A battery has more than one important power number
Home battery comparisons often focus on usable capacity in kWh and one headline power number in kW. For outage planning, at least four separate limits matter: usable energy capacity, continuous discharge power, short-duration surge power and charging power.
Those numbers answer different questions. A system can have enough stored energy but too little discharge power for the loads, enough discharge power but too little motor-start surge, or enough solar on the roof but too little charging power to refill storage as quickly as expected.
Continuous discharge power determines what can run at once
Continuous discharge power is the sustained kW the battery system can deliver to backed-up loads within the manufacturer's operating conditions. It matters when several appliances run at the same time.
This is separate from energy capacity. A battery with many kWh can still be power-limited if the simultaneous household load exceeds its continuous output rating.
Surge power is a short-duration starting constraint
Motors such as air conditioners and pumps can briefly demand more power when they start than they use after they are running. Manufacturers may publish peak power, short-duration power or motor-start capability separately from continuous output.
Do not treat a short surge rating as if the battery can deliver that power continuously. The duration and test condition matter.
Charge power limits how quickly surplus solar can refill storage
During an outage, available solar generally has to support the backed-up household load before the remaining surplus can charge the battery. Even when the array has more surplus power available, the battery or storage system may impose a charging-power limit.
That means adding more PV does not necessarily shorten recharge time once the available surplus exceeds the system's effective charge limit. Charging efficiency, state of charge, controls and the usable solar window also affect the result.
A missing charge-power specification is a data gap, not zero
Manufacturer documents do not always publish every metric in the same format. If Home Solar Atlas cannot verify a separate continuous charging-power value from the current source set, the correct treatment is to label that value as a data gap and require a current manufacturer or installer system assumption.
A missing published number should not be converted into zero, copied from the discharge rating or inferred from a different product generation. This is why some Home Solar Atlas outage tools use a clearly labeled manual fallback rather than inventing a catalog charge limit.
Multi-battery and expansion systems may not scale linearly
Adding complete battery units often increases storage and output capability, but gateways, controllers, conductors, expansion modules and product-specific architecture can change how charging and discharging limits scale. A per-unit value should not automatically be multiplied across every possible expansion configuration.
Use catalog per-unit values as screening assumptions only where the architecture supports that interpretation, then verify the final system limit with current manufacturer documentation and the installer design.
What to verify in an installer quote
A serious battery proposal should make the energy, discharge, surge and recharge assumptions clear enough that the homeowner can test the design instead of relying on a whole-home-backup label.
- Exact battery model and number of full battery units.
- Usable storage capacity in kWh for the installed configuration.
- Continuous backed-up output in kW.
- Short-duration peak or motor-start capability and the published duration or test condition.
- Maximum continuous charging power or the relevant system charging limit.
- Whether rooftop solar can recharge the battery while the system is islanded from the grid.
- Which circuits are backed up and whether load-management controls are part of the design.
Use all four limits in outage planning
Use battery kWh to screen runtime, continuous kW to check simultaneous loads, surge capability for motor starts and charging kW to understand recovery from solar. A multi-day resilience model should combine those constraints rather than relying on one headline specification.
Home Solar Atlas separates these checks so a scenario can pass the energy test without being incorrectly described as a complete electrical-design pass.