Begin with appliances and operating hours
A room count or building size cannot determine a complete solar system. List the appliances you need, their power ratings, when they run and which are essential during an outage. Bills and measured consumption can help establish a baseline.
Separate output from backup duration
The inverter must be appropriate for the loads operating together, including relevant starting demands. Battery storage must be considered against the required backup period and usable capacity. Panel capacity must be assessed against the energy requirement and the site resource.
A simple energy calculation is power multiplied by operating time. For illustration, a constant 100-watt load running for five hours uses 0.5 kilowatt-hours before any system losses. This example is not a system recommendation.
Let the site complete the picture
Shading, available space and site conditions affect the design. An energy assessment can also reveal opportunities to reduce consumption before increasing the system size.
Ask for a design based on your actual loads and priorities. The result should distinguish essential backup from whole-property supply and explain the assumptions behind capacity and operating duration.