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.

The fine print that changes your sizing math

Three details buried in spec sheets and warranty documents can quietly break a sizing plan that looks perfect on paper.

Voltage, not just watts. Most well pumps and central AC units in US homes run on 240V, while many portable power stations output 120V only. If your largest load is 240V, the sizing math above is irrelevant until you confirm the battery has a 240V output or supports pairing two units for split-phase, which manufacturers list on the spec sheet and which roughly doubles the budget.

Cold-weather charging cutoffs. LFP cells will not accept a charge below freezing, and manufacturer spec sheets typically list a charge range starting at 32°F unless the unit has built-in cell heating. Discharge keeps working in the cold, but a battery sitting in an unheated garage during a winter outage may run down and then refuse to recharge from your generator or panels until it warms up. If winter storms are your outage scenario, check the charging temperature line before the capacity line.

Warranty throughput clauses. Home battery warranties are rarely a plain number of years. Many are written as years or a cycle count or a total energy throughput figure, whichever comes first, per the warranty documents themselves. If you plan to cycle the battery daily for time-of-use savings on top of backup duty, that throughput cap arrives years earlier than the calendar date, which effectively shrinks the capacity you can count on late in the battery’s life. Oversizing by one step protects both the outage math and the warranty math.

FAQ

How many kWh do I need to back up just a fridge, lights, and Wi-Fi?

Plan on roughly 2 to 3 kWh per day. ENERGY STAR listings put modern full-size refrigerators at about 1 to 2 kWh per day, and a router, modem, phone charging, and a few LED lights add several hundred watt-hours more. A 3 kWh battery covers one day of that load with margin; double it for a two-day storm.

Can a home battery start my central air conditioner?

Usually not without help. A 3-ton central AC can demand on the order of 20 kW for an instant at compressor startup, per HVAC locked-rotor references, which exceeds the surge rating of nearly every portable unit and most single wall-mounted batteries. A soft-start kit installed on the condenser cuts that inrush dramatically and is the standard fix installers quote for backup setups.

Do I need a transfer switch, or can I run extension cords from the battery?

Extension cords work fine for plug-in loads like the fridge, lamps, and electronics, and they cost nothing extra. Hardwired loads such as a well pump, furnace blower, or ceiling lights require a transfer switch or critical-loads subpanel installed by an electrician, and backfeeding a panel without one is both illegal and dangerous to line workers. Budget for that installation if your critical list includes anything without a plug.

How long will a 10 kWh battery actually run my house?

Divide usable capacity by your measured daily load. Using the roughly 80 percent usable planning figure from this guide, a 10 kWh pack delivers about 8 kWh, so a 3.5 kWh per day critical-loads list runs a bit over two days, while a household averaging the US norm of about 29 kWh per day, per the EIA’s residential consumption data, would drain it in under seven hours. That gap is exactly why sizing from measured critical loads beats sizing from your utility bill.

Is it cheaper to just buy a generator instead?

For pure outage insurance, usually yes: a portable inverter generator costs a fraction of a comparable battery and runs as long as you have fuel. Batteries win on silence, indoor safety, zero maintenance, and instant automatic switchover for things like a CPAP, and they can earn money year-round on time-of-use rates where a generator only sits in the garage. Many storm-prone households end up with a small battery for instant coverage plus a generator to recharge it during long outages.