How Battery Backup Sizing Works: A Homeowner's Guide to the Conversation
When homeowners start researching home battery systems, they often expect to find a clear answer: "a system this large covers a home this size." That answer doesn't exist, and once you understand why, the path to the right system becomes clearer.
Battery backup sizing is determined by two things: how much power you want to keep running, and for how long. Both of those are personal decisions that vary enormously from one household to the next. An electrician can't determine them without your input — but you can walk into that conversation prepared.
Start With Your Critical Loads
A critical load is any appliance or circuit that matters most to you during an outage. Your list will be different from your neighbor's.
For some households, keeping the refrigerator and a few lights running is all that matters. For others, it's a medical device that cannot go without power. For others still, it's a home office with a router, a few computers, and a monitor — because working from home doesn't stop when the grid does.
Before meeting with an electrician, spend ten minutes writing down what you'd genuinely miss in an outage. Be specific: not "the kitchen" but the refrigerator, the microwave, one or two countertop outlets. Not "comfort" but a specific fan or space heater.
That list becomes the foundation of the sizing conversation.
Think About Duration Goals
Once you know what you want to power, you need to think about how long. Four hours gets you through a brief daytime outage. Overnight is a different requirement. Three to five days — common in serious weather events — is a different requirement still.
Duration goals interact with your load list directly. Keeping a refrigerator running for 24 hours requires significantly less storage capacity than keeping a refrigerator, a well pump, a router, and several lights running for the same period.
Be honest about your realistic scenario. If you're in an area with a reliable grid and occasional short outages, your duration goal can be modest. If you're concerned about extended outages — whether from summer thunderstorms, winter storms, or general grid vulnerability — plan for more.
What Affects Sizing
An electrician doing a proper sizing assessment will look at a few things:
Your existing electrical panel. The panel's current configuration affects how a battery system connects and which loads can be served. In some cases, a dedicated critical load subpanel is part of the installation — this is the mechanism that isolates the circuits you want backed up from those you don't.
Whether you have solar. A battery paired with solar can be recharged during the day, which fundamentally changes the duration equation. A standalone battery draws down over time; a solar-plus-battery system can sustain itself through multi-day outages under the right conditions.
The appliances on your critical load list. Motors — refrigerators, well pumps, HVAC compressors — draw a burst of power at startup called inrush current. The battery system needs to be able to handle that surge. An electrician can look up or estimate these values for your specific appliances.
There's No One-Size-Fits-All Answer
You'll see battery systems described in kilowatt-hours (kWh). That number represents stored energy capacity, but it doesn't tell you how long the system will last — that depends entirely on what you're running. The same battery could last a day in one home and three days in another.
This is why manufacturers and installers both resist giving a simple answer. The honest answer requires knowing your load list and duration goal. Anyone who skips those questions and quotes you a system size on the phone is guessing.
Questions to Bring to Your Electrician
- What's on my critical load list, and how does it translate to power demand?
- What duration of outage am I planning for, and is my list realistic for that duration?
- Does my panel need any modifications to support a battery installation?
- If I'm adding solar at the same time, how does that change the sizing?
- What battery brands do you work with, and what are the warranty terms?
A good electrician will work through these questions with you rather than presenting a pre-packaged system. The sizing process is a conversation — your job is to show up prepared.
Request a quote and we'll start with your critical load list.