Ah to Watt-Hours Calculator

This calculator converts Amp Hours (Ah) to Watt-Hours (Wh) using the battery voltage. It's helpful when you want to understand total energy capacity in terms of usable power.

What Are Watt-Hours and Why Convert from Amp-Hours?

Amp-hours (Ah) tell you how much electric charge a battery can deliver over time. However, Ah alone doesn't fully describe how much energy a battery can store or provide—because energy is also dependent on voltage. This is where Watt-hours (Wh) become important.

Watt-hours measure total energy capacity. It tells you how many watts of power a battery can deliver over a given time. By converting Amp-hours to Watt-hours, you're effectively translating stored charge into actual usable energy, making it easier to compare systems, size power setups, or plan battery runtimes across different voltages.

Basic formula:

Watt-hours (Wh) = Amp-hours (Ah) × Voltage (V)

So a 12V battery with 100Ah capacity provides:

100 Ah × 12 V = 1200 Wh

When to Use Ah to Wh Conversion?

  • Solar power systems: Estimating how many panels or batteries you need to store and use energy.
  • UPS design: Understanding how long a battery bank can support critical loads based on energy consumption.
  • Mobile and off-grid setups: Calculating how long your device or appliances can run on a battery.
  • Energy cost estimation: Converting battery capacity into actual energy units (Wh/kWh) for billing and comparison.
  • Battery comparisons: Wh is a universal unit that allows you to compare batteries of different voltages fairly.

Real-Life Examples

  • Example 1: A 12V, 200Ah battery
    Wh = 12 × 200 = 2400 Wh
    That’s 2.4 kilowatt-hours (kWh), enough to run a 100W device for 24 hours.
  • Example 2: A 24V, 100Ah battery
    Wh = 24 × 100 = 2400 Wh
    Same energy as above, but with half the current and twice the voltage.
  • Example 3: A 3.7V lithium battery rated at 2Ah
    Wh = 3.7 × 2 = 7.4 Wh
    Common in smartphones or USB battery packs.

Voltage Matters

It's important to understand that voltage has a direct effect on power. Two batteries with the same Ah rating will not provide the same energy if their voltages differ. This is why Wh is the true measure of total energy:

  • 100Ah @ 12V = 1200 Wh
  • 100Ah @ 24V = 2400 Wh
  • 100Ah @ 48V = 4800 Wh

The higher the voltage, the greater the power delivery—assuming the same Ah. Systems like solar inverters and electric vehicles often operate at higher voltages to reduce current and improve efficiency.

Accounting for Efficiency

Real-world power systems aren’t perfectly efficient. You may lose energy through:

  • Inverter conversion (DC to AC)
  • Cable resistance
  • Battery discharge inefficiency

Typical inverter efficiency ranges from 85–95%. So if your battery has 1000 Wh, you may only get 850–950 Wh of usable energy. Always apply a safety factor when sizing your system:

Usable Wh = Total Wh × Efficiency (%)

Battery Chemistry and Wh Ratings

Different types of batteries may behave differently even at the same Ah rating. For example:

  • Lead-acid batteries have a lower usable capacity (only 50–60% of rated Ah) to avoid damage.
  • Lithium-ion batteries typically allow 80–100% usage and maintain higher voltage throughout discharge.

Always check the manufacturer's datasheet to know the real usable energy in Wh or kWh.

Frequently Asked Questions (FAQ)

What’s the difference between Ah and Wh?

Ah measures capacity (electric charge), while Wh measures energy. Think of Ah as the size of a fuel tank, and Wh as how far you can go with that fuel. The higher the voltage, the more energy (Wh) you get from the same number of Ah.

Can I compare two batteries with different voltages by just looking at Ah?

No. You should always convert to Watt-hours first for a fair comparison. A 100Ah 12V battery (1200 Wh) has half the energy of a 100Ah 24V battery (2400 Wh).

How do I calculate runtime from Wh?

If you know your load's power consumption in Watts, you can estimate runtime as:
Runtime (hours) = Battery Wh / Load (W)
For example: 1200Wh battery running a 100W device = 1200 / 100 = 12 hours (before efficiency losses).

Can I convert Wh back to Ah?

Yes. Use the formula:
Ah = Wh / V
Just divide the Watt-hours by the voltage to find the amp-hour value.

Is this conversion safe for all battery types?

Yes, the math applies universally. But real-world discharge behavior, safety limits, and usable energy vary by chemistry. Lithium batteries are generally more efficient and tolerate deeper discharge than lead-acid batteries.

Tips for Accurate Energy Planning

  • Measure actual voltage: Use nominal voltage (e.g. 12V, 24V), but real voltage may vary during charge/discharge.
  • Apply efficiency correction: Multiply Wh by 0.85 or 0.9 to estimate usable energy.
  • Use proper units: 1000 Wh = 1 kWh, which is the billing unit used by energy companies.
  • Watch for battery degradation: Aging batteries may deliver less than their rated Wh over time.

Conclusion

Converting Ah to Watt-hours helps translate battery capacity into a more universal, energy-based unit. Whether you’re planning a UPS backup, sizing a solar array, or comparing different batteries, understanding Wh allows you to evaluate and predict real performance more accurately. Remember that actual usage depends on efficiency, discharge rates, and voltage stability, so use this calculator as a guide—but test in your own system for best results.