LiFePO4 vs AGM vs Flooded Lead-Acid

This is the decision most off-grid budgets turn on, and the one where the sticker price is most misleading.

For the underlying concepts, see Off-Grid Battery Basics. This page is the direct comparison.

The summary table

	Flooded lead-acid	AGM	LiFePO4
Usable depth of discharge	50%	50%	80% or more
Typical cycle life	500 to 1,200	400 to 1,200	3,000 to 6,000
Cost per rated Ah	Lowest	Moderate	Highest
Weight for same usable energy	Highest	High	About a third
Maintenance	Watering, ventilation	None	None
Tolerates high discharge rates	Poorly	Moderately	Well
Charge below freezing	Yes	Yes	No, without a heater
Tolerates partial charging	Poorly	Poorly	Well

Ranges vary by manufacturer and by how well the bank is treated. Check the datasheet for the battery you are actually buying.

Cost, done properly

Comparing price per rated amp-hour is the mistake that makes lead-acid look cheap. Two corrections change the answer.

Correction one: usable capacity. Lead-acid gives you half its rating. LiFePO4 gives you 80% or more. Before anything else, lead-acid needs roughly 1.6 times the nameplate capacity to deliver the same usable energy.

Correction two: cycle life. This is the larger effect.

Using round illustrative figures rather than any specific product, for a nominal 100 Ah 12 V battery:

	Lead-acid	LiFePO4
Nameplate energy	1,200 Wh	1,200 Wh
Usable per cycle	600 Wh	960 Wh
Cycles	600	3,500
Total energy delivered	360 kWh	3,360 kWh

The lithium battery delivers roughly nine times as much energy over its life. Even at three or four times the purchase price, the cost per kilowatt-hour delivered is substantially lower.

Run this arithmetic on the two batteries you are actually choosing between, using their real prices and datasheet cycle life. It frequently reverses the intuition the sticker prices create.

Where lead-acid still wins

Two cases, and they are real.

Occasional use. Cycle life only pays off if you use the cycles. A cabin visited six weekends a year might run 60 cycles annually. Reaching 3,500 cycles would take nearly sixty years, and no battery lasts that long on the calendar regardless of use. Lithium also ages while sitting, particularly if stored at full charge in heat. For genuinely intermittent use, the cheaper chemistry is often the rational choice.

Charging below freezing without a heater. If a bank lives somewhere unheated in a cold climate and you are not buying a battery with an internal heater or adding a heated enclosure, lead-acid is the chemistry that tolerates the situation. See battery safety.

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A third, weaker case: budget now versus budget later. If the choice is a lead-acid system this year or no system at all, lead-acid is better than nothing. Just size the array properly, because chronic undercharging will kill it far faster than the cycle count suggests.

Where LiFePO4 wins decisively

High or intermittent loads. Lead-acid delivers less than its rated capacity when discharged quickly, an effect known as Peukert’s. Pull it hard for a well pump or a power tool and the usable capacity shrinks. LiFePO4 is close to immune, holding near its rated capacity across normal discharge rates.

Partial state of charge. Lead-acid is damaged by sitting partly charged. Sulfation begins whenever it fails to reach a full charge, and repeated partial charging is the most common cause of premature death in off-grid banks. This matters enormously in winter, when short days may mean the bank does not reach full for weeks. LiFePO4 is untroubled by it.

Weight and space. Roughly a third the weight for the same usable energy. Decisive for tiny houses and anywhere the bank has to be carried in.

Voltage stability. Lead-acid voltage sags steadily as it discharges, so lights dim and inverters approach cutoff early. LiFePO4 holds a nearly flat voltage until close to empty, meaning consistent performance and a more meaningful state-of-charge reading.

No maintenance and no venting. Flooded batteries need watering and real ventilation because they release hydrogen while charging. LiFePO4 needs neither.

AGM, in the middle

AGM is sealed lead-acid. It removes the maintenance and most of the venting from flooded, while keeping essentially the same depth of discharge and broadly similar cycle life, at a higher price.

It makes sense when you want lead-acid economics without watering a bank, or when the batteries must be mounted somewhere awkward. It does not solve the deeper limitations: you still get half the capacity, still suffer under high discharge rates, and still damage it by leaving it partly charged.

Gel is similar to AGM in practice, slightly fussier about charge voltage.

If you switch

Replacing lead-acid with LiFePO4 is not always a straight swap.

Charge profiles differ. Lithium wants a different absorption voltage and does not want a float stage held indefinitely. Your charge controller and any inverter/charger must have a lithium setting. Many ship configured for lead-acid.

Temperature compensation must be off. It is correct for lead-acid and wrong for lithium.

Check the BMS interaction. A battery management system disconnecting under load can produce a voltage spike that damages an inverter. Some inverters cope; some do not.

Do not mix chemistries in one bank. Ever. Different voltages, different charge acceptance, and the result is that one chemistry is always being mistreated.

The short version

For a system in regular use, LiFePO4 is usually the right answer, and the cost analysis supports it once you compare usable energy across the whole life rather than nameplate capacity at purchase.

For a system used a handful of times a year, or one that will be charged below freezing without a heater, lead-acid remains a defensible choice, and AGM is worth the premium over flooded if you would rather not maintain it.