The formula
Lifetime energy (kWh) = Voltage × Ah × Depth of discharge × Cycles ÷ 1,000
Cost per kWh = Price ÷ Lifetime energy
Worked example
A £200 100 Ah AGM used to 50% for 500 cycles delivers 300 kWh in its life: about 67p per kWh. A £350 100 Ah LiFePO4 used to 80% for 3,000 cycles delivers 2,880 kWh: about 12p per kWh, more than five times cheaper, before counting the weight saving and faster charging.
When lead-acid can still make sense
- Occasional use, where the battery will age out from time rather than wear out from cycles.
- Very cold locations where you can't keep lithium above freezing for charging.
- Starter batteries and standby systems that rarely cycle.
Prices in the defaults are examples only. Enter current prices and the cycle ratings from each battery's datasheet.
Frequently asked questions
Is LiFePO4 cheaper than AGM in the long run?
For batteries that are cycled regularly, almost always. LiFePO4 typically delivers 5–10 times more lifetime energy for 1.5–3 times the price.
What is cycle life?
The number of full charge and discharge cycles a battery can do at a stated depth of discharge before its capacity falls to a set level, usually 70–80% of new.