Off-Grid Battery Bank Calculator

A 100 amp-hour battery does not give you 100 amp-hours, and the gap is larger than almost anyone expects on their first build. Four separate deductions apply, and because they multiply rather than add, their combined effect is severe.

Depth of discharge is the largest. Lead-acid chemistry degrades rapidly if routinely taken below about half charge — a flooded battery cycled to 50% might manage 500 cycles, while the same battery cycled to 80% depth manages perhaps 200. The convention of treating lead-acid as 50% usable is not conservatism; it is the condition under which the manufacturer’s cycle life applies. Lithium iron phosphate changes this fundamentally, tolerating 80% depth and often more with cycle lives in the thousands.

Round-trip efficiency is the energy lost as heat pushing charge in and pulling it back out. Flooded lead-acid returns roughly 80% of what went in; LFP returns about 95%. On a solar system this compounds with everything upstream, because that lost 20% has to be collected by panels that cost money.

Temperature is the one people discover in their first winter. Battery capacity is rated at 77°F, and lead-acid loses roughly 0.6% for every degree below it — a fifth of the bank gone at 40°F, a third at 20°F. A cabin battery in an unheated crawlspace is not the battery that was bought.

Multiply them and the picture is stark. Flooded lead-acid at 40°F requires you to buy about 3.2 times the energy you plan to use. LiFePO4 at the same temperature requires about 1.4 times. Anyone comparing the two on price per nameplate amp-hour is comparing the wrong quantity by a factor of more than two.

Autonomy is how long the system runs with no charging at all, and it is a weather and lifestyle question rather than an electrical one. Getting it wrong in either direction is expensive: too little and the system fails in the first bad week, too much and a great deal of money sits idle for years.

System voltage is chosen once and constrains everything afterwards, and the argument turns on a single relationship: power is volts times amps, so the same power at a higher voltage means proportionally less current.

A 3,000 watt inverter at 12 volts draws about 250 amps from the battery at full output — which needs 4/0 cable, a 300 amp fuse, and lugs the size of a thumb. The same inverter at 48 volts draws about 63 amps and runs happily on 4 AWG. The copper alone can differ by hundreds of dollars, and the voltage drop problem largely disappears.

Against that, 12 volts is the native voltage of the entire recreational vehicle and marine accessory market. Fridges, fans, pumps, lights, USB outlets and chargers are all available in 12 volt form and mostly not in 48. A 48 volt system needs a DC-DC converter to feed a 12 volt accessory bus, which is another component and another conversion loss.

The rough division in practice: 12 volts for vans and small RVs where accessory availability dominates and total power is modest; 24 volts for larger rigs and small cabins; 48 volts for house-scale off-grid, where the loads are big enough that copper cost decides it. Anything with an inverter above about 3,000 watts should be looking hard at 48.

The comparison is frequently made badly in both directions, so it is worth setting out what actually differs.

Lithium iron phosphate wins on usable capacity per nameplate amp-hour (80% against 50%), on round-trip efficiency (95% against 80%), on cycle life by a factor of five or more, on weight by roughly half, on charge acceptance — LFP will take charge as fast as you can supply it, where lead-acid tapers badly in its absorption phase — and on the absence of maintenance. Combined, a 100 Ah LFP battery does the work of roughly 200 to 250 Ah of lead-acid, several times over.

Lead-acid wins on purchase price per nameplate amp-hour, on tolerance of cold charging, on the simplicity of the charging equipment, and on the fact that a dead lead-acid battery can be replaced at any auto parts store anywhere. It also fails gracefully rather than shutting off abruptly when its battery management system decides to protect it.

The cold-charging point deserves emphasis because it is a genuine safety and longevity issue rather than a preference. Charging a lithium iron phosphate cell below freezing plates metallic lithium onto the anode, which is permanent and can eventually cause an internal short. Batteries with a low-temperature cutoff in the BMS, or with internal heating, exist precisely for this and are what any cold-climate installation needs.

For most builds the arithmetic now favours lithium clearly on cost per usable cycle. Lead-acid remains sensible for a system used a few weekends a year, where cycle life will never be reached and the lower purchase price dominates.

Sizing errors in off-grid systems are overwhelmingly load-estimation errors rather than battery-arithmetic errors, and they run in one direction.

The reliable method is a shunt-based battery monitor and a week of living with the system, which gives real amp-hours consumed rather than an estimate. Failing that, a cheap inline watt-meter on individual circuits will settle the arguments about the fridge.

Two categories mislead consistently. Cycling loads — fridges, water pumps, heater fans — draw their rated power only part of the time, so using the nameplate overstates them substantially; use a duty-cycle average instead. And parasitic loads — the inverter’s own idle draw, a battery monitor, a propane detector, an always-on charge controller — are individually trivial and collectively significant. An inverter left switched on with nothing plugged in can consume 20 to 40 watts continuously, which over a day is comparable to the fridge.

The other trap is scope creep after the fact. A system sized before the owner adds satellite internet, an induction hob or a rooftop air conditioner is a system that will disappoint, and retrofitting battery capacity is far more expensive than including it at the start. If any of those are plausible within a couple of years, size for them now.