Backup Battery Sizing

Key concepts

• Usable capacity (kWh): The energy you can actually use from the battery. Manufacturers may list total capacity; usable is often less because a reserve is kept to protect the battery. Size your expectations on usable kWh.
• Inverter power (kW): The maximum power the system can deliver at once. If your critical loads add up to 3 kW, the inverter must be rated at least that high or some loads cannot run together.
• Round-trip efficiency: The fraction of energy you get back when you charge and then discharge. Losses occur in charging, conversion, and discharging. Typical systems are in a range that makes runtime calculations straightforward if you use usable kWh and average load.
• Critical loads: The circuits or appliances you choose to back up (e.g., fridge, lights, internet, furnace blower). The sum of their power draw and the hours you want to run them determines how much capacity you need.

Step-by-step sizing framework

1. Define critical loads. List every device or circuit you want to power during an outage and estimate or look up watts for each. Add them for the worst case when several run at once. That total is your peak power (kW).
2. Choose target runtime. Decide how many hours you want those loads to run on battery (e.g., overnight, or until the grid returns). Runtime goals vary by region and how often outages last.
3. Estimate energy needed (kWh). Multiply average power (kW) of your critical loads by hours. For example, 1.5 kW average for 8 hours is 12 kWh. Use usable capacity, not nameplate, and account for round-trip efficiency if you want to be conservative.
4. Match inverter to peak load. The battery’s inverter must support the peak power of all critical loads running at once. If your peak is 3 kW, the inverter should be rated at least 3 kW (and often more for motor surges).

Common mistakes

• Using total nameplate capacity instead of usable capacity when planning runtime.
• Ignoring inverter limits: a large kWh battery with a small inverter cannot run a high total load.
• Assuming the battery will run the whole house; most systems back up only selected circuits.
• Expecting multi-day runtime from a single small battery without solar recharge; for long outages, multiple units or a generator may be more realistic.

Cost drivers (general)

Battery cost scales with capacity and inverter size. Installation, electrical work, and any panel or meter upgrades add to the total. Incentives and utility programs can reduce net cost in some areas. Compare quotes and clarify what is included (permits, commissioning, warranty).

Safety and permitting

Grid-connected battery systems must be installed per local electrical and fire codes and utility interconnection rules. Proper disconnects and labeling are required. Have a licensed installer handle design and installation and verify requirements with your building department and utility.

FAQ