What Is an Amp Hour Calculator and How Are Amp Hours Calculated?
An amp hour (Ah) is a unit of electric charge equal to the current in amps flowing for one hour — a 100Ah battery can supply 100 amps for one hour, 10 amps for ten hours, or 5 amps for twenty hours before it is empty. An amp hour calculator works backward from your electrical load to tell you how many amp hours of battery capacity you actually need.
The amp-hour sizing formula used by solar, RV, and marine installers is: Required Ah = (Load (W) × Hours per day × Days of autonomy) ÷ (System Voltage (V) × Efficiency) ÷ Depth of Discharge. Watt-hours convert directly to amp-hours by dividing by system voltage: Ah = Wh ÷ V.
A 12V, 100Ah lead-acid battery stores roughly 1,200 watt-hours (Wh) of energy, but according to the Battery Council International, a standard flooded lead-acid battery should not be discharged below 50% state of charge, so only about 600Wh is safely usable before recharging is needed.
How to Use This Amp Hour Calculator
Enter your electrical load and usage pattern, and the recommended battery bank size updates instantly. Here's what each field means:
- Load (Watts): The power draw of the device or devices you're running — check the appliance's nameplate or spec sheet, or add up multiple loads.
- Hours Used Per Day: How many hours per day the load actually runs. A fridge might run 24 hours (cycling on and off), while lights might run 4–6 hours.
- System Voltage: The nominal voltage of your battery bank — 12V is common for RVs and small setups, 24V and 48V are typical for larger solar or off-grid systems.
- Days of Autonomy: How many days you need the system to run without any recharging (e.g. cloudy days for solar).
- Depth of Discharge (DoD): The percentage of rated capacity you plan to use before recharging — lower for lead-acid, higher for lithium.
- System Efficiency: Accounts for inverter conversion losses and wiring resistance, typically 85–95%.
This calculator supports 12V, 24V, 36V, and 48V systems, covering the voltage standards used worldwide for solar off-grid cabins, RVs, campervans, marine trolling motors, and battery backup systems.
Depth of Discharge by Battery Chemistry
Lithium iron phosphate (LiFePO4) batteries can typically be discharged to 80–100% of rated capacity without meaningfully shortening their 2,000+ cycle lifespan, according to battery manufacturers such as Battle Born and Renogy — far deeper than lead-acid chemistries tolerate.
| Battery Type | Recommended DoD | Usable Capacity (100Ah battery) |
|---|---|---|
| Flooded Lead-Acid | 50% | 50 Ah |
| AGM / Sealed Lead-Acid | 50–60% | 50–60 Ah |
| Gel | 50–70% | 50–70 Ah |
| Lithium (LiFePO4) | 80–100% | 80–100 Ah |
Frequently Asked Questions
How many amp hours do I need for my RV or solar system?
It depends on your daily energy use: multiply your total load in watts by the hours you run it, divide by your system voltage, then divide by your depth of discharge percentage. A typical RV running a fridge, lights, and a fan on a 12V system with lead-acid batteries usually needs 100–200Ah of usable capacity per day of autonomy.
How accurate is this amp hour calculator?
This calculator applies the standard energy-budget formula used by solar and RV installers, giving a reliable planning estimate. It does not account for Peukert's effect (reduced usable capacity at high discharge rates on lead-acid batteries), battery age, or temperature, all of which can lower real-world capacity. Always round up to the next commercially available battery size.
What is the difference between amp hours (Ah) and watt hours (Wh)?
Amp hours measure electric charge at a specific voltage, while watt hours measure total energy regardless of voltage. They convert directly with Wh = Ah × V — a 100Ah battery at 12V stores 1,200Wh, while the same 100Ah battery at 24V stores 2,400Wh. Watt hours are the better unit for comparing battery banks of different voltages.
How do I extend my battery bank's runtime?
Reduce your load (switch to LED lighting, a more efficient fridge, or DC-direct appliances that skip inverter losses), add more battery capacity, or switch to a lithium chemistry that allows a deeper depth of discharge. Improving system efficiency by shortening cable runs and using properly sized wire also reduces losses.