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Sizing residential battery storage

You size battery storage off two things: the critical load (or your daily consumption) and how much autonomy you want, measured in hours or days. The base formula is kWh = daily_consumption · autonomy_days / (DoD · η_inverter). DoD here is the usable depth of discharge, which runs around 80-90% for LiFePO4 and only 50% for lead-acid, while inverter efficiency lands at 90-95%. A few products to anchor the numbers: the Tesla Powerwall 3 at 13.5 kWh, the BYD Battery-Box Premium HVS that stacks from 5 to 25 kWh, and the LG Chem RESU 9.8. Homes tend to go with LiFePO4 (lithium iron phosphate) chemistry because it's safer thermally and good for 6,000+ cycles at 80% DoD, where lead-acid taps out near 500. Work an example: a 10 kWh/day load, 1 day of autonomy, DoD 0.9, η 0.95, and you get 10 / (0.9 · 0.95) ≈ 11.7 kWh.

Applications

Off-grid backup when the power goes out. Peak shaving, where you bank cheap off-peak energy and discharge it once the peak tariff kicks in. Bidirectional EV charging through V2H/V2G. And a self-consumption boost for PV systems in places where net metering is capped or just not worth much.

FAQ

How long does a LiFePO4 battery last? Figure 10-15 years of useful life and 6,000+ cycles at 80% DoD. By the time the warranty runs out it still holds 70-80% of its nameplate capacity.

Do I need batteries with net metering? Not if your goal is a smaller bill, since the grid is already doing the job of a virtual battery. They earn their keep for backup, peak shaving, or anywhere the compensation hits a ceiling.

What's the cost per kWh? As of 2026, a residential turn-key LiFePO4 system runs roughly USD 600-900 per kWh installed, and that figure keeps sliding about 10% a year.