Battery basics

A battery lifts your self-consumption: daytime solar is used in the evening instead of being exported cheaply. Five terms are enough to read offers.

Capacity: nominal vs usable

A battery has a nominal capacity (e.g. 13.8 kWh) and a smaller usable capacity. The difference is the depth of discharge (DoD) — you never fully empty a battery, to protect it. In an offer, look at the usable kWh figure, not just the nominal one.

Chemistry: LFP vs NMC

• LFP (lithium iron phosphate): safer, very long cycle life, cobalt-free. The home-storage default.
• NMC: higher energy density but hotter-running and shorter-lived. Usually not the first choice at home.

Almost all current home batteries are LFP — which is good.

SoH and warranty

SoH (State of Health) = remaining capacity vs new. Warranties guarantee a minimum remaining capacity at end of term (e.g. "at least 70% after 10 years"). Ask for years and guaranteed cycles.

Modular = future-proof

Many batteries (e.g. Sungrow SBR, RCT, Sigenergy, SMA) are modular: start small and expand later in steps. If you might add an EV or more load later, modularity is valuable.

Sizing — the simple rule

As rough guidance: usable battery kWh ≈ your kWp (or a bit less). For a house with ~4,000 kWh base use + heat pump + possibly an EV, 10–15 kWh is usually sensible. Too big adds little (rarely full in winter, rarely empty in summer); too small wastes solar surplus.

In short: watch usable (not nominal) kWh, LFP is the default, check warranty in years + cycles, buy modular if you might expand, and match the battery roughly to the array size.

Now we switch to heat: how a heat pump works.