Battery sizing goes wrong in both directions — too small to matter, too large to pay. The discipline is to size from loads, not from round numbers.
List the loads you actually protect
Backup is a list, not a feeling: refrigeration, network gear, heating controls, a few lights and sockets. Sum their running watts and surge watts. That list sets the inverter’s power rating before anyone discusses kilowatt-hours.
Size energy from the evening, not the year
For self-consumption, the useful question is how much energy the house draws between sunset and sunrise on a typical day. A battery that covers that window cycles daily and earns its keep; capacity beyond it sits idle most of the year.
Respect the chemistry’s working range
LFP cells cycle happily but dislike heat and deep cold. A garage wall beats a sun-struck exterior wall; a conditioned utility room beats both. Placement is a sizing decision — a hot battery ages like a bigger one shrinking.
Leave the extension path open
Loads grow — heat pumps, EVs, workshops. Choose a platform that stacks or parallels with modules of a later production year, and wire the distribution board so the second module is an afternoon’s work, not a redesign.
