# Watt Hours Calculator

Calculate energy in watt-hours and kilowatt-hours from watts and time. Estimate electricity cost for any appliance or device.

## What this calculates

Watt-hours measure total energy consumed over time. A 100-watt light bulb running for 8 hours uses 800 watt-hours (0.8 kWh). At $0.12 per kWh, that costs about 10 cents. This calculator converts watts and time into watt-hours, kilowatt-hours, and gives you the cost at your electricity rate.

## Inputs

- **Power** (W) — min 0 — Power consumption in watts.
- **Time** (hr) — min 0 — Duration of use in hours.
- **Electricity Rate** ($/kWh) — min 0 — Cost per kilowatt-hour (US average is about $0.12).

## Outputs

- **Energy** (Wh) — Energy consumed in watt-hours.
- **Energy** (kWh) — Energy consumed in kilowatt-hours.
- **Cost for This Usage** — formatted as currency — Cost of this energy at your electricity rate.
- **Monthly Cost (30 days)** — formatted as currency — Projected cost if repeated every day for a month.
- **Energy** (J) — Energy in Joules (1 Wh = 3,600 J).

## Details

**The formula:**

Energy (Wh) = Power (W) × Time (hours)

Divide by 1,000 to get kilowatt-hours (kWh), which is the unit on your electric bill.

**Common appliance costs (at $0.12/kWh):**
- **LED bulb (10W)** for 8 hours: 0.08 kWh = $0.01/day
- **Laptop (50W)** for 8 hours: 0.4 kWh = $0.05/day
- **TV (100W)** for 5 hours: 0.5 kWh = $0.06/day
- **Space heater (1,500W)** for 8 hours: 12 kWh = $1.44/day ($43/month)
- **Window AC (1,200W)** for 10 hours: 12 kWh = $1.44/day
- **Electric oven (2,500W)** for 1 hour: 2.5 kWh = $0.30

**Battery capacity:**

Watt-hours also measure battery storage. A laptop battery rated at 60 Wh can power a 15W load for 4 hours. Portable power stations are rated in Wh (300Wh, 500Wh, 1000Wh). An electric car battery is measured in kWh, typically 40-100 kWh.

**Converting units:**
- 1 kWh = 1,000 Wh
- 1 Wh = 3,600 Joules
- 1 kWh = 3,412 BTU

## Frequently Asked Questions

**Q: What is the difference between watts and watt-hours?**

A: Watts measure the rate of energy use (power), while watt-hours measure the total amount of energy used over time. A 100-watt bulb uses 100 watts of power at any instant, but over 10 hours it consumes 1,000 watt-hours (1 kWh) of energy.

**Q: How do I find my electricity rate?**

A: Check your electric bill for the rate per kWh. In the US, the national average is about $0.12/kWh, but it ranges from $0.08 in some states to over $0.30 in Hawaii and parts of New England. Your bill may also include tiered rates where the price increases with usage.

**Q: How many watt-hours does the average home use per day?**

A: The average US household uses about 30 kWh (30,000 Wh) per day, or roughly 900 kWh per month. This varies widely by climate, home size, and appliance efficiency. Heating and cooling are usually the biggest contributors.

**Q: How do I convert watt-hours to amp-hours?**

A: Divide watt-hours by the voltage. A 100 Wh battery at 12V has a capacity of 8.33 Ah (100 / 12). This is useful for sizing batteries in solar, RV, and marine electrical systems.

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Source: https://vastcalc.com/calculators/physics/watt-hours
Category: Physics
Last updated: 2026-04-08