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An example, with typical numbers
Three 100 Ah batteries cycled 250 times a year. The prices are examples, and the cycle lives are the ranges commonly quoted by manufacturers.
| Battery | Price | Upfront cost per usable kWh | Lasts | Delivers in its life | Cost per kWh delivered |
|---|---|---|---|---|---|
| Lithium (LiFePO4) 100 Ah | $300 | $293 | 10.0 years | 2,560 kWh | $0.12 |
| AGM lead-acid 100 Ah | $240 | $400 | 1.6 years | 240 kWh | $1.00 |
| Flooded lead-acid 100 Ah | $150 | $250 | 2.0 years | 300 kWh | $0.50 |
The cheapest battery to buy is the most expensive per kWh you get out of it. Lead-acid batteries can only use about half their capacity and wear out in a few hundred cycles, while lithium (LiFePO4) uses most of its capacity for thousands of cycles.
How it is calculated
- Energy (kWh) = Ah × volts ÷ 1,000. Usable energy = that × usable depth of discharge.
- Lifetime cycles = the smaller of the cycle life and the calendar life × cycles per year.
- Energy delivered over its life = usable energy × lifetime cycles.
- Cost per kWh delivered = price ÷ energy delivered over its life. The lowest number is the best value.
It ignores capacity fade, so real lifetime energy is a little lower, and it leaves out chargers and installation. Use the same assumptions for every battery you compare.
When the cheap battery makes sense
- Very occasional use. If you cycle a battery only a few times a year, the calendar life limits it. Even then lithium often wins on cost per kWh, but the upfront gap matters more.
- A tight budget today. If you cannot afford the lithium price, a lead-acid battery gets you running. Know that you will replace it sooner.
- Very cold places where charging lithium needs heating.
Using it anywhere
- Prices use dollars, but you can type any currency. The result is in the same currency.
- Ah and volts work at any system voltage. Compare batteries by kWh, not Ah alone.
Common mistakes
- Comparing price per amp-hour. A 24 V Ah holds twice the energy of a 12 V Ah.
- Ignoring depth of discharge. A 100 Ah lead-acid battery gives about 50 Ah of regular use.
- Trusting a cycle claim without its conditions. Cycle life depends on the depth of discharge and temperature.
- Forgetting replacement. A cheaper battery that you replace three times is not cheaper.
Common questions
Is a lithium battery really cheaper than lead-acid?
Per kWh you actually get out of it, usually yes. With the example numbers above, lithium costs about $0.12 per kWh delivered against about $1.00 for AGM, even though it costs more to buy. The gap comes from lithium’s much longer life and deeper usable capacity.
How many cycles does a LiFePO4 battery last?
Manufacturers commonly rate LiFePO4 batteries for 3,000 to 6,000 cycles at 80% depth of discharge, and some state more. Real life depends on temperature, charge rates and how deeply you discharge, so use the datasheet for your battery and be a little conservative.
How many cycles does a lead-acid battery last?
AGM and flooded lead-acid batteries are commonly rated for about 300 to 600 cycles at 50% depth of discharge. Discharging deeper shortens their life further, and leaving them discharged damages them.
Why is lead-acid worse for occasional use than it looks?
At 30 cycles a year, a lead-acid battery’s calendar life limits it to about 5 years, so it delivers only about 90 kWh for its price, around $2.67 per kWh. Lithium still works out at about $0.98 per kWh.
How do I compare batteries with different voltages?
Convert each to kilowatt-hours: amp-hours × volts ÷ 1,000. A 100 Ah battery at 12.8 V holds 1.28 kWh, and a 100 Ah battery at 24 V holds 2.4 kWh. Always compare cost per kWh, never cost per amp-hour.
Should I just buy the cheapest battery?
Only if cash today matters more than cost over time. The cheapest battery usually costs the most per kWh delivered. If your budget is tight, consider buying a smaller lithium battery and growing the system later.
Related calculators
To see how long a battery lasts, use the battery runtime calculator. To size a bank for a van or cabin, see the solar and battery calculator, and to recharge it, the solar charge time calculator.