LiFePO4 vs Lead Acid for Backup Power
Figures last verified
A 100Ah lead acid battery at 12V holds 1,200Wh on paper and delivers about 600Wh in practice, because discharging below half destroys it quickly. A 100Ah lithium iron phosphate battery at the same voltage delivers roughly 1,080Wh and survives thousands of cycles rather than hundreds. LiFePO4 costs two to three times more per nameplate amp-hour and considerably less per usable kilowatt-hour delivered across its service life.
Lead acid still wins in a few specific situations, and they are worth knowing rather than dismissing.
Usable capacity is the headline difference
Depth of discharge is how far a battery can be drained without damage, and it is where the comparison is decided.
Flooded lead acid and AGM should not routinely go below 50 percent state of charge. Doing so repeatedly cuts cycle life dramatically. So half of what you paid for is structurally unavailable.
LiFePO4 tolerates 80 to 100 percent depth of discharge routinely, with most battery management systems allowing 90 percent or more while protecting the cells at the bottom.
The practical consequence is that comparing amp-hour ratings across chemistries compares nothing. A 100Ah LiFePO4 battery replaces roughly a 200Ah lead acid bank at equivalent usable energy, which also halves the weight and the footprint. The underlying units and why amp-hours mean little without voltage are covered in watts versus watt-hours.
Cycle life
Flooded lead acid delivers roughly 300 to 700 cycles at 50 percent depth of discharge with good maintenance. AGM lands in a similar range, sometimes slightly better. Deep-cycle golf cart batteries at the higher end of the lead acid quality range can exceed 1,000 cycles when treated carefully.
LiFePO4 delivers 2,000 to 6,000 cycles at 80 percent depth of discharge, with quality cells commonly rated at 3,000 to 4,000 to 80 percent remaining capacity.
For a backup system that cycles a handful of times a year, both chemistries will likely reach calendar-age limits before cycle limits, which narrows this advantage considerably. For a system cycling daily, whether off-grid or as daily bill offset, the difference is decisive.
Calendar life and self-discharge
Lead acid degrades whether used or not, particularly when stored partially discharged, where sulfation permanently reduces capacity. A lead acid battery kept on a shelf at half charge for a year may be largely ruined. Self-discharge runs roughly 3 to 5 percent per month, more in heat.
LiFePO4 self-discharges at roughly 1 to 3 percent per month and tolerates long storage well, ideally at 50 to 60 percent state of charge. Calendar life is typically quoted at 10 years or more.
For backup equipment that sits unused between outages, this matters more than cycle life. A lead acid bank requires a maintenance charger and periodic attention to be trustworthy when needed. A LiFePO4 pack can sit in a closet for six months and still work.
Charging behavior
LiFePO4 accepts charge much faster, commonly at 0.5C or 1C, meaning a 100Ah battery can absorb 50 to 100A. Lead acid typically accepts 0.1 to 0.2C and slows dramatically in its absorption phase, so the last 20 percent takes disproportionately long.
That difference shows up directly in solar recharging, where daylight is limited. A lead acid bank may be unable to absorb what the panels produce during peak sun and then be unable to finish charging before dark, which compounds the sulfation problem. The arithmetic of daily harvest against battery capacity is in how many solar panels it takes to recharge a power station in one day.
Chargers and charge controllers must be set to the correct chemistry profile. A lead acid profile applied to LiFePO4 overcharges it, and a lithium profile applied to lead acid undercharges it into early failure. Most modern MPPT controllers have selectable profiles.
Temperature
This is where lead acid has a genuine advantage.
LiFePO4 must not be charged below freezing. Charging lithium cells below 0C causes lithium plating on the anode, which is permanent damage and a safety issue. Discharging below freezing is fine, with reduced capacity. Quality packs include low-temperature charge cutoff in the BMS, and better ones include self-heating, which draws from the pack to warm the cells before accepting charge.
Lead acid charges at sub-freezing temperatures without damage, though at reduced capacity and with adjusted voltage setpoints.
For an unheated garage, shed, or vehicle in a northern winter, this is the single strongest argument for lead acid, or for a LiFePO4 pack with documented self-heating and a controller that respects the cutoff. It is also exactly the season during which most long outages occur, which makes it a planning issue rather than a footnote.
At high temperatures the advantage reverses. Lead acid life falls sharply above 25C, roughly halving for every 10 degrees, while LiFePO4 handles heat better.
Weight, safety, and maintenance
A 100Ah LiFePO4 battery weighs roughly 25 to 30 pounds. A 100Ah AGM weighs 60 to 70. For anything portable this decides the question on its own.
Flooded lead acid vents hydrogen during charging and requires ventilation, periodic water topping, and terminal cleaning. AGM is sealed and maintenance-free but still vents under fault conditions. LiFePO4 requires no maintenance.
On safety, LiFePO4 is the most thermally stable common lithium chemistry, considerably more so than the NMC cells used in most consumer electronics and electric vehicles, and it does not enter thermal runaway under the conditions that trouble other lithium types. Lead acid's hazards are acid spillage and hydrogen accumulation rather than fire.
Cost, honestly
Nameplate cost per amp-hour favors lead acid by a factor of two to three at purchase.
Cost per usable kilowatt-hour over service life favors LiFePO4 substantially, because of the depth-of-discharge halving and the cycle life difference compounding together. A lead acid bank replaced three times over the period a LiFePO4 pack serves once erases the purchase price advantage and then some, before counting the labor of replacement and disposal.
The case where lead acid wins financially is a system that cycles rarely, sits somewhere temperature-controlled, and has a low enough capacity requirement that the weight and footprint do not matter. A small sump pump backup battery is the archetype, and the options there are compared in the sump pump backup comparison.
What this means for buying a power station
Nearly every current portable power station uses LiFePO4, and the ones still using NMC are older designs sold on price. NMC packs are lighter and rated for fewer cycles, commonly 500 to 800, and the difference in expected life is large enough to notice on the spec sheet.
If a station does not state its chemistry, that is informative. Check the specification table for LiFePO4 or LFP, and check the rated cycle count and the state of charge it is rated to, since 3,000 cycles to 80 percent and 3,000 cycles to 60 percent are different claims.
For building a bank rather than buying a station, the same chemistry logic applies with the addition that a BMS is mandatory for lithium and the charge controller profile must match. The wider question of whether a battery is the right backup at all, versus a generator or a home battery, is in which backup actually fits your house.