle | 2,200–3,000W | Standard (13A UK) |
| Tumble dryer | 2,000–5,000W | Standard/dedicated |
| Electric shower (UK) | 7,500–10,800W | Dedicated 40–45A circuit |
| Air conditioner | 1,000–4,000W | Dedicated |
| EV home wallbox (7kW) | 7,000W | Dedicated 32A circuit |
| EV rapid charger (50kW) | 50,000W | Three-phase commercial |
Watts, Kilowatts, and Megawatts — Scale Guide
| Unit | Value | Typical context |
|---|---|---|
| 1W | 1 watt | LED indicator light, USB power |
| 100W | 100 watts | Old incandescent bulb, laptop |
| 1kW | 1,000 watts | Small electric heater, microwave |
| 10kW | 10,000 watts | Household solar array, EV charger cluster |
| 1MW | 1,000,000 watts | Small wind turbine, large data centre block |
| 1GW | 1,000,000,000 watts | Large nuclear power station |
Your electricity bill charges you for kilowatt-hours (kWh) — the energy used by a 1kW device running for 1 hour.
Generator Sizing Example
"My generator produces 30A at 240V — how many watts is that?"
P = V × I = 240 × 30 = 7,200W = 7.2kW
At the 80% continuous load rule: 7,200 × 0.8 = 5,760W maximum safe continuous output.
That generator can run:
- A 2,500W tumble dryer (continuously) + a 2,200W oven (continuously) + 1,060W of other loads
- Or a 5kW air conditioning unit with 760W headroom
- But NOT a 7kW EV charger (that would exceed rated output)
AC vs DC Power Calculations
For DC circuits, P = V × I is exact.
For AC circuits, apparent power (VA) is V × I, but real power (watts) depends on the power factor:
Real power (W) = Apparent power (VA) × Power factor
- Resistive loads (heaters, kettles): power factor = 1.0, so W = VA
- Motors and compressors: power factor = 0.7–0.9
- Electronics with switching supplies: power factor = 0.6–0.99 (varies widely)
For home appliance calculations using P = V × I, you'll get the apparent power. For billing purposes, your meter measures real power (kWh).
Frequently Asked Questions
What is a watt, exactly?
One watt is one joule of energy per second. It's the SI unit of power, named after James Watt (the Scottish engineer who improved the steam engine). When a 100W light bulb runs for one hour, it uses 100 Wh = 0.1 kWh of energy.
How do I convert watts to amps?
Rearrange P = V × I to get I = P ÷ V. A 2,400W appliance on a 240V UK circuit draws 2,400 ÷ 240 = 10 amps. The same appliance on a 120V US circuit would draw 20 amps — double the current for the same power.
What's the difference between watts and volt-amps (VA)?
Watts (W) measure real power — energy that actually does work. Volt-amps (VA) measure apparent power — the product of voltage and current. They're equal for resistive loads. For inductive loads (motors, transformers), VA > W because some current flows without doing useful work. UPS systems are rated in VA, not W, for this reason.
How many watts does a house use?
The average UK home uses around 3,100 kWh per year — about 354 watts average power. The average US home uses about 10,500 kWh per year — about 1,200 watts average. Peak demand during evening hours (5–9pm) can be 2–5× higher.
Why do some appliances show watts and others show amps?
In the US, appliances often show amperage because 120V is standard and amps determine wire/outlet sizing. In the UK and Europe, watts are more common because 240V is standard and power consumption is more immediately useful. Either way, P = V × I converts between them.
Related Calculators
- Ohm's Law Calculator — solve for voltage, current, or resistance
- Amps to Watts Calculator — convert current and voltage to power
- Electricity Cost Calculator — calculate appliance running costs
- Battery Life Calculator — estimate how long a battery will power a device