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Amp Hour Calculator
Work out exactly how many amp-hours (Ah) of battery capacity your load needs — for solar setups, RVs, backup power, or any 12V/24V/48V system. Accounts for battery chemistry and depth of discharge, not just a raw watts-to-amps conversion.
Your load
Battery capacity needed
How this calculator works
A common mistake is buying a battery whose rated capacity exactly matches the raw Ah calculation — that only works if you plan to fully drain the battery every cycle, which drastically shortens its life (especially for lead-acid and AGM). This calculator divides by your chemistry's safe depth of discharge so the number you get is what to actually buy, not just what you'll consume.
Why depth of discharge (DoD) matters
| Battery type | Safe DoD | Typical cycle life |
|---|---|---|
| Flooded lead-acid | ~50% | 300–500 cycles at 50% DoD |
| AGM | ~50% | 400–600 cycles at 50% DoD |
| Lithium (LiFePO4) | ~80% (some rated to 100%) | 2,000–5,000+ cycles |
Discharging lead-acid or AGM batteries deeper than 50% regularly cuts their usable lifespan sharply — it's the single biggest reason backup batteries "die early" on job sites. Lithium (LiFePO4) tolerates much deeper discharge without the same penalty, which is why it needs a smaller rated capacity for the same job — worth the higher upfront cost on systems that cycle daily (solar, daily-use equipment).
Don't forget system losses
If you're running AC loads through an inverter, expect 10–15% loss from inverter inefficiency alone. Add wiring resistance over longer cable runs and connector losses, and a 15–20% overall loss factor is a realistic default for most real-world installs — pure DC loads wired directly (like a 12V camera or alarm panel) can use a lower figure, closer to 5–10%.
Related tools
Sizing backup power specifically for an alarm panel, NVR, or PoE switch with a standby + alarm current spec? Use the Battery Backup Calculator instead — it's built around the standby-hours-plus-alarm-minutes method used in fire and security code compliance.
Already own a battery and want to know how long it'll run your load instead of what size to buy? Use the Battery Amp Hour Calculator — it works in the opposite direction from this tool.
Not sure whether to wire batteries in series or parallel once you've got your Ah figure, or which chemistry to pick? See our guide: How to Choose the Right Battery Size for a Security Panel.
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