A 16kWh wallmount battery, a common whole-home backup size class, runs a typical fridge overnight and barely notices. The fridge alone would take about a week to drain it. A window unit cooling one bedroom makes it to morning with charge left over. The wifi could run for a month. The answer that stings is central AC: a typical central unit pulls that same battery down in about four hours, so a battery-only plan for whole-house cooling quits before breakfast.
Those answers stretch from hours to a month off the same battery because of one division, and the bottom number is yours to change.
The only math in this guide
Battery size in kilowatt-hours, divided by the average load you put on it in kilowatts, equals hours of runtime.
16kWh ÷ 0.4kW = 40 hours. 16kWh ÷ 4kW = 4 hours. Same tank, different drain.
The word doing the quiet work there is average. Almost nothing in your house runs flat out. A typical fridge draws 100W to 200W while the compressor runs, then cycles off, so across an evening it averages far less than its label suggests. A window unit cycles the same way. Runtime cares about the average across the hours you are counting, and averages come in lower than nameplate arithmetic.
Two honest deductions before you trust the napkin. The inverter takes a small cut, and you should keep a reserve rather than riding the tank to zero, so do the division and round down. Every number here is a typical range, and your appliance's own label beats any table on the internet, including ours.
Two houses, one battery
Both examples run on round hypothetical numbers, labeled typical, against that same 16kWh battery.
House one backs up the essentials: the fridge, LED lights (a few watts per bulb), a couple of ceiling fans at 30W to 75W each, the modem and router at 10W to 30W, and the phone chargers. Add the cycling up honestly and that load averages around 400W. The division says 16kWh ÷ 0.4kW = 40 hours on paper. Round down for the inverter's cut and a reserve, and you still have a solid day and a half before the sun contributes anything. Add a window unit for the night (window units and mini-splits run from hundreds of watts to a couple kW) and the evening average might land near 1kW. That is about 16 hours, through a hot night with margin.
House two backs up everything, central AC included. A typical central unit draws 3kW to 5kW while it runs, and on an August evening it runs a lot. Stack the rest of the house on top and the average sits around 4kW. The division says four hours. The battery did nothing wrong. The division did exactly what it always does.
That gap is the whole critical-loads conversation. A short list of circuits on a modest battery rides out a long outage. The full house on the same battery is a sprint. The critical-load backup kit is built for the first story, and the whole-home backup kit is what the second one takes.
Catch one: motors hit harder than their label
A well pump or an AC compressor pulls several times its running draw for a second or two at startup, so a pump that draws 1kW or more while running asks for far more in that first moment.
The surge is mostly an inverter problem. The battery's kWh number is the tank, and the inverter's kW number is a ceiling on any single moment. You can have forty hours of runtime in the tank and still fail to start the well pump if the surge asks for more than the inverter can pass at once. When your keep-running list has a motor on it, the inverter gets sized to the surge and the battery gets sized to the hours. Skip the first decision and the system disappoints on day one.
Catch two: panels change the whole question
Everything above is battery-only math, a countdown that only goes down. Pair the battery with panels and a hybrid inverter and it becomes a cycle instead. The panels carry the daytime loads and refill the tank while the sun is up, and the battery carries the night. An essentials-sized load can ride through a multi-day outage this way. The question shrinks from "how long does 16kWh last" to "can the battery carry the night," and a 400W load overnight is a few kWh against a 16kWh tank.
A Florida weather note. Heavy storm clouds cut solar production way down, so the battery does most of the work while the weather is still on top of you. The recharge earns its keep in the days after, when the sun is back and the grid is still out. That cycle is the entire logic behind an off-grid system, and it is why solar-plus-battery houses measure outages in days.
What this means for your build
Almost everyone guesses their essentials load high. The fridge feels like a big appliance and draws like a light bulb. The loads that actually eat a battery are the ones that make heat or cool the whole house, and most of those can sit out a backup plan without anyone suffering.
Measuring beats guessing. Your power bill's monthly kWh divided by 720 gives your whole-house average draw, seasons included. For the keep-running list, add up running watts with honest hours, or put a plug-in power meter on the fridge for a day. A battery sized to a measured essentials load, plus a margin, is usually smaller and cheaper than the one sized to the guess. The system builder runs this against real equipment, and if you are earlier than that, the estimate is the two-minute version.
What Ape Solar would check first
Three things, before any equipment talk. Your power bill, because twelve months of real kWh beats every assumption here. Your keep-running list, because that list sizes the battery. And whether anything on that list has a motor, because one well pump can set the inverter size for the whole build.
We design the system and run the project. A state-licensed contractor does the electrical work.
A note on safety
Backing up circuits means work at your main electrical panel, and that is a job for a licensed electrician, permitted and inspected through your local building department. This guide is for planning and buying smart. It is a starting point for a conversation, never a set of instructions.
FAQ
Will a battery run my fridge overnight?
Easily. A typical fridge uses 1kWh to 2kWh across a full day, so a night is well under 1kWh. Against a 16kWh battery that is small change.
Can I run my AC all night on a battery?
A window unit or a mini-split cooling one room, yes, with sizing. Central AC is the hard case: at a typical 3kW to 5kW running draw it empties a modest battery in a few hours. Whole-house cooling on battery takes a big bank and a big inverter, which is why many storm-country homes cool one room and let the rest wait for the grid.
How long will a battery run a CPAP?
Longer than you would think. Most CPAP machines draw tens of watts, more with a heated humidifier, so a full night is typically well under 1kWh. Even a small battery covers many nights. Check your machine's label, and tell us when sizing, because a medical load moves a design from nice-to-have toward must-hold.
Can a battery really last for days?
On its own, only if the load is tiny. Paired with panels, days is the normal case for an essentials load, because the array refills the battery every day the sun cooperates. The battery only has to bridge the nights and the worst of the cloud cover.
How do I find my actual essentials number?
Add up the running watts of your keep-running list with honest hours, or put a plug-in power meter on the biggest ones for a day. Measured numbers run smaller than guessed ones, and they buy a smaller battery.
Talk to Ape Solar before you guess on battery size. We will run this division against your real loads, and if a smaller battery covers you, we will say so.