Battery Backup Sizing: Keeping the Essentials On Through an Outage

When the power goes, you do not actually need the whole house running. You need the things that make an outage bearable: the fridge holding its cold, a few lights, a fan or two, the router so the phones still work, and a way to charge a phone. Size a battery to do exactly that and you keep life normal for a fraction of the cost of backing up everything. Size it badly and you either run out of power at 9pm or pay for a battery bank you will never fully use. Here is how the sizing actually works.

Step one: decide what “essentials” means for you

Before any numbers, make a list. Be honest about the difference between what you need and what you would like. For most homes the essentials are the fridge and freezer, some lights, fans, the internet router, and phone or laptop charging. Add a water pump if your supply depends on one, or any medical equipment that has to keep running.

The big decision on that list is almost always the air conditioner. We will come back to it, but for now, know that it is the line most people should not cross on a battery backup, because it changes everything about the size and the price.

Step two: add up the energy

Battery capacity is measured in kilowatt-hours, the same unit as your JPS bill, so the goal is to work out how many kilowatt-hours your essentials use in a day.

For each item, take its power draw in watts and multiply by the hours it runs. A 10-watt LED bulb running 5 hours is 50 watt-hours. The one that trips people up is the fridge: it does not pull its full wattage all day, it cycles on and off, so a typical fridge uses somewhere around 1.5 to 2 kilowatt-hours over 24 hours, not its nameplate wattage times 24. Lights, fans and a router are all small. Add them up and a sensible essentials list often lands around 3 kilowatt-hours a day. Yours might be more or less, but that is the order of things.

Step three: decide how long you need to last

Now the question that doubles or halves your battery: how long do you need to ride through? Just the evening peak until the power usually returns? A full overnight? Or a multi-day stretch after a storm?

Multiply your daily essentials by the days you want to cover. Three kilowatt-hours a day for two days is six kilowatt-hours you need to deliver. Simple so far. Then comes the catch that trips most people up.

Step four: usable capacity is less than the label

You cannot use every kilowatt-hour a battery is rated for. Two things eat into it.

The first is depth of discharge, how far you can safely drain the battery. Modern lithium (LFP) batteries let you use most of their capacity, around 80 to 90 percent, while older lead-acid types should only go to about half before you damage them. The second is system losses, in the inverter, the wiring and the conversion from battery to AC power, which together cost you somewhere between 15 and 30 percent. Add those up and a battery’s real delivered energy is well below its sticker number. The simple “capacity divided by load” sum overestimates real runtime by a third or more.

So you size up. If you need to deliver six usable kilowatt-hours, you are realistically looking at a battery nameplate nearer eight to ten, depending on chemistry, to have it comfortably and not kill the battery early. Round up, not down. We cover depth of discharge and the rest when we plan a system.

Step five: the inverter has to handle the surge, not just the average

This one catches people who only thought about the battery. Your battery stores the energy, but the inverter delivers it, and some appliances need a big gulp of power for the first second or two when they start. A fridge compressor can surge to several times its running wattage at startup, and a water pump even more.

If the inverter is sized only for the steady running load, it will trip the instant the fridge kicks in, even with a full battery sitting there. So the inverter has to be sized for the largest starting surge among your essentials, not just their normal draw. Getting this wrong is the classic reason a backup that “should work on paper” keeps cutting out.

The air conditioner question

Here is why we keep steering you away from backing up the AC. An air conditioner is a heavy, motor-driven load that runs for hours, so putting it on your backup multiplies both the battery size and the inverter rating you need, and the cost with them. For the same money that backs up your AC for an hour, you could keep your fridge, lights and fans running for a day or more. Unless cooling through outages is genuinely worth a much bigger spend to you, leave the AC off the critical-loads panel.

Solar changes the maths

If your battery is paired with solar, it does not just sit and drain, it recharges every day the sun is out. That means a modestly sized battery can carry you through a long outage, because each day refills what the night drew down. In a multi-day storm situation, that recharge is the difference between a battery that lasts and one that is flat by the second morning. It is also why backup and solar are so often planned together.

A worked example

Put it together. Say your essentials come to about 3 kilowatt-hours a day, and you want two days of cover. That is 6 usable kilowatt-hours, which after depth of discharge and losses points to a battery rated around 8 to 10 kilowatt-hours. Pair it with an inverter rated to handle the fridge and pump surge, and if you have solar to recharge it, you could stretch well past those two days. Your numbers will differ, but that is the shape of the calculation every time.

What we do

We build this around a critical-loads panel, the set of circuits that stay alive when the power is out, and size the battery and inverter to your real essentials rather than a guess. That keeps you powered through an outage without paying for capacity you do not need. We handle it in battery and inverter support.

Related reading

• Solar with batteries vs grid-tie only for whether you need a battery at all.
• Protecting your electrics through hurricane season for backup as part of storm readiness.
• Battery and inverter support for how we size and install storage.

The next step

A good battery backup is sized to your life, not to a brochure. Get a free quote or message us with the things you cannot stand to lose in an outage, and we will work out the battery, the inverter and the critical-loads setup that keeps them running, without overspending.