A home battery system has two jobs: store enough energy to last through an outage (that's your battery, measured in kWh) and deliver enough power at once to run what you turn on (that's your inverter, measured in kW). Size both correctly and your lights, fridge and well pump keep running. Get either one wrong and the system shuts off when you need it most. Here's how to size yours in four steps.
Step 1: Decide what you want to back up
This single decision drives your cost more than anything else.
- Essentials only: refrigerator and freezer, lights, internet and phone charging, furnace blower, a well pump or sump pump, and a few outlets. This is the most affordable option and covers what most families truly need during an outage.
- Whole home: everything above plus central air conditioning, an electric range, dryer or water heater. These big loads multiply the battery and inverter size you need.
- Off-grid: the system is your only power source every day, so it must cover your full daily use plus a reserve for cloudy days, and it needs solar panels to recharge.
Step 2: Estimate how much energy you use (battery size)
Battery capacity is measured in kilowatt-hours (kWh). To estimate how many you need, add up each appliance's daily energy use. The fastest way is to check the label on each appliance for its wattage, then multiply by how many hours a day it actually runs.
| Appliance | Typical running watts | Typical daily use |
|---|---|---|
| Refrigerator (modern) | 100 to 250W while running | 1 to 2 kWh |
| Chest freezer | 50 to 150W while running | 0.5 to 1.5 kWh |
| LED lighting (whole house) | 100 to 300W | 0.5 to 1.5 kWh |
| Internet modem, router, phone charging | 20 to 50W | 0.5 to 1 kWh |
| Gas furnace blower fan | 300 to 800W | 2 to 6 kWh in winter |
| Well pump (240V) | 750 to 2,000W | 1 to 3 kWh |
| Sump pump | 500 to 1,000W | Varies with rainfall |
| Window AC unit | 500 to 1,500W | 4 to 10 kWh |
| Central AC (240V) | 3,000 to 5,000W | 15 to 40 kWh in summer |
| Electric water heater (240V) | 4,500W | 6 to 12 kWh |
These are typical ranges. Your appliances' labels and your electric bill are always more accurate.
A shortcut: use your electric bill
Your bill shows monthly kWh. Divide by 30 for your average daily use. A home using 900 kWh a month uses about 30 kWh a day for everything. If you're only backing up essentials, plan on a fraction of that.
Fridge 1.5 + freezer 1 + lights 1 + internet 0.5 + furnace fan 4 + well pump 2 = 10 kWh per day
For a two-day outage, plan on about 20 kWh of battery, or 10 to 15kWh paired with solar to recharge during the day.
LiFePO4 home batteries let you use nearly all of their rated capacity, but leave yourself some cushion for cold weather, inverter losses and the unexpected. Adding about 20% to your estimate is a good rule.
Step 3: Find the most you'll run at once (inverter size)
The inverter turns battery power into household AC power, and its rating in kilowatts (kW) is the most it can deliver at any moment. Add up the wattage of everything that might run at the same time, then account for startup surge: motors in well pumps, AC units and refrigerators briefly draw two to three times their running watts when they start.
Fridge 200W + freezer 150W + lights 200W + internet 50W + furnace fan 600W + well pump 1,500W = 2,700W
With well-pump startup surge, a 3.6kW inverter is the minimum and 7kW gives comfortable headroom.
120V or 120/240V?
Many homes need 240V for at least one essential load. Well pumps, central AC, electric dryers, ranges and water heaters usually run on 240V. If any of these are on your backup list, you need a split-phase (120/240V) inverter. A 120V-only inverter works for cabins, sheds and homes where every backed-up load is 120V.
Step 4: Choose grid-tied backup or off-grid
- Hybrid (grid-tied) backup: your home stays on utility power normally, and the system switches over automatically during an outage. With solar, it can also cut your bill. This is the right choice for most homes on the grid.
- Off-grid: no utility connection at all. You need enough solar to recharge every day, enough battery to carry you through cloudy stretches, and often a generator input as backup.
Matching your needs to a system
| Your situation | Battery | Inverter | Complete system |
|---|---|---|---|
| Cabin, shed or small off-grid space (120V) | 10 to 15kWh | 3.6kW, 120V | SUNON3.6 Cabin Starter |
| Off-grid home or essentials with 240V loads | 15 to 30kWh | 7.2kW, 120/240V | SUNON7.2 Off-Grid System |
| Whole-home backup on the grid, with or without solar | 16 to 48kWh | 11.4kW, 120/240V hybrid | RX-12K + Lithi2-16 Hybrid Backup |
| Large off-grid home | 16 to 48kWh | 12kW, 120/240V | SUNON12 + Lithi2-16 Off-Grid |
| Adding storage to an existing system | 32 to 64kWh | Your current inverter | Lithi2-16 Multi-Pack |
Every system in our home backup and off-grid line pairs a matched inverter and battery bank, so the components are designed to communicate with each other out of the box. You can also split larger orders into monthly payments with Shop Pay Installments at checkout.
Common questions
How long will a battery run my house?
Divide your battery's kWh by your daily use. A 16kWh battery backing up 10 kWh of daily essentials lasts about a day and a half without solar. With solar recharging during the day, it can run indefinitely in good weather.
Can I start small and add batteries later?
Yes. The batteries in these systems are modular, so you can add more as your budget allows. Size your inverter for your long-term plan, since it's harder to upgrade later.
Do I need solar panels?
Not for short outages. A battery charged from the grid will carry your essentials through most outages. For outages longer than a day or two, or for off-grid living, solar is what keeps the battery going.
Want us to size it for you?
Tell us what you want to power and for how long. We'll recommend the right battery and inverter, free and with no pressure.
Call 833-664-7433
or email info@homesteadsupplyonline.com