Battery round-trip efficiency explained
Energy out ÷ energy in. ~90% for modern lithium home batteries: store 10 kWh of solar, get about 9 kWh back.
Every charge-discharge cycle loses some energy as heat in the battery and its inverter. Modern home batteries are typically rated around 89–92% AC round-trip.
What this means for you: Efficiency matters for daily cycling economics, much less for occasional backup.