Home batteries aren't complicated at their core. Here's a plain-English explanation of what's happening inside the box.
The four main components
A modern home battery system has four key parts:
- The battery cells — where energy is actually stored, typically lithium iron phosphate (LFP) chemistry
- The battery management system (BMS) — electronics that monitor each cell's voltage, temperature, and state of charge to keep the battery operating safely
- The inverter — converts DC electricity (from solar panels and battery) to AC electricity (used by your appliances)
- The energy management system (EMS) — the "brain" that decides when to charge the battery, when to discharge it, and whether to export to the grid
The daily cycle
On a typical sunny day, here's what happens:
Morning. The sun rises and your solar panels start generating electricity. If you're running appliances, the solar powers them directly. If generation exceeds consumption, the surplus starts charging the battery.
Midday. Peak solar generation. If the battery is already full, surplus energy is exported to the grid at the feed-in tariff.
Afternoon. As the sun gets lower, generation drops. The battery continues charging from any remaining surplus.
Evening. After 4–5 pm, solar generation typically drops below household consumption. The battery starts discharging to power your home instead of drawing from the grid.
Night. Once the battery is depleted (or reaches its minimum state of charge, typically 10–20%), the home switches to grid power for the remainder of the evening.
AC-coupled vs DC-coupled systems
There are two main ways a battery connects to your solar system:
DC-coupled means the battery connects directly to the solar panels via a hybrid inverter, before the DC power is converted to AC. This is more efficient (no conversion losses) and is the approach used by systems like Tesla Powerwall 3 and Sungrow SBR.
AC-coupled means the battery has its own inverter and connects to your home's AC circuit. This is more flexible — you can add a battery to any existing solar system regardless of what inverter it uses. Enphase IQ batteries use AC coupling.
What is backup power?
Many batteries can provide backup power during a grid outage. When the grid goes down, the battery and solar system form a local "island" that continues powering your home.
Not all batteries support backup mode, and those that do may only back up a sub-panel (selected circuits) rather than your whole home. Tesla Powerwall and Enphase IQ batteries are well-regarded for their backup capability.
Battery capacity vs usable capacity
You'll see batteries marketed with a nominal capacity (e.g., 13.5 kWh) but only a slightly lower "usable" capacity. This is because batteries are not discharged to 0% or charged to 100% — doing so accelerates degradation. The BMS maintains a buffer at both ends.
When comparing batteries, always use the usable capacity figure for sizing calculations.