Battery Capacity Calculator

Step-by-Step Guide

1. 1Enter load power: Input the total power in watts (W) that the connected equipment draws.
2. 2Enter battery voltage: Input the nominal battery voltage — commonly 12V, 24V, or 48V for backup and solar systems.
3. 3Enter desired runtime: Input how many hours you need the load to run on battery power.
4. 4Select battery type: Choose lead-acid, lithium-ion, or LiFePO4 — each has a different default efficiency assumption built in.
5. 5Read the required capacity: View both ideal capacity (Ah) and efficiency-adjusted capacity, along with total energy (Wh) and discharge current (A).
6. 6Apply a preset or export results: Use a built-in preset for common scenarios like LED backup, router backup, or solar systems, or export the full calculation as a text file.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓Ideal and efficiency-adjusted capacity (Ah)
• ✓Total energy required (Wh)
• ✓Discharge current (A) calculation
• ✓Battery type-specific default efficiency (lead-acid, Li-ion, LiFePO4)
• ✓Full step-by-step derivation
• ✓Six built-in presets (LED, solar, RV, more)
• ✓Calculation history (saved locally)
• ✓Export calculation as a text file
• ✓No signup required

Pro Tips

• 💡Add 20-30% margin beyond the calculated adjusted capacity, especially for lead-acid batteries — regularly discharging to 100% of rated capacity shortens cycle life significantly compared to keeping some reserve.
• 💡Use higher voltage battery banks (24V or 48V rather than 12V) for larger loads — this reduces both the Ah requirement and the current draw, which in turn reduces the cable gauge needed to safely handle the connection.
• 💡Choose LiFePO4 for applications with frequent deep discharge cycles despite the higher upfront cost — its longer cycle life and higher efficiency often result in a lower total cost of ownership over the system's lifetime.
• 💡Always check the discharge current against your battery's maximum continuous discharge rating, not just its Ah capacity — some batteries have capacity to spare but can't safely deliver high current for the load you're planning.
• 💡Use this calculator's ideal Wh figure to compare battery options across different voltages and chemistries on an even footing, since Wh represents total stored energy independent of voltage.

Common Mistakes to Avoid

• ✕Sizing to the ideal (unadjusted) capacity figure and ignoring efficiency losses. This under-sizes the actual battery needed, especially for lower-efficiency lead-acid systems where the gap between ideal and adjusted capacity can be 20% or more.
• ✕Using a generic 'average' power figure for loads that have variable or peak draw, like refrigerators with compressor startup surges. Size for the realistic peak or average-over-time figure, not just nameplate wattage.
• ✕Assuming all battery chemistries share the same efficiency. Lead-acid, lithium-ion, and LiFePO4 have meaningfully different round-trip efficiencies — always select the correct battery type for an accurate adjusted capacity.
• ✕Forgetting that battery capacity ratings often assume a specific discharge rate (like a 20-hour or 100-hour rate for lead-acid). Actual usable capacity can be lower at faster discharge rates than the manufacturer's rated Ah figure suggests.
• ✕Ignoring temperature effects on battery capacity. Cold temperatures significantly reduce usable capacity for most battery chemistries, particularly lead-acid — size with margin if the battery will operate in cold conditions.