A system should be sized around how much power your household actually uses, not a rough guess. Pull up a full year of utility bills, either from paper statements or your online account, so you can see how usage rises and falls with the seasons.

Summer air conditioning, winter electric heat, a pool pump or an electric vehicle can all push usage up in certain months. If you expect changes soon, such as adding an electric car, switching to a heat pump or having someone move out, mention that to the installer so they can factor it in.

Be cautious about a system that seems much larger than your history supports. Oversizing can mean paying for panels whose extra output your utility may credit at a low rate. Undersizing has its own trade-off, since you'll keep buying more power from the grid. Matching the system to realistic use is usually the safest starting point.