How Much Battery Backup Do I Need?

The Short Answer, by Outage Length

These rows use the worked example below: a fridge labeled 600 kWh a year, a 20W modem and router, about 100 Wh a day of lights and phone charging, and 20W of self-consumption. Swap in your own labels; the unit column is a planning estimate, as Step 3 explains.

Step 1: List What Has to Stay On

Write down only the loads you’d miss during the outage you’re planning for. A typical list holds the refrigerator, the modem and router, a few lights and phone chargers. Add a freezer, a CPAP machine, a sump pump or a well pump if your house has one. Leave off anything you can live without for the length of the outage, because every item on the list costs battery.

Step 2: Turn Each Load Into Watt-Hours

Watt-hours are watts times hours. A 20W router running 24 hours uses 480 Wh. Steady loads are easy: read the watts on the device or its power adapter and multiply.

Refrigerators cycle on and off, so their nameplate watts overstate a day’s use. The yellow EnergyGuide label gives estimated kWh per year instead. Divide by 365: a fridge labeled 600 kWh a year uses about 1,644 Wh a day. If your fridge has no label, a plug-in watt meter reads the real number after a day.

Step 3: Add Losses and Self-Consumption

A battery never delivers its full rated capacity to your outlets. Bluetti’s Apex 300 manual gives the formula we use for planning: usable energy is capacity times 95 percent depth of discharge times about 90 percent inverter efficiency, and the unit itself draws about 20W while it runs. Ninety-five percent of 90 percent is 0.855.

So the worked example runs like this for one day:

Round up to about 3,200 Wh. Three days is three times 3,163, about 9,500 Wh, before any solar recharging. The formula is Bluetti’s, written for the Apex 300, and its figures are approximate, so treat every result as a planning estimate.

Step 4: Check Output Separately

Add the running watts of everything that might be on at once, then add the startup draw of the largest motor. Bluetti’s manual says motors and compressors may start at 2 to 4 times their rated power. A fridge that runs at 150W can ask for 600W for a moment.

The 120V units in the table put out 1,800W (DELTA 3 Classic) to 2,600W (Elite 400) continuously, three or more times that fridge’s startup, with room left for the router and lights. A well pump or central AC needs a unit that makes 240V and enough surge to start the motor. The DELTA Pro 3 makes 120V and 240V from the unit and is rated by EcoFlow for an 8,000W surge, so we pick it for well pumps. The other two 240V units we cover, the Bluetti Apex 300 and the Anker SOLIX F3800 Plus, have no published surge figure to check a pump against.

Step 5: Decide How You’ll Recharge

A battery sized for three days has to come back up before the next outage, and during a long one it has to come back up while the grid is down. Solar input decides how fast. The Elite 400 accepts 1,000W of solar, the HomePower 2000 Plus v2 800W, and the DELTA Pro 3 2,600W. Jackery’s figure for the v2 base unit is a 3.2-hour refill at its full 800W. That makes the expanded Jackery in our three-day row a stored-energy plan: it reaches 9,500 Wh with packs, but its 800W solar input is the lowest of the three figures above, and refilling 9,500 Wh at 800W takes at least 12 hours of full-rate sun. If panels will carry you through the outage, the DELTA Pro 3’s 2,600W refills much faster. All six units are compared in our best home battery backup systems guide.

Planning for a smaller home? Our best battery backup for small homes guide applies this math to four units. For panel-level backup of a whole house, see the whole-home battery backup guide.

Frequently Asked Questions