Watt-Hours Calculator

Understanding Watt-Hours and Energy Capacity

A watt-hour (Wh) represents the energy consumed when a device drawing 1 watt operates for 1 hour. Unlike amp-hours alone, which describe only charge capacity, watt-hours account for the electrical pressure (voltage) at which that charge flows. A 3000 mAh battery at 3.7 V delivers significantly different energy than a 3000 mAh battery at 12 V—the latter stores over three times more energy despite identical charge ratings.

Energy capacity matters because batteries with identical amp-hour specs can perform very differently. Laptop batteries typically range from 22 to 100 Wh depending on design; smartphone batteries cluster between 10 and 20 Wh. For renewable energy systems, solar panels are often sized by kilowatt-hour (kWh) output, where 1 kWh = 1000 Wh. Understanding this relationship prevents underestimating a battery's true power delivery.

Energy Calculations from Charge and Power

Watt-hours can be computed using two fundamental relationships. The first combines electric charge with voltage, while the second multiplies power by time duration.

Wh = Ah × V

Wh = W × t

Where charge is in amp-hours, divide milliamp-hours by 1000:

Wh = (mAh / 1000) × V

Wh — Energy in watt-hours
Ah — Charge capacity in amp-hours
mAh — Charge capacity in milliamp-hours
V — Nominal voltage in volts
W — Power rating in watts
t — Operating time in hours

Practical Examples: Batteries and Appliances

Battery Energy Capacity: A lithium-ion battery rated 2500 mAh at 3.7 V contains (2500 / 1000) × 3.7 = 9.25 Wh. Compare this to a 2500 mAh battery at 7.4 V (two cells in series): it holds 18.5 Wh—double the energy despite identical charge. This is why nominal voltage is critical when comparing batteries.

Appliance Consumption: A 60-watt incandescent bulb left on for 8 hours uses 480 Wh of energy. A modern 10-watt LED achieving similar brightness uses only 80 Wh over the same period. For larger loads, a 1500-watt space heater running 6 hours consumes 9 kWh (9000 Wh)—substantially more than most household devices.

When evaluating portable power stations or uninterruptible power supplies (UPS), manufacturers always list capacity in Wh or kWh because this figure directly tells you how long a given load can run before discharge.

Common Pitfalls When Working with Watt-Hours

Avoid these mistakes when calculating or comparing energy capacity.

Forgetting to account for voltage — Two batteries with identical amp-hour ratings at different voltages store completely different energy amounts. Always confirm the nominal voltage before comparing capacities. A 10 Ah battery at 48 V stores 480 Wh, while a 10 Ah battery at 12 V holds only 120 Wh.
Confusing Wh with kWh — Kilowatt-hours (kWh) are used for grid-scale energy; household meters measure consumption in kWh. Dividing watt-hours by 1000 gives kWh. A home using 30 kWh per day consumes 30,000 Wh—roughly 1250 W average power.
Ignoring discharge efficiency — Real-world batteries and power systems lose 5–20% of stored energy as heat during discharge. Inverters and power electronics add further losses. A battery rated 100 Wh may deliver only 80 Wh of usable energy to your device.
Mixing units in calculations — Ensure all units match before multiplying: convert mAh to Ah (divide by 1000), express voltage in volts, and time in hours. Sloppy unit conversion is the most common source of calculation error.

Reversing the Calculation: From Watt-Hours to Power or Charge

If you know watt-hours and need to find power or charge, rearrange the formulas:

Finding power from energy and time: Power (W) = Wh / t. A 500 Wh battery discharged over 5 hours provides an average 100 W of power.
Finding charge from energy and voltage: Ah = Wh / V. A 100 Wh battery at 50 V contains 2 Ah of charge capacity. Conversely, 500 Wh at 8 V equals 62.5 Ah.

These rearrangements are useful when you have energy storage capacity and want to estimate device runtime or compare battery pack specifications across different voltage platforms. Professional technicians routinely convert between these quantities when sizing UPS systems, solar arrays, and battery banks for off-grid applications.

Frequently Asked Questions

How do I determine watt-hours if I know the amp-hours and voltage?

Multiply amp-hours by voltage: Wh = Ah × V. For milliamp-hours, first convert to amp-hours by dividing by 1000. Example: a 2400 mAh battery at 3.7 V contains (2400 / 1000) × 3.7 = 8.88 Wh. This method works for any battery or capacitor system where voltage is stable.

What's the difference between watt-hours and kilowatt-hours?

One kilowatt-hour equals 1000 watt-hours. Utilities measure household consumption in kWh because the numbers are smaller and more convenient; a typical home uses 20–30 kWh per day. Batteries and portable devices use Wh because capacities are usually in the single-digit to few-hundred watt-hour range.

How long will a 10,000 Wh battery run a 500-watt device?

Divide energy by power: 10,000 Wh ÷ 500 W = 20 hours of runtime at constant draw. In practice, actual runtime depends on power electronics efficiency (typically 85–95%) and whether the device draws exactly 500 W continuously. Variable loads and inverter losses reduce runtime by 10–20%.

Is a 20 Ah battery at 48 V better than a 50 Ah battery at 12 V?

Compare their watt-hour ratings: 20 Ah × 48 V = 960 Wh versus 50 Ah × 12 V = 600 Wh. The 48-volt system stores 60% more energy despite having lower amp-hours. This illustrates why voltage matters as much as charge capacity when evaluating battery systems.

Why don't manufacturers just use amp-hours for batteries?

Amp-hours alone don't describe energy; you must also know voltage. A 5 Ah battery at 3.7 V stores 18.5 Wh, while a 5 Ah battery at 48 V holds 240 Wh. Watt-hours eliminate ambiguity and let customers directly estimate runtime for their devices without looking up nominal voltage separately.

Can I convert watt-hours back to watts?

Yes, but you need time duration. Watts = Wh ÷ time (in hours). A 100 Wh battery discharged over 10 hours provides an average 10 W. Over 2 hours, the same battery delivers 50 W average. Power and energy are linked through time.