Batteries: sizing & coupling

Storage is its own design problem with two independent numbers and a couple of wiring choices. Get these straight and battery sizing stops being mysterious. (For the “why” and the economics, see the batteries & backup guide; this module is the design mechanics.)

Two numbers: power and capacity

• Power (kW) — how much it can deliver at once. This decides what it can run simultaneously (does it have the surge to start your AC?).
• Capacity (kWh) — how much energy it holds, i.e. how long it lasts.

A battery can have plenty of kWh but too little kW to start a big load — size both to the job.

Scaling capacity: parallel units

With most modern home batteries you add complete units in parallel to get more capacity — each unit is already at the system voltage, so two units ≈ double the kWh at the same voltage. You generally don’t stack cells in series yourself; that’s done inside the factory unit (true series strings are a high-voltage exception). Remember Module 1: parallel adds capacity, series adds voltage.

Hybrid inverter & coupling

A battery stores DC, so it needs battery-capable electronics:

• DC-coupled — battery shares the solar (hybrid) inverter. Slightly more efficient for storing your own solar; cleanest for new installs.
• AC-coupled — battery has its own inverter and ties in on the AC side. Easiest way to add storage to an existing solar system.

Sizing for backup vs. daily use

• Backup: decide whole-home vs. essential circuits (fridge, internet, a few outlets), then size capacity for the hours you want to ride through. Your solar recharges it by day.
• Daily cycling / arbitrage: size to store your typical daily surplus so you can use it at night or sell into evening peaks.

Next: put the whole design together and read what the simulation tells you.