Home Battery Storage: Compare Usable Capacity With Installation Scope

Size a home battery around the loads you need during an outage and the utility rate problem you are trying to solve. The nameplate kWh is only one input. Usable energy, continuous output, motor-start capability, backup controls, electrical work, and the utility’s interconnection rules determine what the system can actually do. A battery that can store energy for evening use may not run an entire electric home through a long outage.

Tesla’s Powerwall 3 specifications list 13.5 kWh nominal energy, output options up to 11.5 kW, and a 10-year warranty; the same page says multiple units and expansion units can increase capacity. Read the manufacturer specification. Tesla’s support page also identifies the Gateway or Backup Switch and describes solar charging and outage operation. See the installation documents. These are product facts for a named system, not a delivered installation price.

Make a load and capacity worksheet

List each desired backup load, its running watts, starting watts if it has a motor, and hours per day. Separate “must run” circuits such as refrigerator, medical equipment, internet, lights, and a gas-furnace blower from optional loads such as an electric range, resistance heat, pool pump, or EV charger. Sum the running watts to test continuous output. Multiply each load’s watts by hours, add them, and divide by 1,000 for daily kWh. Then allow for inverter losses, reserve, weather, and the fact that a battery’s usable rating is not the same as its total chemical capacity.

A labeled example

Assume a backup panel contains a refrigerator averaging 100 W for 12 hours, a modem and router totaling 20 W for 24 hours, LED lighting averaging 60 W for 5 hours, and a furnace blower averaging 400 W for 4 hours. Daily energy is (100×12 + 20×24 + 60×5 + 400×4) ÷ 1,000 = 3.58 kWh. Add a hypothetical 15% conversion and reserve allowance: 3.58 × 1.15 = 4.12 kWh. Two outage days would therefore require about 8.23 kWh of usable energy under these assumptions. Actual duty cycles, starting surges, temperature, battery age, and backup settings can change the result.

Power is a separate check. A refrigerator compressor and furnace blower may start at more than their running wattage. Ask the installer to document starting capability and which circuits are backed up. Do not infer whole-home backup from a large kWh figure.

Price the scope, not just the cabinet

Request separate lines for battery, inverter if not integrated, gateway or transfer equipment, critical-load panel, wiring, permits, interconnection, labor, wall or pad work, monitoring, sales tax, and optional solar integration. The installation document says installation should be performed by a Tesla Certified Installer and lists Gateway equipment. That scope explains why a web product price cannot represent a finished backup system.

For bill savings, record the utility’s export compensation and time-of-use rates. A battery may shift energy from a cheap hour to an expensive hour, but round-trip losses reduce delivered kWh. For resilience, compare the value of specific backed-up circuits with the cost of extra capacity. Check current state and utility incentives separately; do not subtract a 30% federal residential battery credit from a new September 2026 installation without current IRS eligibility. The IRS Form 5695 instructions state that residential clean-energy expenditures after December 31, 2025 fall outside the ended credit rules.

The next useful step is an installer quote with a one-line electrical diagram, load list, usable kWh, continuous and surge output, warranty conditions, and utility approval path. Compare those lines across bids before comparing battery brand names.