Calculate watts from amp hours, voltage, and discharge time, and convert Ah to W instantly for any battery or solar setup.
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Amp Hours to Watts Formula
Amp hours measure stored charge and watts measure power, so you cannot convert one into the other without two extra values: the system voltage and the time over which the charge is delivered. The main formula is:
W = Ah * V / h
A useful intermediate step is the energy in watt hours, which only needs voltage:
Wh = Ah * V
The same relationship can be rearranged to solve for any of the four quantities:
Ah = W * h / V
V = W * h / Ah
h = Ah * V / W
- W = power in watts (W)
- Ah = battery or cell capacity in amp hours (Ah)
- V = system voltage in volts (V)
- h = discharge time in hours (h)
- Wh = energy in watt hours (Wh)
The calculator multiplies amp hours by voltage to get the total energy in watt hours, then divides by the discharge time to return the average power in watts. The solve-for selector lets you pick which value you are missing, and the inputs you do not need are hidden so you only fill in the three values the formula requires. The watt hour figure is always shown alongside the watt result so you can see both the energy and the power for your battery.
Watts From a 100 Ah Battery at Common Voltages
The table below shows how the same 100 Ah battery produces very different watt figures depending on its voltage and how quickly it is drained. Watt hours stay the same for a given voltage; watts change with discharge time.
| Voltage | Energy (Wh) | Watts over 1 h | Watts over 5 h | Watts over 10 h |
|---|---|---|---|---|
| 12 V | 1,200 | 1,200 | 240 | 120 |
| 24 V | 2,400 | 2,400 | 480 | 240 |
| 48 V | 4,800 | 4,800 | 960 | 480 |
Common System Voltages
If you are not sure what voltage to enter, these are the nominal voltages used by most batteries and packs.
| System | Nominal Voltage |
|---|---|
| Single lithium (LiFePO4) cell | 3.2 V |
| AA / AAA alkaline cell | 1.5 V |
| Car / motorcycle battery | 12 V |
| Larger solar and RV banks | 24 V or 48 V |
Example Problems
Example 1. You have a 100 Ah, 12 V battery and you fully drain it over 1 hour. The power is:
W = 100 * 12 / 1 = 1,200 watts
The same battery holds 100 * 12 = 1,200 watt hours of energy.
Example 2. You run a 50 Ah, 24 V battery steadily for 4 hours. The average power the battery can supply is:
W = 50 * 24 / 4 = 300 watts
FAQ
Can you convert amp hours straight to watts?
No. Amp hours measure charge and watts measure power, so they describe different things. You also need the voltage and the discharge time. Multiply amp hours by voltage to get energy in watt hours, then divide by the number of hours to get the average power in watts.
What is the difference between watts and watt hours?
A watt is a rate of energy use at one moment, while a watt hour is an amount of energy used over time. A 100 watt device running for 2 hours uses 200 watt hours. For a battery, watt hours come straight from amp hours times voltage, and watts depend on how fast you draw that energy out.
What voltage should I use for a battery?
Use the nominal voltage of the battery or pack, such as 12 V for most car and small solar batteries or 3.2 V for a single LiFePO4 cell. Real voltage drifts up and down during charge and discharge, but the nominal value gives a close estimate for sizing and planning.
