Backup power systems use two main battery chemistries: lithium iron phosphate (LiFePO4, LFP) and lead-acid — AGM and gel. Lead is almost always cheaper on the price tag, but the right comparison isn't the price of the battery — it's the price of each kilowatt-hour it delivers over its whole life. Here's the difference, point by point.
The comparison in one table
| Parameter | LiFePO4 | AGM / gel |
|---|---|---|
| Cell voltage | 3.2 V | 2 V (a 12 V block is six cells) |
| Usable depth of discharge | 80–90 % | about 50 % to preserve life |
| Cycle life (manufacturer data) | 3,000–6,000 cycles at 80 % | 300–800 cycles at 50 % |
| Weight per 1 kWh | 8–12 kg | 25–30 kg |
| Round-trip efficiency | about 95 % | 80–85 % |
| Voltage under load | nearly flat until the end | sags noticeably |
| Charging in frost | not below 0 °C without heating | possible, but slower |
| Battery management system (BMS) | required | not needed |
| Up-front price | higher | lower |
| Price per cycle | lower | higher |
Why LiFePO4 became the standard
Depth of discharge. From a 100 Ah lead-acid battery you can regularly use only about half — deeper discharges kill it quickly. LiFePO4 works at 80–90 % without noticeable harm. For the same usable energy you need almost twice as much lead.
Cycle life. With frequent outages, the battery cycles every day. A 500-cycle lead battery is used up in a year and a half to two years on that schedule; a 4,000-cycle LiFePO4 will last more than ten.
Stable voltage. LiFePO4 voltage hardly drops until the very end of discharge, so the inverter doesn't cut out early because of sag under a heavy load — a pump starting, for example.
Weight and size. A 5 kWh LiFePO4 battery weighs about 45–50 kg and fits in one rack; lead-acid with the same usable energy means eight heavy blocks.
The cost of a kilowatt-hour over the battery's life
Let's compare in relative units. Say 1 kWh of nominal AGM capacity costs 100 units and LiFePO4 costs 250 — two and a half times more.
| AGM | LiFePO4 | |
|---|---|---|
| Price per 1 kWh nominal | 100 | 250 |
| Usable per cycle | 0.5 kWh | 0.9 kWh |
| Cycle life | 500 cycles | 4,000 cycles |
| Lifetime energy | 250 kWh | 3,600 kWh |
| Price per kWh delivered | 0.4 | 0.07 |
Even at 2.5 times the up-front price, every kilowatt-hour from lithium comes out roughly six times cheaper. Plug in your own prices — the ratio is usually similar.
When AGM still makes sense
- Outages are rare and the battery mostly sits fully charged — the cycle life isn't being spent.
- A small UPS for a boiler or router used a few hours a year.
- An unheated room in winter where a lithium battery can't be insulated or heated.
- A very tight budget right now — knowing the battery will need replacing much sooner.
Don't mix up lead-acid types
- AGM and gel (deep-cycle) are sealed, need no topping up and are built for cycling.
- Car starter batteries are designed to deliver a large current for a few seconds to crank an engine. They can't take deep discharges and degrade within a few dozen cycles in a UPS.
Safety
LiFePO4 is the most thermally stable of the common lithium chemistries: unlike the nickel-manganese-cobalt cells in power banks and EVs, it is far less prone to thermal runaway. That doesn't make protection optional — a BMS, a fuse on the positive cable and correct charge settings are mandatory.
Safety: lead-acid batteries release hydrogen when overcharged. Even sealed AGM batteries belong in a ventilated room and must never be charged above the voltage the manufacturer specifies.
Never mix batteries
Don't combine different chemistries, capacities or ages in one bank. A new battery paired with an old one performs at the old one's level, and because of voltage differences one of them will always be under- or overcharged.
Summary
For a system that works through regular outages, the choice is practically settled — LiFePO4: more usable energy, many times the cycle life, and a lower price per kilowatt-hour. AGM remains a niche option for rare outages and cold rooms. How to set up lithium charging in your inverter so the battery reaches its rated life is in LiFePO4 charge settings, and how much capacity you need is in calculating battery capacity.
