Photovoltaics may not keep running during an outage either.
A standard grid-connected PV system may disconnect for safety reasons when the grid is lost. If it is to generate during an outage too, the inverter, wiring, switching and control must be designed for it.
In the design, we therefore check the whole system, not just the battery's state of charge.
Back up what you really need.
The basis can be the fridge, internet, lighting and heating controls. Pumps, a heat pump or other appliances increase both power and energy requirements.
Supplying the whole home is a different task from backing up selected circuits. Even then, it is not unlimited power for an unlimited time.
How much at once and for how long?
Power in kW determines which appliances the system can supply at the same time. Energy in kWh together with consumption determines the available backup time.
We also take into account the start-up requirements of motors and pumps. Switching off large non-critical appliances can extend the supply to the important ones.
The sun can help, but it cannot be relied on without limits.
With a suitable architecture, photovoltaics can supply consumption and recharge the battery during an outage. At night or when generation is low, however, the available energy is limited.
For sensitive equipment, we also consider the switchover time. Battery backup is not automatically a UPS – Uninterruptible Power Supply.
Everyday use and reserve
A battery can routinely help use photovoltaics and at the same time hold a reserve for outages. However, a larger reserve means less energy available for everyday use.