Off-Grid Battery Basics

The battery bank is where off-grid systems succeed or fail. Panels are cheap and last decades. Batteries are expensive, wear out, and are unforgiving of mistakes made when the system was designed. Most people who end up disappointed with an off-grid setup bought the wrong bank, or the right bank and treated it badly.

This page covers what the specifications actually mean, so the numbers on a product page tell you something useful.

Rated capacity is not usable capacity

A battery’s rating is the energy it contains. What you can actually take out is smaller, often much smaller, and the gap is the single most common source of undersized systems.

Depth of discharge (DoD) is how far down you can draw a battery before you start doing real damage. Lead-acid chemistries are conventionally limited to about 50%. LiFePO4 tolerates 80 to 100%, with 80% the usual recommendation for long life.

So a 100 amp-hour battery at 12 volts holds 1,200 watt-hours nominally, but:

Chemistry	Typical DoD	Usable from 1,200 Wh
Flooded lead-acid	50%	600 Wh
AGM	50%	600 Wh
LiFePO4	80%	960 Wh

Two batteries with identical labels can therefore deliver very different amounts of power. When you compare prices, compare cost per usable watt-hour, never per rated amp-hour.

The chemistries

Flooded lead-acid

The oldest and cheapest option per rated amp-hour. Flooded cells need topping up with distilled water, need ventilation because they vent hydrogen while charging, and must be kept upright. Handled well they are a reasonable budget choice; neglected, they fail quickly and predictably.

AGM and gel

Sealed lead-acid variants. No watering, no free liquid, far more tolerant of being mounted in odd positions. You pay more than flooded for the convenience and get similar depth of discharge and broadly similar cycle life.

LiFePO4 (lithium iron phosphate)

Now the default for most new off-grid builds. More usable capacity per rated amp-hour, several times the cycle life, a fraction of the weight, and near-flat voltage under load. It costs substantially more up front and has one hard constraint that catches people out. See the cold-weather section below.

Note that LiFePO4 is a specific lithium chemistry and is not the same as the lithium-ion cells in laptops and phones. It is markedly more thermally stable, which is why it dominates stationary storage.

Cycle life and what it really costs

A cycle is one discharge and recharge. Cycle life ratings state how many cycles a battery survives before its capacity falls to some percentage of new, usually 80%.

Rough figures, which vary widely by manufacturer and by how well the bank is treated:

Chemistry	Typical cycle life at rated DoD
Flooded lead-acid	500 to 1,200
AGM	400 to 1,200
LiFePO4	3,000 to 6,000

The comparison that matters is lifetime cost per usable kilowatt-hour: purchase price divided by (usable capacity × cycle life). Run that on any two batteries you are choosing between and the answer is often the opposite of what the sticker prices suggest.

For the full comparison, including where lead-acid still wins, see LiFePO4 vs AGM vs flooded lead-acid.

To make the shape of it concrete, using round illustrative numbers rather than any specific product: a 100 Ah 12 V lead-acid battery delivering 600 Wh usable over 600 cycles supplies roughly 360 kWh across its life. The same nominal size in LiFePO4, delivering 960 Wh usable over 3,500 cycles, supplies roughly 3,360 kWh, nine times as much energy. Even at three or four times the purchase price, the lithium option costs less per kilowatt-hour delivered.

Where that logic breaks down: cycle life only pays off if you use the cycles. A cabin visited six weekends a year will never reach 3,500 cycles, because calendar aging will retire the bank first. For genuinely intermittent use, cheaper lead-acid can be the rational choice.

Discharge rate changes how much you get

Lead-acid batteries deliver less than their rated capacity when discharged quickly. The rating is normally quoted at a gentle 20-hour rate; pull the same battery hard and the usable capacity shrinks. This is Peukert’s effect, and it means a lead-acid bank running a heavy load underperforms its label.

LiFePO4 is largely immune. Its capacity stays close to rated across normal discharge rates, which is a significant practical advantage on any system with big intermittent loads like a well pump or a power tool.

Cold weather

Cold matters more off-grid than anywhere else, because the season with the least sun is also the season with the coldest battery box.

All chemistries deliver less capacity when cold. But most LiFePO4 batteries must not be charged below freezing. Charging a cold lithium cell causes lithium plating, which permanently damages it. Manufacturers handle this in one of three ways: a built-in heater, a battery management system that refuses charge until the cell warms, or nothing at all, in which case protecting the battery is your problem.

If your bank lives anywhere unheated in a cold climate, confirm which of those three you are buying. It is the difference between a battery that lasts a decade and one that quietly dies its first winter.

Lead-acid has the inverse problem worth knowing: a discharged lead-acid battery freezes at a much higher temperature than a charged one. Leaving a flooded bank flat over winter can literally freeze and split the cases.

Cold climates affect far more than the battery bank. See off-grid solar in cold and difficult climates.

Bank voltage: 12, 24, or 48

Batteries are wired in series to raise voltage and in parallel to raise capacity. Choosing a system voltage is one of the earliest decisions and one of the most consequential, because it sets the current everything else must carry.

Higher voltage means lower current for the same power, which means thinner and cheaper cable, smaller fuses, and less energy wasted as heat. A 2,000 W load draws about 167 A at 12 V but only about 42 A at 48 V. That is the difference between heavy, expensive cable and something manageable.

As a rough guide: 12 V suits small systems and anything sharing parts with vehicle equipment; 24 V suits mid-size cabins; 48 V suits full-time off-grid homes and anything with substantial loads. Work out your wire sizes at each voltage before committing, because the cable cost difference is often decisive.

Sizing a bank

The method, step by step, is on its own page: how to size an off-grid battery bank.

Rules that save banks

• Never mix batteries of different ages, capacities, or chemistries in one bank. The weakest cell sets the behavior of the whole thing.
• Size for the worst month, not the average. A bank sized on annual-average sun hours will be chronically undercharged in December.
• Chronic undercharging kills lead-acid. Sulfation from repeatedly failing to reach a full charge is the most common cause of premature death.
• Ventilate flooded banks. They vent hydrogen while charging, which is explosive in an enclosed space.