About This Tool
The Battery Capacity Calculator estimates how long a battery will run a device or how much energy it stores in Watt-hours (Wh). Whether you're sizing a backup power system for your home, choosing a portable power station for camping, or figuring out if your phone battery will last through a workday, this tool gives you the numbers you need. It works with common battery ratings like Amp-hours (Ah) and voltage to calculate total capacity in Wh, then estimates runtime based on the load (watts). It's essential for DIY solar projects, RV owners, and anyone using rechargeable batteries. Knowing your battery's true capacity helps avoid over-discharging and extends battery life.
How It Works
The calculator uses the formula: Energy (Wh) = Voltage (V) × Amp-hours (Ah). Runtime (hours) = Energy (Wh) / Load (W). For example, a 12 V battery rated at 100 Ah stores 12 × 100 = 1,200 Wh. If powering a 50 W device, runtime = 1,200 / 50 = 24 hours. For lead-acid batteries, you should only use 50% of capacity to avoid damage, so usable Wh = 600 Wh.
Examples
- Example 1: A 48 V e-bike battery rated at 14 Ah stores 48 × 14 = 672 Wh. Running a 250 W motor gives a runtime of 672 / 250 = 2.69 hours (about 2 hours 41 minutes).
- Example 2: A 12 V deep-cycle battery rated at 200 Ah (usable 100 Ah for lead-acid) stores 12 × 100 = 1,200 Wh usable. Powering a 100 W fridge runs for 1,200 / 100 = 12 hours.
Pro Tips
- For lead-acid batteries, use only 50% of rated capacity to prevent damage—calculate with usable Ah (half the rating).
- For lithium-ion batteries, you can safely use 80-90% of rated capacity, so adjust accordingly.
- Always factor in inverter losses (about 10-15%) if using AC devices from a DC battery—add 15% to the load wattage for accurate runtime.