Battery Bank & Backup Time

The arithmetic everyone does

Ah = Load (W) × Hours / System voltage

A 1500 W load for 4 hours on a 48 V bank: 6000 Wh ÷ 48 V = 125 Ah. So one 150 Ah battery string will do it. It will not. That figure is off by more than a factor of two, and every term you left out makes it worse.

Four things that eat the capacity

1. Depth of discharge. A lead-acid battery taken to flat repeatedly will not last a year. Tubular batteries are designed around roughly 60 % discharge for reasonable cycle life; VRLA around 50 %. Only about half the nameplate is yours to use.
2. The C10 rating. This is the one nobody accounts for. An Indian tubular battery marked 150 Ah is rated at the C10 rate — 15 A for ten hours. Discharge it in two hours instead and the plates cannot keep up: you get roughly 82 % of the nameplate. In one hour, about 70 %. Short backups at high current are punished hardest, which is exactly the case people size for.
3. Inverter efficiency. 90 % is typical, and it is a loss on everything you draw.
4. Battery round-trip efficiency. Another 15 % gone in lead-acid, about 4 % in lithium.

Put them together and the honest equation is:

Ah = (W × h) / (V × η_inverter × η_battery × DoD × k_rate)

For that same 1500 W for 4 hours on tubular cells: 6000 Wh becomes about 7800 Wh at the DC bus, which at 60 % DoD and a 0.9 rate factor needs roughly 300 Ah at 48 V. Two and a half times the naive answer, and the whole gap is in terms most calculators leave out.

Series and parallel

Series adds voltage, parallel adds capacity. A 48 V bank is four 12 V blocks in series; wanting 300 Ah at 48 V means two such strings side by side, eight blocks in total.

Prefer fewer, larger strings. Parallel lead-acid strings never share current perfectly, the weakest string is worked hardest, and it degrades faster — which makes the sharing worse. Three parallel strings is the practical limit; beyond that, move to a higher system voltage instead.

Higher voltage helps twice over: at 1500 W a 12 V bank pulls 140 A through the cabling, while a 48 V bank pulls 35 A. Same power, a quarter of the current, a sixteenth of the cable loss.

Is lithium worth it?

The nameplate comparison flatters lead-acid and the usable comparison does not: a 100 Ah lithium block genuinely replaces a 150 Ah tubular one. On daily cycling — solar self-consumption, a shop that runs on backup every evening — lithium usually wins over the life of the installation. For a bank that sits float-charged and is called on twice a month, tubular is still hard to beat on price.

Do not forget the charger

Lead-acid wants a charge current around C/10 — a 300 Ah bank needs a 30 A charger, and it will need ten hours or more to recover. If your outage pattern does not leave that much charging time between events, the bank never reaches full charge, sulphates, and dies early. In areas with long daily cuts this, not the sizing, is what kills batteries.

Temperature matters too. Capacity falls roughly 1 % per °C below 25 °C, and at 40 °C the battery delivers slightly more but ages far faster. A battery box in direct sun is a bank you will replace early.

Depth of discharge is a purchase decision

A battery bank has two capacities: the one printed on the case and the one you are willing to use. The gap between them is depth of discharge, and it trades directly against how many cycles the bank will deliver before it is finished.

A flooded lead-acid battery taken to 80 per cent discharge every day might manage 300 to 500 cycles — under two years of daily use. The same battery held to 50 per cent typically reaches 1000 to 1200, and at 30 per cent it can exceed 2000. The energy you draw per cycle falls, the number of cycles rises faster, and total lifetime energy goes up. Sizing a lead-acid bank at 50 per cent usable capacity is not conservatism; it is the cheapest configuration over the life of the installation.

Lithium iron phosphate changes the arithmetic rather than removing it. LFP tolerates 80 to 90 per cent discharge for 3000 to 6000 cycles, so the usable fraction of the nameplate is far higher and the penalty for using it is far smaller. That is most of why an LFP bank costing twice as much per nominal kilowatt-hour is frequently cheaper per delivered kilowatt-hour.

Temperature, and the capacity that was never there

Battery ratings are quoted at 27 degrees Celsius, and capacity falls as the battery gets colder. At 15 degrees a lead-acid bank delivers around 85 per cent of rated capacity; near freezing it is closer to 70 per cent. The battery is not damaged and recovers fully on warming, but a bank sized on the nameplate will not hold up a winter night in north India.

Heat does the opposite and is far more expensive. Capacity rises slightly, and life collapses — the working rule is that every 8 to 10 degrees above 27 halves the service life. A battery cabinet in an unventilated room that sits at 40 degrees through a Chennai summer is running at roughly a quarter of its rated life, which is why a bank quoted for five years is regularly replaced in two.

None of that appears in a backup-time calculation, and all of it decides whether the bank still meets the calculation a year later. Ventilate the enclosure, keep batteries off hot walls and away from the inverter heat sink, and treat any manufacturer figure as conditional on a temperature the installation has to actually provide.