How Many Amp Hours Do I Need?
Size a battery bank for your project. Enter your loads and runtime (or your daily energy and days of autonomy), pick system voltage, and set Efficiency and Usable % (depth of discharge). The calculator returns recommended amp-hours.
Common Settings
Loads & Runtime
| Device (optional) | Watts | Hours | Qty | Wh |
|---|
Each row adds **Watts × Hours × Qty** to total energy.
Daily Usage
Total required energy = (Daily energy − Offset) × Days. Offset is optional recharge you expect each day.
Results
Total energy to supply
Required battery capacity
Rounded recommendation
Inputs summary
Formula: Ah needed ≈ Total Wh ÷ (Volts × Efficiency × Usable). Example: 600 Wh, 12 V, 90% eff, 80% usable → 600 ÷ (12 × 0.9 × 0.8) ≈ 69.4 Ah.
Frequently Asked Questions
Should I size for peak load or average load?
Size your battery for the energy you’ll use (average over time), but make sure your inverter/DC system can handle your peak power. In practice, total your daily watt-hours (or device × hours) to get amp-hours, then check the biggest “all-on” moment to ensure your inverter and wiring can supply that surge without excessive voltage drop.
How do I include duty cycle (on/off cycling) in the calculation?
Multiply a device’s watts by the hours it’s actually on, not the full day. If a fridge averages 45 W but only runs 30% of the time, estimate its daily use as 45 W × 24 h × 0.30 ≈ 324 Wh. Add that to your other loads and convert to Ah.
What Efficiency and Usable % should I pick if I’m unsure?
For mixed AC/DC systems, a safe default is 90% efficiency. For LiFePO₄, start with 80–90% usable; for AGM/lead-acid, ~50% usable to protect cycle life. If you notice frequent deep discharges, either raise battery size or reduce usable %.
Do higher system voltages really reduce the amp-hours I need?
They reduce Ah, but not Wh. Energy is what your loads consume (Wh), and that doesn’t change with system voltage. At higher voltage, the current is lower for the same power, which helps with cable sizing and voltage drop; the calculator will show fewer Ah simply because Ah = Wh ÷ V.
How much safety margin should I add to the result?
A typical buffer is 15–30% above the calculated Ah to cover cold weather, inverter idle draw, aging, and usage spikes. Seasonal or mission-critical setups (medical devices, essential refrigeration) may justify 40–50% reserve.
How do inverter idle draw and standby loads change my sizing?
Inverters and devices often sip power even “off.” Add a line item for inverter idle watts × hours/day and any parasitic draws (trackers, routers, monitors). Those extra Wh can easily add 5–15% to your daily energy and push your Ah requirement up.