Lithium vs lead-acid batteries for backup power
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The battery is usually the most expensive part of a backup system, and the choice between lithium and lead-acid shapes how big it has to be, how long it lasts and how you look after it.
Here “lithium” means lithium iron phosphate (LiFePO4), the chemistry used in most home backup batteries. “Lead-acid” covers flooded, tubular, AGM and gel batteries.
The difference that matters most: usable capacity
A battery’s label shows its full capacity, but you shouldn’t use all of it every day.
- Lead-acid batteries last much longer if they are only drained about halfway before recharging. Regularly going deeper shortens their life sharply.
- Lithium batteries are commonly cycled to about 80% of their capacity, and many manufacturers allow more.
So to get 5 kWh of usable energy, you need about 10 kWh of lead-acid, or about 6.25 kWh of lithium. Comparing price per labelled kWh therefore flatters lead-acid. Compare price per usable kWh instead.
Side by side
| Lithium (LiFePO4) | Lead-acid (tubular, AGM, gel) | |
|---|---|---|
| Typical daily depth of discharge | About 80% | About 50% |
| Cycle life, as commonly quoted by makers | Several thousand cycles | Hundreds to about 1,500 cycles, depending on type |
| Energy lost in charging and discharging | Small, around 5% | Larger, often 15–20% |
| Weight for the same usable energy | Much lighter | Heavy |
| Maintenance | Built-in battery management system; little to do | Flooded types need topping up with distilled water; all need full recharges |
| Partial charging | Fine | Harmful if it becomes a habit (sulphation) |
| Upfront price | Higher | Lower |
Cycle-life figures vary by manufacturer and depend on temperature and depth of discharge. Check the data sheet of the specific battery you are offered, not a general claim.
Why lead-acid hates being half-charged
In places with long or frequent outages, batteries often spend days without a full recharge. Lead-acid batteries left partly charged build up lead sulphate crystals on their plates, which permanently reduces capacity. They need regular full charges, ideally from the grid or a well-sized solar array. Lithium batteries don’t suffer from this, which makes them a better match for unpredictable power.
Heat and cold
- Heat shortens the life of every battery. Keep batteries in a shaded, ventilated place, never in direct sun or a hot roof space.
- Cold: standard LiFePO4 batteries should not be charged below about 0 °C (32 °F). Many have heaters or a battery management system that blocks charging when cold. Check this if your batteries will sit in an unheated garage in winter.
- Flooded lead-acid batteries release hydrogen while charging and must be in a ventilated space.
Which should you choose?
Lithium usually makes sense when:
- you cycle the battery most days, as with daily outages or solar self-use;
- you expect to keep the system for many years;
- space or weight matters;
- your power is unpredictable and batteries often can’t fully recharge.
Lead-acid can still make sense when:
- outages are rare and short, so the battery mostly sits full;
- the budget today is the hard limit;
- your inverter or charger only supports lead-acid charging profiles.
Whichever you choose, check that the inverter and solar charge controller have a charging profile for that chemistry. Mixing old and new batteries, or different types in one bank, shortens the life of all of them.
The cost per cycle test
To compare quotes fairly, work out cost per usable kWh per cycle:
price ÷ (rated kWh × depth of discharge × expected cycles)
A battery that costs twice as much but lasts four times as many cycles at a deeper discharge is the cheaper battery over its life.
See how big a battery you need for each chemistry: switch the battery type in step 3.