LiFePO4 for solar: what the spec sheet doesn't say
Lithium iron phosphate has become the default for solar storage and it deserves to be. But the numbers that sell it — 5,000 cycles, 100% depth of discharge — come with conditions that decide whether you get them.
Usable capacity is the only capacity that matters
A 100Ah 12.8V LiFePO4 battery stores 1.28 kWh nominally. Discharge it to 90% and you have about 1.15 kWh before inverter losses of 10-15%. The same rating in lead-acid gives you roughly half that in daily use without shortening its life. So when comparing, multiply amp-hours by nominal volts, then by the depth of discharge you will actually run, and compare that usable figure against price.
| Chemistry | Usable DoD, daily | Cycle life at that DoD | Charge efficiency | Cold charging |
|---|---|---|---|---|
| Flooded lead-acid | 50% | 300-500 | ~75-85% | Fine |
| AGM | 50% | 400-700 | ~85% | Fine, capacity drops |
| LiFePO4 | 80-90% | 3,000-5,000 | ~95%+ | Usually blocked below 0°C |
Three constraints that void the warranty
Charging below freezing
Charging a LiFePO4 cell below 0°C (32°F) plates lithium and permanently damages it. Better BMS units refuse the charge; cheaper ones may not. If your battery lives in an unheated shed in a cold climate, look for integrated low-temperature cutoff, or keep the bank above freezing.
Charger voltage
Lithium needs an absorption voltage around 14.2-14.6V for a 12V pack and no equalisation stage. A lead-acid charger that equalises at 15.5V will trip the BMS or cook cells. Set the charger to a lithium profile before connecting anything.
Series and parallel limits
Most 12V batteries allow four in series (48V) or four in parallel, but not both without a balancer. Exceeding the stated limit, or mixing old and new units in parallel, is the most common cause of premature failure — and it is easy to spot, so it is the first thing a warranty claim examines.
What to look for beyond the headline
Grade-A prismatic cells rather than "grade B" or unspecified; an explicit cycle-life figure at a stated DoD and temperature; a low-temperature charge cutoff; Bluetooth or CAN monitoring if you want to see state of health; and a warranty written in years and cycles rather than just years. A 10-year warranty that assumes 50% DoD is not the same product as a 10-year warranty at 90%.
For whole-home backup, the same arithmetic applies at 48V. Compare on cost per usable kWh over the warranty period, and add the cost of the inverter and any required rapid-shutdown or code compliance work — in many jurisdictions that is now a substantial part of the project.