The honest answer is "it depends" — but you can narrow it down fast. Battery size comes down to which loads you want to keep running, for how long, and whether you pair it with solar. Published figures put a typical home around 20 to 30 kWh of daily use, so whole-home backup for a day often lands in the 20 to 40 kWh range, while essentials-only needs far less. Below is a plain-English way to size it without over- or under-buying. Every number here is a starting point — get an itemized quote for your home.
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Work through these in order. Each one moves you from a vague "it depends" to a real kWh target you can take to an installer. Sizing varies by home, loads, and utility — get a quote.
This single choice drives everything. Essentials — lights, fridge, internet, phone charging, a few outlets — need only a few kWh. Whole-home backup that includes central AC, electric heat, or an electric range can need many times more, and often multiple battery units.
Best for: setting your goal before you shop, so you don't over- or under-buy.
The catch: whole-home backup of heavy loads can multiply both the kWh and the cost vs. essentials-only.
List the devices you must keep running and add up their wattage. A fridge, a few LED lights, a router, and phone charging is a modest load; add a window AC, a well pump, or electric heat and the number climbs quickly. This running wattage is the base for everything that follows.
Best for: turning "I want backup" into a concrete watt figure.
The catch: it's easy to forget always-on loads (well pump, sump pump, medical devices) that you'll badly miss in an outage.
Running watts multiplied by backup hours gives a rough watt-hour (then kWh) target. Covering a few essential loads overnight is very different from running much of the house for a full day, which for a typical home commonly lands in the 20 to 40 kWh range.
Best for: matching capacity to a realistic outage length for your area.
The catch: sizing for a worst-case multi-day outage without solar gets very expensive, very fast.
Real systems lose some energy to inversion and round-trip inefficiency, and you don't want to run a battery flat every cycle. Adding roughly 10 to 20 percent on top of your raw kWh target gives a more honest size.
Best for: avoiding a battery that looks big enough on paper but falls short in practice.
The catch: skipping headroom leaves you in the dark sooner than the spec sheet implied.
Capacity (kWh) decides runtime; output (kW) decides what the battery can start and run at the same instant. Motors like AC compressors and well pumps surge hard at startup, so a battery with ample stored energy but low output may not start them at all.
Best for: homes with high-surge motor loads to back up.
The catch: a unit with plenty of kWh but too little kW output can't start a big AC, no matter how full it is.
A battery paired with rooftop solar recharges during the day, so it can cover longer outages with less stored capacity than a grid-only battery. Without solar, your battery must hold the full outage's worth of energy, because it can't refill while the grid is down.
Best for: areas with frequent or long outages — solar pairing reduces the kWh you need to store.
The catch: a grid-charge-only battery stops helping once it's drained and the grid is still out.
Once you have a target kWh, divide by each battery model's usable capacity to get how many units you need. Essentials may fit in one; whole-home or multi-day goals often need two, three, or more. Larger homes targeting a full day of whole-home backup are commonly sized with multiple units.
Best for: translating your kWh target into a real shopping list.
The catch: usable capacity is less than the headline rating — size on usable kWh, not the label.
Use the steps above to land on a kWh target, then get itemized quotes from licensed installers that spell out capacity, output, and the loads the system is sized to back up.
Essential vs. whole-home backup, sizing, and cost ranges for an installed home battery. Prices vary by home and installer — get a quote.
Cheap plug-in box or installed whole-home system? An honest comparison of cost, coverage, install, and runtime.
Whether rooftop solar pays off — and how pairing it with a battery cuts the storage you need for multi-day backup.