A battery backup system keeps your essential home circuits running during grid outages without the expense or complexity of a full off-grid solar setup. By moving critical loads—like your refrigerator, internet router, and a few lights—to a dedicated subpanel, a battery and inverter can seamlessly take over when the power drops. You remain fully connected to the grid for your daily high-power needs, but gain targeted resilience when blackouts strike.
For full grid independence, see off-grid solar cost by system size. For a system that also reduces daily bills with solar, see hybrid solar.
What “Battery Backup Only” Means
A backup-only system is optimized for emergencies, not daily utility bill reduction.
- Targeted Resilience: You power a critical loads panel (fridge, lights, modem, medical device) during grid outages. You do not attempt to run the whole house off the battery.
- Grid Reliance: When the grid is up, you use grid power as usual. The battery sits in standby mode, kept at a full state of charge.
- Flexible Charging: The battery is primarily kept full by the grid. Adding a small solar array allows the battery to recharge during prolonged multi-day outages, but the solar isn't sized to offset your daily household consumption.
How the Hardware Works
To achieve seamless backup without going off-grid, the system relies on a specific wiring architecture:
- Hybrid Inverter/Charger: This unit connects to the grid, the battery, and the critical loads panel. When the grid is active, it passes grid power through to the loads and keeps the battery topped up. When the grid fails, an internal transfer switch automatically disconnects from the grid (islanding) and begins pulling power from the battery in milliseconds.
- Critical Loads Subpanel: An electrician moves the breakers for your essential circuits from your main breaker box to this new subpanel. This physically isolates your backup loads from heavy appliances like electric ovens or central AC, ensuring your battery isn't instantly drained.
- Solar Integration (Optional): If you add solar panels, they wire directly into the hybrid inverter's charge controller. During an outage, the sun can recharge the battery while it simultaneously powers your critical loads.
Crucial Details to Consider for Home Backup
Many homeowners buy a battery based only on its total capacity, but real-world backup sizing requires accounting for hidden drains and electrical realities:
- Inverter Standby Consumption: Inverters consume power just by being turned on. A 5,000W hybrid inverter might draw 40W to 60W continuously. Over a 24-hour outage, that standby draw alone will eat 1 to 1.5 kWh of your battery capacity, even if you don't turn on a single light.
- Motor Surge (Starting Watts): Refrigerators, freezer compressors, and well pumps require two to three times their running wattage for a split second when they start. Your inverter must be sized to handle this instantaneous surge, not just the running watts.
- Shared Neutral Wiring: In older homes, electricians sometimes shared neutral wires across different circuits (multi-wire branch circuits). Moving only one of these circuits to a critical loads panel can cause dangerous wiring faults or tripped breakers. An electrician must identify and handle these carefully.
Illustrative Sizing Example: A 24-Hour Outage
Let's size a battery backup system for a homeowner who wants to survive a 24-hour blackout. Note: This is an illustrative example; actual wattages vary by appliance model.
Step 1: Calculate Daily Energy (Wh)
- Refrigerator: 150W running (cycles on 50% of the time) = 1,800 Wh/day.
- Internet router & modem: 20W continuous = 480 Wh/day.
- LED lighting (4 bulbs): 40W total for 6 hours = 240 Wh/day.
- Phone/Laptop charging: 100 Wh/day.
- Inverter standby draw: 40W continuous = 960 Wh/day.
Total daily energy needed: 3,580 Wh (3.58 kWh).
Step 2: Calculate Maximum Power (Watts)
- Running watts: ~250W (if everything runs at once).
- Maximum surge watts: 1,200W (fridge compressor starting) + 60W (router and lights) = 1,260W.
Step 3: Size the Equipment
- Inverter: A 2,000W or 3,000W inverter easily handles the 1,260W surge.
- Battery: To get 3.58 kWh of usable energy without draining a lithium iron phosphate (LiFePO4) battery past 80% depth of discharge, you need:
3.58 kWh Ă· 0.80 = 4.47 kWhof total battery capacity. A standard 5 kWh server rack battery is a perfect fit.
If this homeowner wanted to survive a 3-day outage without grid power, they would either need three 5 kWh batteries, or they could add a small 1,000W solar array to recharge the single battery each day.
Cost Estimates (2026 Ballpark)
Because you are only backing up critical loads, the hardware costs are a fraction of a whole-home off-grid system:
- Battery: A 5 kWh LiFePO4 server rack battery costs roughly $1,200–$1,800.
- Hybrid Inverter/Charger (3kW–5kW): $800–$1,500.
- Critical Loads Panel & Electrician Labor: $800–$2,000 (highly dependent on local labor rates and main panel complexity).
- Solar (Optional): $1,000–$2,000 for a small 1–2 kW array to extend runtime.
Total Estimated Cost (Grid-charged only): $2,800 to $5,300.
Frequently Asked Questions
Can I use my existing grid-tied solar panels to charge the backup battery during an outage?
Usually, no. Standard grid-tied solar inverters (like older string inverters or basic microinverters) are legally required to shut down during a blackout to prevent backfeeding power into the grid and injuring lineworkers. To use solar during an outage, you must upgrade to a hybrid inverter that can "island" your home, or AC-couple your existing solar into a compatible battery system (like a Tesla Powerwall or Enphase IQ Battery).
How do I prevent my central air conditioner from draining the backup battery?
The most foolproof method is physical isolation. By leaving the heavy 240V breaker for your central AC in your main electrical panel, and only moving your 120V critical circuits (fridge, lights) to the backup subpanel, it is physically impossible for the AC to draw power from the battery during an outage.
Is a transfer switch required if I only want to back up a few outlets?
If your inverter is hardwired to your home's electrical panel, a transfer switch (or an inverter with a built-in automatic transfer switch) is legally required by the National Electrical Code to prevent grid backfeeding. If you are just using a portable "solar generator" power station and plugging your fridge directly into the front of the unit with an extension cord, no transfer switch is needed.
What happens if the grid is down for several days and I don't have solar panels?
If you only have a grid-charged battery, your backup power will simply run out once the battery is depleted (e.g., after 24 to 48 hours, depending on your loads). To survive multi-day outages, you must either add solar panels to recharge the battery daily, connect a gas generator to the hybrid inverter's generator input, or dramatically reduce your power usage to stretch the battery's capacity.
Size your backup with the WattSizing calculator (use only backup loads and desired autonomy), and read hybrid solar and days of autonomy to align battery and optional solar with your goals.
