How to Size a Home Battery Backup From Your Real Measured Loads

By Nacho Iniguez ✦ Updated June 11, 2026

Most battery backup advice starts at the wrong end. It tells you to add up nameplate wattages, pick the biggest number, and buy a unit rated above it. That overshoots on power and undershoots on energy, and it ignores the one figure that actually decides how long the lights stay on during an outage: watt-hours per day.

The honest way to size a backup battery is to measure your real loads, decide how long you want them to run without grid power, and work backward to kWh. This guide walks that path. When you want to skip the spreadsheet, our battery sizing calculator runs the same math for you.

Step 1: Measure each appliance, do not guess

Nameplate ratings are worst-case numbers printed for safety margins, not what a device pulls in normal use. A fridge labeled “700W” might average closer to 120W to 150W over a full day because the compressor cycles on and off. If you size from the label, you pay for capacity you will never use.

Plug a Kill A Watt meter (or any inexpensive plug-in energy monitor) into each critical load and leave it for 24 hours. Read the kWh field, not the instantaneous watts. That cumulative reading already accounts for cycling, standby draw, and duty cycle, which is exactly what you want.

Your “critical loads” list during an outage is usually short:

• Refrigerator and chest freezer
• A few LED lights
• Phone and laptop charging
• Internet router and modem
• Well pump or sump pump, if you have one
• CPAP or other medical devices
• Maybe a window AC or furnace blower

Skip the dryer, electric oven, and whole-home AC unless backup of those is a real goal. They change the answer by an order of magnitude, and most people do not actually want to run them on battery.

For loads you cannot reach with a plug-in meter (hardwired well pumps, furnace blowers), use the published nameplate running watts from the manufacturer and a realistic runtime estimate. Label those as estimates so you remember they are softer numbers than your measured ones.

Step 2: Add up watt-hours per day

Convert every measurement to watt-hours per day (1 kWh = 1,000 Wh) and total them. A typical lights-and-fridge survival load lands somewhere around 2,000 to 4,000 Wh per day. Add a well pump or a window AC and it climbs fast.

A worked example for a modest critical-loads list:

That 3.56 kWh per day is your baseline. Everything from here multiplies off it.

Step 3: Pick your days of autonomy

Days of autonomy is how long you want the battery to carry your critical loads with no recharge at all. A short, common grid blip might be a few hours. A storm that downs lines can run two or three days.

• 1 day: covers most short outages, smallest and cheapest battery.
• 2 days: a sensible default for storm-prone areas.
• 3+ days: for rural homes, frequent multi-day outages, or anyone without a generator backstop.

If you have solar that keeps producing during the outage, you can lean toward the lower end, because the array recharges the battery each day. Without solar, size for the worst outage you realistically expect. We compare this tradeoff in depth in our solar plus battery ROI calculator.

Multiply: 3.56 kWh per day x 2 days = 7.12 kWh of energy you need to deliver.

Step 4: Account for usable depth of discharge

A battery’s rated capacity is not all available. Lithium iron phosphate (LFP) packs, which dominate home backup in 2026, are commonly specified by their makers for deep cycling, but you still leave headroom for inverter conversion losses and cold-weather derating. A practical planning figure is to assume you can use roughly 90 percent of rated LFP capacity, and divide by an inverter efficiency of about 0.90.

Combined, that means your usable fraction is around 0.81 of nameplate. To be safe and keep the math simple, size so your required energy is no more than about 80 percent of rated capacity.

Required rated capacity = 7.12 kWh / 0.80 = 8.9 kWh

So a roughly 9 kWh to 10 kWh battery covers this example home’s critical loads for two full days. Round up, never down, because real outages bring loads you forgot to measure.

Step 5: Size for surge, not just energy

Watt-hours tell you how big the tank is. They say nothing about whether the inverter can start a motor. Refrigerators, freezers, well pumps, sump pumps, and air conditioners all draw a brief startup surge far above their running wattage, because the motor’s locked-rotor current spikes the instant it kicks on.

A fridge that runs at 120W to 150W can spike to 1,200W to 2,200W for a fraction of a second at startup, per appliance and inverter sizing references. Well pumps and AC compressors are worse: locked-rotor current typically runs three to eight times the running current. A 3-ton central AC can demand on the order of 20 kW for a split second to start, which is why most portable batteries simply cannot start one.

This is why a battery has two power numbers: continuous output (what it sustains) and surge output (what it tolerates for a moment). The EcoFlow Delta Pro 3, for example, is rated by the manufacturer at 4,000W continuous and 8,000W surge, which is enough headroom to start a household fridge and a well pump that are not running at the same instant.

Two practical rules:

1. Add up the running watts of everything that could be on at once. Your battery’s continuous rating must clear that.
2. Take your single largest motor load and confirm its startup surge fits under the battery’s surge rating. Stagger startups where you can, since two motors kicking on together stack their surges.

Putting it together

Sizing a backup battery is a sequence, not a single number:

1. Measure daily watt-hours per critical load with a Kill A Watt.
2. Sum them into a daily kWh figure.
3. Multiply by your target days of autonomy.
4. Divide by usable depth (about 0.80) to get rated kWh.
5. Separately confirm continuous and surge power can run and start your loads.

Energy sizing and power sizing are different problems, and a battery has to pass both. A 10 kWh pack that cannot surge to start your well pump is useless during an outage; a 4 kW inverter with only 2 kWh behind it runs the fridge for an hour and quits.

When you are ready to pick a specific unit, our best home backup battery roundup for 2026 ranks current models by usable capacity and surge headroom, and the battery sizing calculator turns your measured numbers into a target kWh in seconds. If time-of-use rates are part of your decision, the TOU arbitrage calculator shows whether a bigger pack pays for itself beyond backup. For more on the fundamentals, browse our guides and reviews.