Solar Backup Power Supply for Home: How to Size It for Outages (2026)

By Lauren Holland

Updated November 2026

A solar backup power supply for home can keep your refrigerator cold, your CPAP running and your sump pump moving when the grid goes down. The key is matching the system to what you actually need to run, for how long, and how much sunlight you can count on. This guide walks you through a simple watt-hour calculation, the factors that change the numbers, and the mistakes that leave people short during a real outage.

What a Solar Backup Power Supply Can and Cannot Do

A solar backup power supply usually combines a battery-based power station with one or more solar panels. The battery stores energy, the inverter converts it to household AC, and the panels recharge the battery when the sun is out. Some setups also accept a wall or vehicle charge as a backup.

Shopping for one? See our guide: Top 5 EcoFlow Power Stations with Solar Panel Options for Home Backup (2026).

Keep the limits in mind. A portable power station rated at 1,000 watt-hours will not run a central air conditioner for an afternoon, and most units cannot start large motors without a surge margin. Power stations are best for essential loads: lighting, phones and laptops, routers, a few medium appliances, and select medical devices. For an overview of what these units handle well, see Home Backup With a Solar Generator: What It Can Power and How to Connect Safely.

Step 1: List Your Essential Loads

Start with a list of everything you want to keep running. Be specific about what you truly need during an outage, because every extra item increases the battery size and cost of the system.

• Refrigerator or chest freezer
• CPAP or other medical device
• Sump pump or well pump
• Internet router and modem
• Phones, laptops and tablets
• LED lights
• Furnace fan or space heater (often impractical on a small system)

If a medical device is involved, confirm its power needs with the manufacturer and your medical provider. Medical equipment decisions should be made with a qualified professional.

Step 2: Find the Wattage and Daily Run Time of Each Load

Look for the wattage on the appliance label or in the manual. Refrigerators often list amps and volts instead; multiply amps by volts to get watts. Then estimate daily run time in hours.

Watt-hours per day equals watts multiplied by hours per day. A few common examples:

These numbers are ballpark. Refrigerators in particular vary widely with age, ambient temperature and how often the door opens. Use your appliance’s energy label or a plug-in watt meter for better accuracy if you can.

Step 3: Add the Daily Total and Apply Losses

Add your watt-hour figures to get a daily total. Then account for system losses. Inverters, battery conversion and wiring all lose some energy, so the usable capacity is less than the rated capacity. A practical rule is to divide the total by about 0.85 to cover typical inverter and conversion losses.

Example: a refrigerator at 1,000 Wh per day, a CPAP at 300 Wh, a router at 300 Wh and lights at 200 Wh adds up to 1,800 Wh. Divided by 0.85, that is roughly 2,120 Wh per day of battery capacity needed.

Step 4: Decide How Many Days of Backup You Need

Most short outages last hours, but storms can knock out power for several days. Decide how many days you want to cover before you buy. Multiply your daily requirement by that number.

For a 2,120 Wh daily load and a two-day target, you would need about 4,240 Wh of usable storage. Many households choose one to three days. For guidance on sizing across different outage lengths, read Home Backup Power Supply: How to Size One for Outages (2026).

Step 5: Size the Solar Panels for Recharging

Storage alone does not solve a multi-day outage. You need enough solar input to refill the battery each day. A useful estimate is:

1. Take your daily watt-hour use, for example 2,120 Wh.
2. Divide by peak sun hours for your location. Many parts of the US average roughly 4 to 5 peak sun hours on a typical day, but winter months are lower. Use a conservative figure of 3 to 4 for planning.
3. Divide by a system efficiency factor around 0.75 to account for panel temperature, angle, dust and charge controller losses.

Using 2,120 Wh, 3.5 peak sun hours and 0.75 efficiency gives about 810 watts of panel capacity. Panels rated around 800 to 1,000 watts total would be a reasonable starting point. For panel-specific choices, see 6 Best Solar Panels for Home Backup Power Picks Compared (2026).

Also check the power station’s maximum solar input. Panels rated above that limit may be clipped, so match the array to the controller’s input spec.

Step 6: Check Surge and Inverter Capacity

Battery size and wattage are separate limits. Your inverter must handle the running watts of everything that runs at the same time, plus the surge when motors start. A sump pump or well pump can draw two to three times its running watts at startup. Confirm the power station’s continuous output and surge rating exceed your combined peak load with some margin.

If you need to run more than a few large appliances, you may be looking at a whole-home approach. See Whole-Home Backup Power Solution: How to Size One for Outages (2026).

Factors That Change Your Sizing

• Season: Winter days are shorter and sun angles lower, so panel output drops.
• Location and shade: Trees, roof pitch and orientation affect real-world harvest.
• Temperature: Batteries perform less efficiently in extreme cold or heat.
• Battery chemistry and depth of discharge: Usable capacity is often stated as a percentage of rated capacity.
• Load growth: Adding a space heater or second refrigerator later can break your original plan.

Practical Checklist Before You Buy

1. Written list of essential loads with wattage and hours
2. Daily watt-hour total with a 15 percent loss margin
3. Target number of backup days
4. Panel wattage matched to local peak sun hours
5. Inverter continuous and surge ratings checked against startup loads
6. Transfer method decided (manual plug-in or automatic transfer switch with an electrician)
7. Outlets or circuits that will connect to the system identified

Common Mistakes

• Ignoring surge loads. A sump pump that starts fine on the grid can trip an undersized inverter.
• Using rated capacity as usable capacity. Losses and depth-of-discharge limits reduce what you actually get.
• Assuming full sun every day. Plan for cloudy stretches, not perfect conditions.
• Overloading one circuit. Power stations often have multiple outlets with different limits.
• Connecting to house wiring improperly. Backfeeding the grid is dangerous to utility workers. Use a properly installed transfer switch or a licensed electrician.

Tips for a Reliable Setup

• Test the system under load once a season, not for the first time during a storm.
• Keep panels clean and check mounting hardware after high winds.
• Store the battery within the manufacturer’s recommended temperature range.
• Plan which appliances to turn off first to conserve energy.

Recommended gear

A plug-in watt meter helps you replace estimates with measured numbers for each appliance. Look for one that displays watts, watt-hours and run time.

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Portable solar panels let you add recharge capacity as your needs grow, and many fold for storage. Match wattage and connector type to your power station.

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A transfer switch allows a generator or solar system to safely feed household circuits. This is installed by a licensed electrician and is the safest route for whole-circuit backup.

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FAQ

How many watt-hours do I need for a refrigerator during an outage?

Most standard refrigerators use roughly 800 to 1,500 watt-hours per day, depending on age, size and ambient temperature. Measuring with a plug-in watt meter over 24 hours gives the most accurate figure.

Can a solar backup power supply run a CPAP overnight?

Many CPAP machines draw 30 to 60 watts, so a modest battery can cover a full night. Confirm the device’s power draw and consult your sleep provider about backup planning.

How much solar do I need to recharge daily?

It depends on your daily watt-hour use and local peak sun hours. Divide daily use by sun hours and a efficiency factor of about 0.75 to estimate panel wattage.

Will a sump pump work on a portable power station?

Often yes, but the surge at startup must be within the inverter’s rating. Check the pump’s locked rotor amps and compare to the station’s surge specification.

Do I need an electrician to connect solar backup to my house?

For plugging appliances directly into the power station, no. For feeding household circuits, a licensed electrician should install a transfer switch to prevent backfeeding the grid.

How long do batteries last in a home backup system?

Lifespan depends on chemistry, depth of discharge, temperature and cycle count. Manufacturers specify cycle ratings, and actual results vary with use.

Is a solar backup power supply enough for a whole house?

Usually not for central air, electric dryers or electric ranges. Most households size for essentials and a few priority circuits rather than the entire home.

Conclusion

A solar backup power supply for home works best when you size it from your real loads rather than from the biggest number on a box. Add up watt-hours for essentials, apply a loss margin, choose your target number of backup days, and size panels for recharging. Keep surge capacity and transfer safety in mind, and test the system before you need it.

About Lauren Holland

Lauren Holland is part of our editorial team and reviews and edits our solar generators for home backup & emergencies guides, checking product details against manufacturer specifications and owner feedback.