⚡ Watt-hour Calculator
Calculate energy consumption in watt-hours
Enter power consumption or generation in watts
How to Use This Calculator
Enter Power
Input the power consumption or generation in watts (W). This is the rate of energy use, typically found on device labels or specifications.
Enter Time
Input the duration of operation and select the time unit (hours, minutes, or seconds). The calculator automatically converts to hours for the calculation.
Calculate
Click "Calculate Energy" to get the energy consumption in watt-hours (Wh) and kilowatt-hours (kWh). Energy = Power × Time.
Formula
E = P × t
Energy from Power and Time
Where:
- E = Energy (Watt-hours, Wh)
- P = Power (Watts, W)
- t = Time (Hours, h)
Unit Conversions:
- 1 Wh = 3,600 Joules (J)
- 1 kWh = 1,000 Wh = 3,600,000 J
- 1 hour = 60 minutes = 3,600 seconds
Example Calculation 1:
For P = 100 W, t = 10 hours:
E = P × t = 100 × 10 = 1,000 Wh = 1 kWh
A 100W light bulb running for 10 hours consumes 1 kilowatt-hour of energy.
Example Calculation 2:
For P = 1,500 W, t = 2 hours:
E = P × t = 1,500 × 2 = 3,000 Wh = 3 kWh
A 1.5 kW heater operating for 2 hours consumes 3 kilowatt-hours of energy.
Example Calculation 3:
For P = 50 W, t = 30 minutes:
E = P × t = 50 × (30/60) = 50 × 0.5 = 25 Wh
A 50W device running for 30 minutes consumes 25 watt-hours of energy.
About Watt-hour Calculator
The Watt-hour Calculator determines energy consumption or generation by multiplying power (watts) by time (hours). A watt-hour (Wh) is a unit of energy equal to the energy consumed when one watt of power is used for one hour. This calculator is essential for estimating electricity costs, sizing batteries, and understanding energy usage patterns.
When to Use This Calculator
- Energy Cost Estimation: Calculate electricity costs from device power and usage time
- Battery Sizing: Determine battery capacity needed for specific loads and durations
- Solar System Design: Calculate daily energy requirements for solar installations
- Energy Audits: Estimate energy consumption of household appliances
- Educational: Understand the relationship between power and energy
Why Use Our Calculator?
- ✅ Flexible Time Units: Supports hours, minutes, and seconds
- ✅ Quick Calculation: Instantly determine energy consumption
- ✅ Multiple Units: Results in Wh and kWh
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Learn power, energy, and time relationships
Common Applications
Electricity Bill Estimation: Calculate energy consumption to estimate monthly electricity costs. For example, a 1,500W heater running 4 hours/day for 30 days consumes: E = 1,500 × (4 × 30) = 180,000 Wh = 180 kWh. At $0.12/kWh, this costs $21.60/month. This helps identify energy-intensive appliances.
Battery Capacity Planning: Size batteries for off-grid or backup power systems. If a load requires 200W for 8 hours, it needs E = 200 × 8 = 1,600 Wh = 1.6 kWh. Accounting for battery efficiency (e.g., 80%), you'd need a 2 kWh battery. This ensures adequate backup power duration.
Solar Panel Sizing: Determine solar system capacity based on daily energy needs. If daily consumption is 10 kWh and average sun hours are 5, you need at least 2 kW of solar panels (10 kWh / 5 h = 2 kW). This helps design properly sized solar installations.
Tips for Best Results
- Use average power consumption for variable loads
- For cost calculation: multiply kWh by electricity rate ($/kWh)
- Account for battery efficiency when sizing batteries (typically 80-90%)
- Consider duty cycle for intermittent devices (actual time on vs. total time)
- 1 kWh is the standard unit for electricity billing
Frequently Asked Questions
What is a watt-hour (Wh)?
A watt-hour is a unit of energy equal to one watt of power used for one hour. 1 Wh = 1 W × 1 h = 3,600 Joules. It measures total energy consumed over time, not instantaneous power. Common multiples: 1 kWh = 1,000 Wh, 1 MWh = 1,000,000 Wh.
How do I calculate electricity cost from watt-hours?
Convert watt-hours to kilowatt-hours (kWh = Wh / 1,000), then multiply by your electricity rate. For example: 2,400 Wh = 2.4 kWh. At $0.12/kWh: cost = 2.4 × 0.12 = $0.288. Electricity bills are typically in kWh.
What's the difference between watts and watt-hours?
Watts (W) measure power (rate of energy use). Watt-hours (Wh) measure energy (total consumption). Power is instantaneous; energy accumulates over time. Example: A 100W bulb uses 100W of power. If it runs 10 hours, it consumes 1,000 Wh = 1 kWh of energy.
How do I size a battery from watt-hours?
Calculate required energy (Wh), divide by battery voltage to get amp-hours (Ah = Wh / V), then account for efficiency and depth of discharge. For example: 1,200 Wh at 12V = 100 Ah. With 80% efficiency and 50% DoD, you need 100 / (0.8 × 0.5) = 250 Ah battery.
Can I use this for solar panel calculations?
Yes! Calculate daily energy needs (Wh), then divide by average sun hours to get required solar power (W). For example: 5,000 Wh/day with 5 sun hours = 1,000 W = 1 kW of solar panels. Account for system losses (typically 20-30%) when sizing.