Tag

#Battery Storage: Sizing, Packages and Overnight Load

Storage sizing is an optimisation problem with an uncomfortable answer: the battery that covers the worst week of the year is almost never the battery that pays for itself.

The honest method starts with a load profile, not a capacity target. Take actual overnight consumption, subtract whatever the array still delivers at dusk, and the remainder is the evening deficit a battery is being bought to cover. For many households that figure sits between 4 and 8 kWh, well below the capacity commonly quoted. Sizing above it buys resilience, which is a legitimate goal, but it should be priced as resilience rather than as savings.

Round-trip efficiency and standby losses are the quiet cost. Every cycle loses something to conversion, and the inverter itself draws power overnight simply to stay awake and monitor the array. That standby figure, often a few watts to a few tens of watts, is small per hour and visible over a year, and it is the usual explanation for an app showing consumption at three in the morning from a system that generated nothing.

Packaged off-grid kits bundle panels, inverter, charge controller and storage into a single order, which removes compatibility risk and removes flexibility at the same time. They are worth comparing against a specified build rather than assumed to be cheaper.

Product-level differences are real. The Generac PWRcell, for one, prices and scales differently from the modular alternatives, and the specification that matters depends on whether the goal is backup during outages or daily cycling for tariff arbitrage.