Why should lead-acid banks be oversized?

Because lead-acid should only be discharged to about 50% for a reasonable life — so the bank must be roughly double the calculated need. Lithium (LFP) allows 80–90%, so its margin is smaller.

Lead-Acid Knowledge · Practical · Last reviewed 2026-09-16

How Much Battery Capacity Do I Need?

Q: How do I calculate what size battery I need?

Energy needed = load in watts × hours. Capacity = energy ÷ voltage, then divide by the usable depth of discharge. A 100 W load for 5 hours needs 500 Wh; on 12 V that is about 42 Ah, or roughly 85 Ah at lead-acid’s 50% usable window.

Sizing a battery bank is one multiplication, one division, and one honest margin — and the margin is the part people skip, which is why so many banks come up short on day three.

Direct answer: Energy needed = load (W) × hours. Then capacity = energy ÷ voltage, divided by the usable depth of discharge. A 100 W load for 5 hours needs 500 Wh; on 12 V that is about 42 Ah — roughly 85 Ah for lead-acid at its 50% usable window, or ~55 Ah for LFP at 80%.

The formula and worked examples

Scenario	Load	Hours	Energy	12V bank (50% DoD)
Fridge + lights (boat)	100 W	5 h	500 Wh	~85 Ah
CPAP overnight	50 W	8 h	400 Wh	~70 Ah
Small off-grid cabin	300 W	6 h	1,800 Wh	~300 Ah

The depth-of-discharge margin

Lead-acid survives longest at 50% depth of discharge, so the bank is built roughly double the calculated need; lithium LFP allows 80–90%, so its margin is far smaller (see cycle life). The margin is the difference between the arithmetic answer and the battery that actually lasts — skip it and the bank cycles too deep, aging early.

The Peukert and temperature caveats

Two more honest adjustments: high loads deliver fewer usable Ah than the C20 nameplate (see runtime calculation), and cold cuts available capacity (see cold weather). A sizing that ignores either is optimistic exactly where real conditions are harsh.

What This Means

My position: Battery sizing is a margin problem disguised as arithmetic — the formula is two lines, and the failures are almost always the depth-of-discharge margin that was skipped to save money, paid back in early replacement.

Why: The arithmetic gives a floor; the 50% rule gives the design. A bank built on the usable window — with a glance at load rate and temperature — matches its math in year three as well as day one, which is the actual definition of the right size.

My editorial view, not a purchasing guarantee.

What buyers should ask

What is the real load in watts, including peaks and idle draw?
How many hours must the bank cover between charges?
Is the depth-of-discharge window right for the chemistry (50% vs 80–90%)?

Sources

Cross-verified from battery engineering references and off-grid system documentation.

Off-grid system documentation — load tables and sizing practice.
Battery engineering references — depth of discharge and cycle life.

Last reviewed: 2026-09-16