Modular or Single-Unit Home Battery: How Much Room to Expand?

A modular battery lets you add capacity later, a single unit is simpler and fixed. The right choice depends on how sure you are about your future demand, and on limits that are easy to miss: the inverter, the warranty and how many modules the maker allows.

When expandability is worth having

Expandable systems suit households whose demand is likely to change: an electric car, a heat pump or a growing family. If you know your needs and they are unlikely to change, a single unit sized to your use avoids paying for room you never use. Our battery capacity guide and the home battery calculator help you size the first purchase.

Remember that more capacity is not always more savings. A battery larger than your daily usable demand only stores energy you will not use.

How makers describe expansion

Tesla. The UK datasheet for the Powerwall 3 lists up to three Expansion units per system, each with 13.5 kWh. Expansions are battery only, and a system can combine up to four Powerwall 3 units and three Expansions, for up to 94.5 kWh in total. The datasheet gives AC charge power configurable up to 5 kW for a single unit and up to 8 kW with Expansions.

GivEnergy. The company's battery page lists high-voltage stackable batteries in 10.2, 13.6, 17.0 and 20.4 kWh configurations and speaks of a stackable architecture for flexible scaling. The page does not say how many modules are allowed, or whether different generations can be mixed, so check the datasheet for your inverter.

Fogstar. Fogstar lists a 5.12 kWh module (51.2 V, 100 Ah) that can be stacked, and describes capacity as scalable. The page does not state a maximum number in parallel. It lists pre-loaded protocols for Luxpower, Victron, Sunsynk and Solis, among others.

The limits that catch people out

Capacity and power are different things, as our kW vs kWh guide explains. Adding modules adds energy storage, while the output you can draw depends on the inverter or inverters. Tesla's datasheet shows the point: output is stated per Powerwall 3 unit, while Expansions are battery only.

• Inverter limit. Check the maximum number of modules the inverter supports and its power rating.
• Warranty. Fogstar's warranty runs six years from the shipment date, with ten years of technical support, and excludes modification and use with an incompatible charging system. We could not confirm how it treats a module added later, so ask before you buy.
• Mixing old and new. Not every maker says whether different generations or capacities can share a stack. If the datasheet does not say, ask the maker in writing.
• Space and weight. A stack grows taller and heavier with each module. Allow room now for the full planned size.

Questions to ask before you buy

Ask your installer which modules the inverter will accept today and in three years, whether the same model will still be sold, and who is responsible for the warranty if you add modules yourself. A fixed unit sidesteps these questions. For how the voltage type affects this, see high-voltage versus low-voltage batteries.

Common questions

Related guides

Keep reading, or check your own numbers:

• High-voltage versus low-voltage home batteries
• Battery capacity guides
• Home battery storage