Buying a home battery is mostly a sizing problem. Get the size right and the system quietly carries your home through an outage. Get it wrong and you either pay for capacity you never use or watch the battery shut off at 2 a.m. This guide walks you through sizing a home battery backup system in five steps, using the two numbers that matter: kWh (how long it runs) and kW (how much it can run at once).
kWh vs kW: the two numbers that decide everything
Kilowatt-hours (kWh) measure stored energy. A 10kWh battery can, in theory, supply 1,000 watts for 10 hours or 2,000 watts for 5 hours. This number decides how long your backup lasts.
Kilowatts (kW) measure power output. The inverter’s continuous rating tells you how many appliances can run at the same time, and the surge rating tells you whether it can start motors such as a well pump or central air conditioner.
A system needs enough of both. A huge battery with a small inverter can run a fridge for days but cannot start your AC. A big inverter with a small battery can start the AC but will drain in an hour.
Step 1: Decide what you want to back up
Most homes fall into one of three groups:
- Essentials only: fridge, freezer, lights, internet router, phone charging, a few outlets. Usually 5 to 10kWh per day.
- Essentials plus comfort: add a furnace blower or window AC, microwave, well pump, TV. Usually 10 to 20kWh per day.
- Whole home: everything, including central AC, electric water heater or EV charging. Often 25 to 40kWh per day or more.
Your utility bill shows your average daily use. Divide the monthly kWh by 30 to get a realistic whole home number.
Step 2: Add up running watts
List what must run at the same time and add the running watts from each appliance label. Typical values:
| Appliance | Typical running watts | Typical daily energy |
|---|---|---|
| Refrigerator | 150 to 300W | 1 to 2kWh |
| Chest freezer | 100 to 200W | 0.5 to 1.5kWh |
| LED lights (whole house) | 100 to 300W | 0.5 to 1.5kWh |
| Wi-Fi router and modem | 20 to 40W | 0.5 to 1kWh |
| Furnace blower | 400 to 800W | 2 to 6kWh |
| Well pump (1 HP) | 1,000 to 1,500W | 1 to 3kWh |
| Central AC (3 ton) | 3,000 to 4,000W | 15 to 30kWh |
| Microwave | 1,000 to 1,500W | 0.3 to 0.5kWh |
These are typical ranges. Your own appliance labels, or a plug-in energy meter, will always give a better number.
Step 3: Check surge (starting) watts
Motors draw two to three times their running watts for a split second when they start. Well pumps, AC compressors and refrigerators are the usual culprits. Your inverter’s surge rating has to cover the biggest motor starting while everything else is already running. A soft starter on the AC can cut its startup draw considerably and is often the cheapest way to make a central AC work on battery.
Step 4: Calculate battery capacity
Take your daily kWh and multiply by the number of days you want to cover without sun or grid. Then add about 15 to 20 percent for inverter losses and a reserve you do not want to drain completely.
Example: essentials plus a furnace blower use about 12kWh per day. For one full day of backup: 12 × 1.2 = roughly 14 to 15kWh of battery.
If you add solar, the panels refill part of the battery every day, so you can cover longer outages with less stored capacity.
Step 5: Match the system to your numbers
Under 5kWh: portable power stations
For essentials and short outages, a portable power station plugged into a transfer switch is the simplest option. The Anker SOLIX F3800 stores 3,840Wh with 6,000W of 120V/240V output, and the EcoFlow DELTA Pro 3 stores 4,096Wh with 4,000W of 120V/240V output. Both can add expansion batteries later. Browse all portable power stations.
5 to 20kWh: expandable power stations and small home systems
Pair a power station with expansion batteries, or start a modular home system. The Anker SOLIX E10 starts at 6kWh with stackable 6kWh modules and 10kW to 30kW of turbo output.
20kWh and up: whole home backup
For central AC, well pumps and the whole panel, look at whole home backup systems. The EcoFlow DELTA Pro Ultra X delivers 12kW per inverter (scalable to 36kW) and grows from 12kWh to 180kWh with 6kWh battery packs.
DIY solar builds
If you are building your own system, combine a hybrid or off-grid inverter with 48V home energy storage batteries. Server rack and wall-mount LiFePO4 batteries typically store about 5kWh per 48V 100Ah unit, so three of them give you roughly 15kWh. Not sure which inverter type you need? Read hybrid vs off-grid solar inverter.
Quick sizing cheat sheet
| Goal | Battery (kWh) | Inverter (kW) |
|---|---|---|
| Fridge, lights, internet for a day | 4 to 6 | 2 to 3 |
| Essentials plus well pump and furnace | 10 to 15 | 4 to 6 |
| Most of the house, no central AC | 15 to 25 | 6 to 10 |
| Whole home with central AC | 25 kWh and up | 10 to 15 and up |
Frequently asked questions
How many kWh does it take to run a house for a day?
It depends on the home, but many US homes use somewhere around 25 to 35kWh per day. Check your utility bill: divide the monthly kWh by 30 for your own number. Backing up only essentials usually needs a fraction of that.
Can a home battery run central air conditioning?
Yes, if the inverter can handle the compressor’s surge and the battery is large enough for the run time you want. A soft starter helps a lot. Systems such as the EcoFlow DELTA Pro Ultra X are built for large AC loads.
Do I need solar panels with a home battery?
No. A battery can charge from the grid and simply wait for an outage. Solar lets it recharge during long outages and lowers your electric bill the rest of the time.
Do I need an electrician?
Any system that connects to your home’s electrical panel, including transfer switches and smart panels, should be installed by a licensed electrician and may need a permit in your area.
Still not sure what size you need? Email support@frozena.shop with your appliance list and we will help you compare systems. Every order ships free in the US with free 30-day returns.