Grid Independence
Grid independence is the percentage of your daily energy needs met by solar and battery storage, without drawing from the utility grid. A 45% grid independence score means your system covers nearly half your consumption — the rest still comes from the grid. True 100% off-grid operation requires both an oversized solar array and enough battery capacity to cover multi-day cloudy periods, which is expensive for most homes.
TOU Arbitrage — The Real Financial Case for Batteries
In most states, the financial justification for home batteries isn't backup power — it's TOU arbitrage. Utilities like PG&E, SCE, ConEd, and Eversource charge peak rates of 35–50¢/kWh in the evening (when your solar panels have stopped producing). A battery charges at 8–15¢/kWh overnight and discharges during that expensive peak window, capturing a 20–35¢/kWh spread on every kilowatt-hour shifted.
Backup Duration — Managing Expectations
A 13.5 kWh battery sounds like a lot until you run the numbers: a central AC unit alone draws 3–5 kW, meaning it can drain a Powerwall in 3–4 hours. The practical approach during outages is load management — turn off the HVAC, skip the dryer and stove, and a single battery can run refrigerator, lights, and devices for 1–2 days. The Backup Mode tab in this calculator shows exact runtime for common appliances.
The 30% Federal Tax Credit
Standalone battery storage systems (not paired with solar) qualify for the 30% Residential Clean Energy Credit under the Inflation Reduction Act through 2032. On a $10,000 Powerwall, this reduces your effective cost to $7,000. Several states layer additional incentives on top: California's SGIP program offers up to $1,000 per kWh, and New Jersey's CEF program provides $200/kWh for qualifying systems.