How do you calculate watts from volts and amps?

Watts = Volts × Amps (P = V × I). For example, 120V × 10A = 1,200W. This is Ohm's Law for power. You can rearrange it to find any missing value: Volts = Watts / Amps, Amps = Watts / Volts.

How many watts is 10 amps at 120V?

10 amps at 120V = 10 × 120 = 1,200 watts (1.2 kW). Running for 1 hour consumes 1.2 kWh of electricity.

How do you convert watts to kWh?

kWh = Watts × Hours / 1,000. A 500W appliance running for 4 hours uses 500 × 4 / 1,000 = 2 kWh. Multiply by 30 for monthly consumption: 60 kWh/month.

Watt Calculator

Calculate watts, volts, or amps using P = V × I. Enter any two values — get the third plus monthly energy usage.

Solve for

Ohm's law — P = V × I

Watts (W)?

Volts (V)?

Amps (A)?

Hours of use per day

Power

1,200.00 W

Daily energy use4.800 kWh

Monthly energy use144.00 kWh

Annual energy use1,752.0 kWh

Monthly cost (~$0.15/kWh)~$21.60

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How to Use This Calculator

Choose what to solve for

Click Watts, Volts, or Amps at the top to set which value you want to calculate. The selected value's input is automatically filled with the calculated result — the other two fields are inputs you enter.

Use the voltage presets

Click any voltage preset button (12V, 24V, 48V, 120V, 240V) to quickly set common system voltages. This is especially useful when working through a series of calculations for different circuits in the same system.

Set hours per day for energy consumption

The hours per day selector combines with the wattage to calculate daily kWh, monthly kWh, and annual kWh. This is useful for estimating how much energy an appliance consumes and how many solar panels you need to power it.

The Formula

Ohm's Law triangle: Watts = Volts × Amps (P = V × I) Volts = Watts ÷ Amps (V = P / I) Amps = Watts ÷ Volts (I = P / V) Energy consumption: Daily kWh = Watts × Hours per day ÷ 1,000 Monthly kWh = Daily kWh × 30 Annual kWh = Daily kWh × 365 Monthly cost = Monthly kWh × electricity rate

The relationship P = V × I is the foundation of all electrical calculations. Watts measure power (instantaneous energy flow). Kilowatt-hours (kWh) measure energy (power accumulated over time). Your electricity bill is in kWh — your appliance label is in watts. The conversion is straightforward: a 100W bulb running for 10 hours uses 1 kWh of energy.

Example

Sizing a solar panel for a specific appliance

You want to run a 150W refrigerator (24 hrs/day) and a 60W laptop (6 hrs/day) from solar. How much daily kWh do you need to generate?

Refrigerator150W × 24h / 1000 = 3.6 kWh/day

Laptop60W × 6h / 1000 = 0.36 kWh/day

Total daily need3.96 kWh/day

Converting to amps at 48V DC

Refrigerator at 48V150W / 48V = 3.125A

Laptop at 48V (via inverter)60W / 48V = 1.25A

Total DC amps needed4.375A continuous

This continuous DC draw (plus peaks during compressor startup) determines your wire gauge, charge controller size, and battery capacity. The kWh figure guides your solar panel sizing — at 5 peak sun hours, you need about 3.96 / 5 / 0.86 = 0.92 kW of solar panels (approximately 2-3 × 400W panels).

FAQ

What's the difference between watts and kilowatt-hours? ▼

Watts (W) measure power — the rate of energy flow at a specific instant. Kilowatt-hours (kWh) measure energy — the total amount of electrical energy consumed over a period of time. Analogy: watts are like speed (km/h), while kWh are like distance traveled (km). A 1,000W appliance running for 1 hour uses 1 kWh. The same appliance running for 30 minutes uses 0.5 kWh. Your electricity meter and bill are in kWh — your appliance labels are in watts. Multiply watts × hours to get watt-hours, then divide by 1,000 to get kilowatt-hours.

How do I find the watts for an appliance if the label only shows amps? ▼

Use this calculator: set "Solve for Watts," enter the amps from the label, and enter the voltage (120V for US appliances, 230V for European). The calculator gives you watts. For example, a US appliance labeled "2.5A" at 120V draws 2.5 × 120 = 300W. Note: some appliances show a VA (volt-amp) rating rather than watts — VA is the apparent power and is always equal to or higher than actual watts (true power). For resistive loads (heaters, lights), VA ≈ watts. For motors and electronics, VA can be 10–30% higher than watts.

How do I calculate the amps drawn by a 1,000W inverter from a 12V battery? ▼

Set "Solve for Amps," enter 1000W, and 12V. Result: 1000/12 = 83.3A. But add inverter efficiency — most inverters are 85–92% efficient, so the actual battery draw is 1000 / 0.88 / 12 = 94.7A at 88% efficiency. This is why short, thick battery cables are so critical for high-wattage inverters on 12V systems — 83–95A through undersized wire causes dangerous voltage drop and heat. Use our DC Wire Size Calculator to find the right wire gauge for this run.