Lithium iron phosphate tolerates 80 to 90 percent depth of discharge routinely, where lead acid should be kept to about 50 percent to reach its rated life. That difference is the single biggest reason lithium banks can be smaller than lead acid banks for the same usable energy.
Depth of discharge is the specification people compare least and the one that decides how much battery you actually need to buy.
Quick Answer
Use 80 to 90 percent DoD on LiFePO4 and about 50 percent on lead acid. Usable capacity is nameplate capacity multiplied by DoD, so a 100Ah lead acid battery gives roughly 50Ah usable where a 100Ah LiFePO4 gives 80 to 90Ah.
Depth of Discharge by Chemistry
| Chemistry | Recommended DoD | Usable from 100Ah | Typical cycle life |
|---|---|---|---|
| Flooded lead acid | 50% | 50Ah | 500 to 1,000 cycles |
| AGM | 50% | 50Ah | 500 to 1,200 cycles |
| Gel | 50 to 60% | 50 to 60Ah | 700 to 1,500 cycles |
| LiFePO4 | 80 to 90% | 80 to 90Ah | 3,000 to 6,000 cycles |
All of those cycle figures assume the stated depth of discharge. Going deeper shortens life on every chemistry, and lead acid punishes it far more severely than lithium ever does.
Why Lead Acid Is So Restricted
Discharging lead acid deeply accelerates sulfation, a process where lead sulfate crystals form across the plates and progressively reduce available capacity.
Shallow cycling reverses most of that during charging, and deep cycling leaves crystals that harden and become permanent.
The relationship is steep rather than anything like linear. A lead acid battery cycled routinely to 50 percent may last several times longer than the identical battery cycled to 80 percent.
That is why a lead acid bank is sized at roughly double the energy you intend to use, which is the hidden cost behind its lower price per amp hour, covered in LiFePO4 against lead acid.
Why Lithium Handles It Better
Lithium iron phosphate has no sulfation mechanism at all, so deep discharge simply does not cause the same kind of progressive damage to the cells.
Manufacturers commonly rate LiFePO4 for thousands of cycles at 80 percent DoD, and many rate them at 90 percent with a modest reduction in cycle count.
The battery management system enforces its own low voltage cutoff, which prevents the genuinely damaging over-discharge rather than leaving that judgment to you.
That combination is why a 100Ah lithium battery frequently replaces a 200Ah lead acid bank in practice, covered in how to choose a solar battery.
Sizing From Usable Capacity
Start with daily energy use
Work out the watt hours you consume per day rather than amp hours, since watt hours compare properly across different system voltages and amp hours do not.
Decide days of autonomy
How many days without any meaningful charging the bank needs to cover. Two to three days is common off grid, and a single day is frequently enough where sun is reliable.
Divide by depth of discharge
Multiply your required usable energy by roughly two for lead acid, or by about 1.2 for lithium, to arrive at the nameplate capacity you need.
Account for inverter losses
Converting DC to AC loses roughly 10 to 15 percent, so the energy your appliances consume is not the energy the bank has to supply.
Add margin for temperature
Cold reduces available capacity on every chemistry, so a bank sized exactly for summer will fall short in winter, covered in battery sizing.
What the Cutoff Voltage Actually Does
Depth of discharge is set in practice by a low voltage cutoff configured on the inverter or charge controller rather than by any percentage you enter directly.
Voltage maps onto state of charge quite differently on each chemistry, and lithium in particular has an extremely flat discharge curve, meaning the voltage barely moves at all across most of its usable range. Our state of charge calculator applies the right curve rather than one generic table.
That flatness makes voltage a poor indicator of remaining capacity on lithium, which is why coulomb-counting battery monitors are far more useful than a voltmeter.
Setting that cutoff too aggressively on lithium can shut a whole system down while meaningful capacity still remains, since the voltage drop happens suddenly right at the bottom, covered in battery monitors for off grid solar.
The Economics Behind the Number
Depth of discharge is really a cost calculation, and running it once explains why lithium wins despite the higher sticker price.
The useful metric is cost per usable kilowatt hour delivered across the battery’s whole life, not the price on the shelf. That figure combines purchase price, usable capacity, and cycle count into one comparison.
Work it as total energy delivered over the life of the battery. Nameplate capacity multiplied by depth of discharge multiplied by rated cycles gives you the number, and dividing purchase price by that gives cost per kilowatt hour.
Lead acid loses on two of those three terms at once. It delivers half its nameplate capacity per cycle and manages a fraction of the cycles, so a lower purchase price is fighting a much larger disadvantage.
Replacement frequency compounds it further. A lead acid bank replaced three times across the life of one lithium bank means three purchases, three installations, and three disposals.
Where lead acid still makes sense is short-term or low-cycle use. A backup bank that sits charged and discharges a handful of times a year is not accumulating cycles, so cycle life stops being the binding constraint. Depth of discharge is exactly where using a car battery instead falls apart.
Cold climates complicate the picture too, since lithium cannot charge below freezing without heating, and that either adds equipment cost or rules it out for unheated installations.
Everyday Use Against Emergency Use
The recommended figures describe routine daily cycling, and an occasional deeper discharge in a genuine emergency is a different situation.
Taking a lead acid bank to 70 percent once during an outage costs some life and is not a disaster.
Doing exactly the same thing every single day is what destroys a bank within a year rather than within five.
Lithium is more forgiving here too, since occasional deep cycles have limited effect on a chemistry rated for thousands of them.
Where People Get This Wrong
Comparing batteries on nameplate capacity rather than on usable capacity, which makes lead acid look considerably cheaper per amp hour than it actually is in practice.
Sizing a lead acid bank for the energy they intend to use rather than roughly double it, which produces a bank that dies early and then gets blamed on poor quality.
Using voltage to judge state of charge on lithium, where the flat curve makes that unreliable across most of the range.
And ignoring temperature derating entirely, since a bank correctly sized at 25C delivers noticeably less capacity sitting in a cold garage in January.
Related Reading
- LiFePO4 against lead acid
- choosing a battery
- battery sizing
- battery monitors
- why batteries degrade
- how many batteries off grid
Frequently Asked Questions
How deep should you discharge a solar battery?
80 to 90 percent for LiFePO4 and about 50 percent for lead acid. Going deeper shortens life on both, and lead acid is punished far more severely.
Why is lead acid limited to 50 percent?
Deep discharge accelerates sulfation, where crystals form on the plates and harden. Shallow cycling reverses most of that; deep cycling makes it permanent.
How much usable capacity do I actually get?
Nameplate multiplied by depth of discharge. A 100Ah lead acid gives roughly 50Ah usable where a 100Ah LiFePO4 gives 80 to 90Ah.
Can I occasionally go deeper?
Yes, in a genuine emergency. Taking lead acid to 70 percent once costs some life. Doing it daily is what destroys a bank in a year.
How is depth of discharge actually set?
By a low voltage cutoff on the inverter or charge controller rather than by entering a percentage anywhere.
Why is voltage unreliable on lithium?
LiFePO4 has a very flat discharge curve, so voltage barely moves across most of its range. A coulomb-counting monitor is far more useful than a voltmeter.
Is lithium actually cheaper despite the price?
Usually, on cost per usable kilowatt hour across the battery’s life. Lead acid delivers half its nameplate per cycle and a fraction of the cycles, so a lower sticker price fights a large disadvantage.
Does cold weather change this?
Yes. Available capacity falls in cold conditions on every chemistry, so a bank sized exactly for summer will fall short in winter.
