Path-Q Solar
How does a battery's usable capacity differ from its advertised capacity?
Answer
Advertised capacity is commonly the battery's nameplate or total stored energy, while usable capacity is the energy the system allows you to charge and discharge under specified conditions. Usable capacity reflects operating limits and configured reserve energy, but the rating may be stated before or after inverter and other conversion losses; temperature and power constraints can also affect the energy actually delivered.
Key points
- Compare usable kilowatt-hours with continuous and surge power ratings; these describe energy duration and instantaneous power respectively.
- A backup reserve may make less than the total usable capacity available for ordinary bill-saving operation.
- Round-trip losses mean the energy delivered may be less than the energy used to charge the battery.
- Ask whether the stated capacity is at installation, under a specified temperature, or at the end of the warranty period.
What it depends on
- Battery and inverter specifications
- Configured reserve and depth-of-discharge limits
- Operating temperature and power level
- Whether the figure is measured before or after conversion losses
Risks and trade-offs
- Sizing from nameplate capacity alone can overstate backup duration.
- High loads may reduce runtime even when enough total energy is stored.
- Usable capacity may decline over time.
Information you may need
- Usable energy and continuous or surge power ratings
- Reserve settings and warranty-end capacity
- Expected critical loads and runtime
Your next step
What to do next
Ask for a runtime calculation based on usable capacity, reserve settings, conversion losses, and your actual critical-load profile.
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