Battery Bank Calculator

Cost per usable kilowatt-hour, over a lifetime

Comparing batteries on price per nominal kilowatt-hour is the mistake that makes lead-acid look cheap. Two corrections change the ranking.

The first is usable fraction. A lead-acid bank yields about half its nominal capacity before cycle life collapses, and loses another 15 to 20 percent to round-trip inefficiency. Net usable is around 40 percent of what you bought. LiFePO4 delivers roughly 85 percent — more than twice as much energy from the same nominal rating.

The second is cycle life. A flooded battery cycled to 50 percent might manage 1,200 cycles; taken deeper it falls off sharply. LiFePO4 commonly reaches 3,000 to 6,000 cycles at 80 percent depth of discharge. On daily cycling that is the difference between three years and a decade or more.

Divide purchase price by usable kilowatt-hours multiplied by cycle count, and lithium generally wins comfortably despite costing two to three times more up front. The exception is a bank that cycles rarely — a backup system that runs a few times a year — where calendar life rather than cycle life governs and the cheaper chemistry can make sense.

Why 48 volts

System voltage does not change how much energy a bank holds. It changes the current needed to move it, and current is what costs money everywhere else in the system.

A 5 kW load at 12 volts draws over 400 amps. At 48 volts it draws around 100. Conductor size scales with current, so the 12 volt version needs cable of a size normally seen in service entrances, along with fuses, busbars and disconnects to match — all of which cost more than the equivalent 48 volt hardware by a wide margin.

Resistive losses scale with the square of current, so the same wiring loses sixteen times more power at 12 volts than at 48. On a system running daily that is a permanent efficiency tax.

Twelve volts persists in vehicles and small installations because the loads and the accessories are already 12 volt. Twenty-four is a compromise seen in mid-size marine and RV systems. Anything residential in scale is 48, and the inverters and charge controllers built for it are correspondingly better developed.

Days of autonomy, and what it really buys

Autonomy is the number people over-specify most, and it is the most expensive input on this page.

For grid backup the relevant question is how long outages last where you live. Most are measured in hours, and a single day of autonomy covers the overwhelming majority. Sizing for three days of autonomy on a grid-tied system triples the battery cost to cover an event that may never occur.

Off-grid is different, because the alternative to stored energy is a generator or going without. Three to five days is the conventional range, chosen to ride through a stretch of overcast weather without starting an engine.

The hybrid answer is usually cheapest for off-grid: size the bank for two or three days and keep a generator for the rare longer stretch. Batteries sized for the worst week of the year sit idle for the other fifty-one.

Also worth checking is whether your loads during an outage are the same as normal. Most households run far less on backup than they do day to day, and sizing autonomy against full normal consumption inflates the bank considerably.