Off-grid by project · Emergency backup
Solar backup power for outages: how much battery and solar you need
Backup power doesn't have to run the whole house. Pick the loads that matter in an outage, size a battery for them, and add solar so it refills every sunny day.
At a glance
Step 1: choose your critical loads
Outage backup is about priorities. Medical devices, food, water, heat and communication come first. Typical values from the appliance wattage chart:
Skip electric heat, water heaters, dryers, ovens and central air. They would multiply the battery size; gas appliances, a wood stove or a generator handle those better.
Step 2: size the battery, solar and inverter
Sized with the off-grid planner's math, 400 W panels and 4 peak sun hours:
| Backup plan | Daily use | Battery for 1 day | Battery for 3 days | Solar to refill daily | Inverter (continuous / surge) |
|---|---|---|---|---|---|
| Essentials Fridge, lights, internet, phones, and a CPAP | 2.61 kWh | 3.91 kWh | 11.75 kWh | 3 panels (1.2 kW) | 382 W / 755 W |
| Essentials + winter heat Adds a gas furnace blower so the furnace can run | 4.61 kWh | 6.91 kWh | 20.75 kWh | 5 panels (2.0 kW) | 1,007 W / 1,305 W |
| Essentials + well water Adds a 1/2 hp shallow-well pump | 3.61 kWh | 5.41 kWh | 16.25 kWh | 4 panels (1.6 kW) | 1,632 W / 3,305 W |
Notice the jump from one day to three: 3.91 kWh becomes 11.75 kWh for the essentials. Rather than buying three days of battery, most people size for about one day and let solar refill it, then use a generator for long, cloudy stretches. A well pump adds a large startup surge, so check the inverter can start it while everything else runs (see inverter sizing).
Build your own outage backup
- Daily use
- 2.61 kWh
- Panels to refill daily
- 3 × 400 W
- Inverter
- 382 W
- Startup surge
- 755 W
Uses the same math as the off-grid planner at 4 peak sun hours. A planning estimate, not a design.
Step 3: choose a setup
- Solar panelsMake power in daylight
- Charge controllerCharges the battery safely
- BatteryStores power for night and storms
- InverterTurns battery power into 120V
- Essential loadsCords or a transfer switch
A portable power station packs the controller, battery and inverter into one box. Every system still needs proper fusing, and hardwired circuits need a transfer switch installed by an electrician.
Portable power station + panels
Best for: renters and essentials-only backup.
How it works: plug appliances into the station with extension cords; folding panels recharge it.
Limits: usually 1–4 kWh per unit; can't power hardwired loads like a furnace or well pump.
Power station or battery + transfer switch
Best for: homeowners who want hardwired essentials such as the furnace or well pump.
How it works: an electrician installs an inlet and a transfer switch or interlock feeding selected circuits.
Limits: professional installation and often a permit.
Home battery system
Best for: frequent or long outages, and homes that already have or want solar.
How it works: a wall or rack battery with a backup-capable inverter runs a critical-loads panel automatically.
Limits: the most expensive option; designed, permitted and installed by professionals.
Battery + generator hybrid
Best for: long storms, winter outages and well water.
How it works: the battery covers nights and short outages; the generator recharges it during long stretches without sun.
Limits: fuel storage and generator maintenance.
- Never back-feed a house through a wall outlet or dryer outlet. It can electrocute line workers and start fires.
- Run generators outdoors only, far from windows and doors, and keep a working carbon monoxide alarm inside.
- Use extension cords rated for the load, and don't run them under rugs or through closed doors.
- For medical devices, check the manufacturer's guidance and plan more than one backup source.
- Hardwired connections, transfer switches and home battery systems need a licensed electrician. See what's safe to DIY.
FAQ
Do my grid-tied solar panels work during a power outage?
Usually not. Grid-tied inverters shut off automatically when the grid goes down so they can't back-feed the lines. To use solar during an outage you need a battery system or an inverter with a backup (islanding) mode.
How big a battery do I need for a power outage?
For essentials like a fridge, lights, internet and a CPAP, plan on roughly 3.91 kWh of advertised battery per day of backup. Solar that refills the battery each day lets a smaller battery last much longer.
Can a portable power station run my refrigerator?
Yes. An efficient full-size fridge uses about 1.2 kWh a day, so a 2 kWh power station typically runs it for a day or more and a 1 kWh unit for part of a day, longer with solar recharging. Check that its continuous and surge ratings cover the fridge's startup.
Can I plug a power station or generator into a wall outlet?
No. Back-feeding through an outlet with a double-ended cord can electrocute utility workers and start fires. Use extension cords to individual appliances, or have an electrician install an inlet with a transfer switch or interlock.
Is a generator better than batteries for outages?
They work best together. Batteries are silent and instant for short outages; a generator covers long storms and cloudy stretches. Never run a generator indoors or in a garage.
Planning for winter storms? Read off-grid solar in winter for low-sun sizing and cold-weather battery limits, and the battery sizing guide for more on backup goals.