Battery quotes usually compare the price per kWh on the label. That figure leaves out three things that decide the real cost: how much of the battery you can use, how many times you can cycle it, and how much energy is lost getting in and out. The fair measure is the cost per kWh delivered over the battery's life.
The formula
- Usable kWh is the nominal capacity multiplied by the depth of discharge you run it to.
- Cycles is the rated cycle life at that depth of discharge, from the datasheet.
- If the battery cycles once a day, its calendar life may run out before its cycles do, so use whichever comes first.
A worked example
The prices here are examples only; use your own quotes and the datasheet figures for the exact models.
| Lead-acid (deep cycle) | Lithium iron phosphate (LiFePO4) | |
|---|---|---|
| Example battery | 12 V 200 Ah, 2.4 kWh nominal | 5 kWh nominal |
| Example price | KES 30,000 | KES 175,000 |
| Price per nominal kWh | KES 12,500 | KES 35,000 |
| Depth of discharge | 50% | 90% |
| Usable per cycle | 1.2 kWh | 4.5 kWh |
| Rated cycles at that depth | about 800 | about 6,000 |
| Round trip efficiency | about 80% | about 92% |
| Cost per kWh delivered | about KES 31 | about KES 6.50 |
On the label, lead-acid looks almost three times cheaper. Cycled every day, it costs nearly five times more per kWh delivered, and at about KES 31 per kWh it is dearer than buying the same energy from KPLC at the domestic all-in rate of KES 28.30. It also needs replacing after a little over two years of daily use.
The lithium battery's 6,000 cycles would take more than 16 years of daily use. If its calendar life is 12 years, it delivers about 4,400 cycles in practice, and the cost rises to about KES 8.90 per kWh: still far cheaper.
Efficiency costs panels too
Every kWh stored in a lead-acid battery needs about 1.25 kWh of solar to put it there, against about 1.09 kWh for lithium. For a battery cycled daily, that means more panels for the same evening energy.
When lead-acid still makes sense
- Backup that is rarely used. A battery that only covers occasional outages cycles a few times a month, so cycle life matters less and a lower upfront price can win. Calendar life and how it is kept charged matter more.
- Very tight budgets for small loads, such as lights and phone charging, where the battery is small anyway.
Even then, keep lead-acid batteries cool and fully charged, never run them below about 50%, and top up flooded types with distilled water.
What to check on a lithium quote
- Chemistry: lithium iron phosphate (LiFePO4) is the safe, long-life standard for stationary storage.
- Usable capacity and the depth of discharge the warranty allows.
- Warranty in years and in cycles or total energy throughput.
- Battery management system and communication with your inverter, so the inverter charges it correctly.
- Operating temperature: keep any battery out of direct sun, in a ventilated space.
Size your battery
- Our battery backup guide sizes a battery from the loads you keep on and the hours you need.
- The free solar calculator adds the battery and the solar needed to recharge it.
- The battery backup and storage guide covers usable capacity and charge limits in more depth.