Installers ask for amp-hours—but energy planning starts in watt-hours. This guide shows exactly how many Ah your bank must deliver at your system voltage to cover the load for the hours you need.
Benefits
- Derives Ah from first principles: Wh = W × h, then Ah = Wh ÷ V at your nominal bus.
- Clarifies why the same load needs fewer Ah at 48 V than at 12 V for identical energy.
- Pairs with safety margins for DoD, inverter loss, and cold-weather derating before you order cells.
How it works
- Sum continuous load watts on the inverter or DC bus during the autonomy window.
- Multiply by hours of backup or overnight runtime to get required watt-hours.
- Divide by system voltage to get baseline Ah; add 20–50% margin per your DoD policy.
FAQ
How many amp-hours do I need for my battery bank?
Compute Wh = load (W) × time (h), then Ah = Wh ÷ voltage (V). A 300 W load for 10 h needs 3,000 Wh; at 12 V that is 250 Ah before margins. At 48 V the same energy is about 62.5 Ah.
Is nameplate Ah the same as usable Ah?
No. Lithium banks often allow 80–90% usable DoD; lead-acid may be 50%. If you need 250 Ah delivered, divide by your allowable DoD fraction when sizing nameplate capacity.
Do I count inverter loss in Ah?
Yes for AC loads. Either inflate watts at the battery by inverter efficiency (e.g., divide load W by 0.9) or apply a 1.1–1.2× multiplier on the final Ah figure.
Technical specifications
- Baseline: Ah_required = (load_W × hours) ÷ voltage_V.
- Usable Ah: Ah_nameplate ≈ Ah_required ÷ allowable_DoD.
- Loss margin: +10–20% for inverter conversion on AC-dominated loads.
- Validation: compare to manufacturer max continuous discharge current at planned Ah.
12 V vs. 48 V amp-hour math
Energy is conserved; voltage changes the current and Ah label on the sticker. A 48 V bank needs one-quarter the Ah of a 12 V bank for the same watt-hours—do not compare Ah across different buses without converting to Wh first.
When Ah alone is not enough
High surge loads (pumps, compressors) may require a bank that can deliver peak amps even if average Ah is sufficient. After sizing Ah for energy, check inverter and BMS current limits against startup draws.