Instrument № 10 - Power

Amp Hours
Needed

Size a battery bank from your loads and runtime - or from daily energy use and days of autonomy. With realistic efficiency and depth-of-discharge corrections built in.

See the math behind this calculator

LiFePO₄ usable
~ 80–100% DoD

Lead-acid usable
~ 50% DoD

Inverter efficiency
~ 85–95%

How to size a bank

There’s a clean formula for sizing battery storage. It just requires honesty about how much energy you actually use and how much of a battery’s nameplate capacity is real.

Add up your daily watt-hours. Multiply by days of autonomy. Divide by system voltage to get amp-hours of raw energy. Then divide again by efficiency and usable depth-of-discharge to get the nameplate capacity you actually need.

Ah_needed = total_Wh ÷ (V × efficiency × usable_DoD)

Frequently asked

Why divide by efficiency?

Because the energy that leaves your battery isn’t the energy that reaches your load. An inverter eats 5–15% as heat. DC-DC conversions, wiring losses, and BMS overhead all add up. Sizing for 100% efficiency gives a system that comes up short.

What’s a good safety margin?

Round up to a stock battery size - most pros round to the next 50 or 100 Ah. Hard-to-replace systems (boats, remote cabins) often spec 1.5–2x the calculated minimum to handle aging and unexpected loads.

What about temperature?

Capacity drops in cold weather. LiFePO₄ loses 10–15% at freezing, 30% below 0°F. Lead-acid is worse. If your system sees winter, account for it explicitly or oversize.

Show the working

This works backwards from your actual loads to the battery bank that would carry them, including the losses between the two. If any of it looks wrong, it might be - tell us what we got wrong.

Daily energy from a device list

watt-hours = watts × hours per day × quantity
total = sum of every device

Daily energy entered directly

watt-hours = kWh × 1000
total = (daily Wh − solar offset) × days of autonomy

Battery bank size amp-hours = total watt-hours ÷ (volts × efficiency × depth of discharge)

Rounded to a buyable size recommended Ah = ceiling(amp-hours ÷ 10) × 10

Constants used

1000 — watt-hours in a kilowatt-hour
Efficiency — inverter and wiring losses, default 90%
Depth of discharge — default 80%, the usual lead-acid limit
Rounded up to the nearest 10 Ah — because batteries are not sold in arbitrary sizes

Efficiency and depth of discharge both divide, so they compound. At 90% efficiency and 80% depth of discharge you need roughly 39% more nameplate capacity than your raw energy figure suggests.