Battery Backup Sizing Calculator
Add your own appliances with their exact wattage, or use the presets, to find exactly how much battery capacity you need to keep them running.
Backing Up What Matters, Not Everything
The instinct when sizing a home battery is to back up the whole house, but that's rarely the cheapest or smartest approach. A full-home backup for a few days can require 30-40 kWh of storage, which gets expensive fast. Most homeowners get far better value backing up a defined critical circuit — fridge, lights, WiFi, a few outlets — and accepting that big draws like central air or an electric oven stay off during an outage.
This calculator works off a simple checklist so you can see exactly how adding or removing one appliance changes your battery requirement. Air conditioning and water pumps are usually the biggest single swings — including a 1-ton AC unit can more than double the battery size needed compared to a fridge-and-lights-only setup, which is worth seeing before you commit to a spec.
Depth of Discharge — the Number Most People Skip
Battery capacity on a spec sheet isn't all usable. Lithium batteries are typically rated for 80-90% depth of discharge, meaning manufacturers reserve the last 10-20% to protect long-term battery health. Sizing against the full rated capacity — ignoring depth of discharge — is the most common mistake in DIY battery planning, and it leads to a backup that runs out earlier than expected during an actual outage.
Once you have a target kWh figure, check it against real product specs — most residential battery units (like Tesla Powerwall or LG equivalents) come in roughly 10 kWh increments, so your final unit count usually rounds up rather than landing on an exact number. If you're pairing this battery with a new solar system, size the inverter next using the Inverter Sizing Calculator to make sure it can handle your peak and surge loads.
Sizing for a Typical Outage, Not a Worst-Case One
Most grid outages resolve in a few hours, not days — utility data in most regions shows the median outage lasting well under 8 hours, even though the rare multi-day event is what people remember and plan around. Sizing your battery for a realistic typical outage, then accepting that a genuine multi-day event might mean rationing power to just the fridge and phone chargers, usually gets you a far more reasonable and affordable system than sizing for the worst case you can imagine. If backup during multi-day outages genuinely matters for your situation — medical equipment, well-water pumps, extreme climate — that's a legitimate reason to size larger, just go in knowing you're paying a real premium for a scenario that may rarely happen.
Frequently Asked Questions
How big of a battery do I need to back up my house?
It depends entirely on which appliances you want to keep running and for how long. A fridge, WiFi, and a few lights for 8 hours might need only 3-5 kWh, while backing up air conditioning as well can push that to 15-20 kWh.
What does depth of discharge mean?
Depth of discharge is the percentage of a battery's total capacity that can safely be used. Most lithium batteries are rated for 80-90% depth of discharge, meaning you shouldn't plan to use the last 10-20% to preserve battery lifespan.
Should I back up my air conditioner during an outage?
AC units draw significantly more power than most other appliances, so backing them up multiplies your required battery capacity. Many homeowners choose to back up only critical loads — fridge, lights, WiFi, medical equipment — and skip AC and heating to keep battery costs manageable.
How many battery units will I need?
This depends on your chosen battery's unit capacity. Home battery systems commonly come in 10 kWh units, so a 25 kWh requirement would need three units, rounded up.
What if my appliance isn't on the preset list?
Use the "Add Your Own Appliance" field above the checklist to enter its exact name and wattage from the nameplate or user manual. It's included in the backup calculation exactly like a preset item.