Solar batteries store the energy produced by solar panels and make it available when sunlight is limited or unavailable. Proper storage sizing determines how long a system can operate without recharge and how stable it performs under load.
Capacity: Wh vs Ah
Battery capacity is commonly expressed in:
- Amp-hours (Ah) at a specific voltage
- Watt-hours (Wh), which reflect actual stored energy
For system planning, watt-hours provide clearer comparisons because they include voltage.
Battery types
- Lithium (LiFePO4): High usable capacity, long cycle life, lighter weight.
- AGM / Sealed Lead Acid: Lower upfront cost, heavier, lower usable depth of discharge.
The usable portion of a battery depends on its recommended depth of discharge and chemistry.
Depth of discharge and cycle life
Battery lifespan depends on how deeply it is discharged during each cycle. Lithium systems typically allow deeper discharge while maintaining long cycle life. Lead-acid systems generally require shallower discharge to preserve longevity.
Storage sizing logic
Battery capacity should be based on:
- Daily energy consumption (Wh per day)
- Required runtime without sun
- System voltage
- Expected seasonal variation
For complete system calculation, see solar system sizing methodology.
Interaction with the inverter
The inverter draws power from the battery. Continuous load and surge requirements must align with both inverter rating and battery discharge capability. See inverter fundamentals for details.