Battery Backup Capacity: A Framework for Thinking About Your Needs
One of the most common stumbling points in battery backup conversations is that homeowners and electricians are trying to answer a question without the inputs that would make it answerable. "How much battery do I need?" is unanswerable without knowing what you want the battery to run and for how long.
The following framework helps you develop those inputs before you talk to an electrician — so the conversation can move from general questions to specific options.
Step One: Define "Critical" Loads
Every household has a different definition of what's essential during a power outage. The first step is writing down yours — honestly, not aspirationally.
Think through your home circuit by circuit, room by room, and ask: if the grid goes out tonight and I have a battery, what absolutely needs to stay on?
Common answers include:
Medical necessity loads. If anyone in the household uses a CPAP, home oxygen, infusion equipment, refrigerated medication, or any other medically necessary powered device, this is the top priority and should be the first item on the list.
Refrigerator and freezer. Food safety is a significant practical concern for outages beyond a few hours. Most homeowners include the refrigerator as a critical load.
Lighting in key areas. Full house lighting isn't usually necessary — a few circuits covering common areas and one or two bedrooms serve most families adequately during an outage.
Communication and information. Internet router, modem, and a means of keeping phones charged. These keep you connected to information, emergency services, and work during an extended outage.
Heating or cooling. Depending on the season and your system, this may or may not be on the critical list. As discussed in other posts, AC is a heavy load that significantly affects battery duration.
Sump pump or well pump. If your home has a basement sump or a private well with an electric pump, these may be essential.
Step Two: Estimate Duration Goals
Now ask: for how long do you need these loads to run?
This is a more personal question than it might seem. Consider:
- What are typical outage durations in your area? A utility that restores power in two to four hours most of the time is a different planning context than one with documented multi-day outages.
- What seasonal outage scenarios concern you most? A summer afternoon outage with AC on is different from a winter overnight event with heating concerns.
- Are there scenarios where solar recharge would be available during the outage? If so, duration constraints are relaxed considerably.
A homeowner in central Albuquerque where outages typically resolve within a day has different duration needs than a homeowner in a rural area served by a long distribution line where extended outages are more common.
Step Three: Rough Energy Estimation
Once you have a load list and a duration goal, you can begin to think about energy in rough terms. Energy is measured in kilowatt-hours (kWh), which is the product of power (watts) and time (hours).
Appliances have published wattage ratings — often on a label inside the door or on the back of the unit. Multiplying the watts by the hours of operation gives the energy consumption in watt-hours; divide by 1,000 for kWh.
The math gets more nuanced in practice (refrigerators cycle on and off, so their full wattage doesn't run continuously), but this rough exercise gives you a starting order of magnitude for what you're asking the battery to do.
A simple spreadsheet with load, wattage, daily hours of use, and days of duration gives you a total kWh estimate to bring to the electrician. This number doesn't have to be precise — it just has to be in the right ballpark.
Step Four: Acknowledge the Trade-Offs
Before the electrician conversation, be honest with yourself about what you're willing to trade off:
- Are you willing to modify behavior during an outage (turning off non-essential loads to extend battery duration)?
- Is a smaller battery that covers you for a typical outage acceptable, or is multi-day resilience the goal?
- If covering AC is important to you, are you open to a larger system or pairing with a generator?
Homeowners who have thought through these trade-offs in advance have much more productive conversations with electricians, because they can say "here's what I care about, here's how long I need it, and here's what I'm willing to give up."
Ready to Size Your System?
Request a quote and bring your load list — we'll turn it into a specific system recommendation.