Small Solar for Backup: What 400–800 Watts Can Actually Run

With 400 watts of panels feeding a battery, you can count on about 1,500 watt-hours a day in summer and roughly 600 in December. That covers phones, LED lights, a radio, a CPAP without the heated humidifier, and part of a fridge. Double it to 800 watts and a winter day gets you about 1,200 watt-hours, enough for a modern fridge or the small stuff, but usually not both. Neither size will run a space heater, a well pump or central air.

The gap between June and December is the whole story. Here's the math so you can run it for your own house.

01The one formula you need

Panel watts × peak sun hours × 0.75 = watt-hours you can actually put in the battery.

A "peak sun hour" is one hour of full, overhead-strength sunlight. A cloudy six-hour day might add up to two peak sun hours. NREL-based figures put most Midwest states at about 4 to 4.5 peak sun hours a day averaged across the year (Illinois about 4.3, Ohio about 4.1, Michigan about 4.0).

That yearly average hides a lot. December usually gives 40 to 60 percent of what summer does. Across the northern states that means planning on about 2 peak sun hours a day in December, sometimes less, and close to nothing on gray, snowy days.

The 0.75 is a planning haircut. NREL's PVWatts calculator assumes 14 percent losses for a permanent roof system, and that's before the inverter. A folding panel leaning on a lawn chair loses more to bad angles, heat, dirt and battery conversion. Plan on 75 percent and enjoy the good days when you beat it.

02What 400 watts refills in a day

• June, clear (about 5.5 sun hours): 400 × 5.5 × 0.75 ≈ 1,650 Wh
• October, decent day (about 3.5): 400 × 3.5 × 0.75 ≈ 1,050 Wh
• December (about 2): 400 × 2 × 0.75 ≈ 600 Wh
• Overcast December day: a small fraction of that; don't count on it

03What 800 watts refills in a day

• June: about 3,300 Wh
• October: about 2,100 Wh
• December: about 1,200 Wh

Those are averages. One clear cold day in January can beat them, and a week of lake-effect gray can drop way below them. Your battery covers the bad days, and the panels catch it back up on the good ones.

04What the loads actually cost

Here's what typical outage loads use over a day. Check the label or a plug-in watt meter for your own stuff.

• Refrigerator: Energy Star models average about 493 kWh a year, roughly 1,350 Wh a day. Older or side-by-side units can run a lot higher.
• CPAP, no heated humidifier: usually 15 to 30 watts, or 120 to 240 Wh for an 8-hour night. Turn the humidifier and heated hose on and that can double or more.
• LED bulbs: a 60-watt-equivalent LED pulls about 9 watts. Three of them for 5 hours is about 135 Wh.
• Phones: a full phone charge is well under 20 Wh. Charging four phones is a rounding error.
• Space heater: a standard plug-in heater on high pulls 1,500 watts. One hour eats more than an 800-watt array makes on a December day.

05Putting it together: a December day

With 400 watts (about 600 Wh coming in):

• CPAP, no humidifier, one night: 200 Wh
• Three LED lights, evening: 135 Wh
• Phones, radio, a tablet: 80 Wh
• Left over: about 185 Wh

That's a comfortable light-and-communication setup, and it keeps a CPAP user sleeping. It does not keep a fridge going. Run the fridge off the battery and you'll drain it in a day or two, with the panels only putting back about half.

With 800 watts (about 1,200 Wh coming in):

• Fridge, plugged in 24 hours: about 1,350 Wh

That's already more than the panels made. The fix most people use is to run the fridge in shifts: a few hours on, a few off, doors shut. That cuts its draw a lot and leaves room for lights and phones. Use a fridge thermometer and keep the inside at 40°F or below. Our post on what in the fridge has to go on day two covers the food-safety side. If insulin lives in that fridge, read insulin when the fridge is off too.

In summer, 800 watts covers a fridge and everything small with room to spare. Fall and winter are what you need to plan for.

06What it won't run

Small solar is a trickle. These loads need a generator or a much bigger system:

• Space heaters, electric heat and heat pumps
• Well pumps (big starting surge, see starting vs. running watts)
• Central air and window units for more than short bursts
• Electric stoves, ovens, dryers and water heaters

We go through why in what a power station won't run. For heat in a winter outage, the plan is one warm room, not solar.

07Getting more out of fall and winter sun

The sun sits low from October through February and the days are short. A few things help a lot:

1. Tilt the panels steep. The usual rule for winter is your latitude plus about 15 degrees. Across most of the Midwest that's somewhere around 55 to 60 degrees, nearly upright. Steep panels also shed snow.
2. Face them due south and chase shadows. A low sun throws long shadows. A tree that's harmless in July can shade your panels all afternoon in December. Walk the yard at noon now and find a clear spot.
3. Move them at lunch. With portable panels you can turn them toward the sun once or twice a day. It's free output.
4. Brush off snow and frost first thing. Even a thin layer cuts output hard.
5. Buy more panel than the battery's input limit. Panels rarely make their full rating in winter, so 600 watts of panels on a 400-watt input still helps from November to February. Check the spec sheet first.
6. Watch the voltage limit. Panel voltage goes up when it's cold. If you wire panels in series, stay well under the power station's max input voltage, or a bitter clear morning can push it over.

08Keep the battery warm and the cables out

Most lithium batteries shouldn't be charged below freezing, and many power stations will slow or stop charging when they're cold. Keep the power station inside the house and run the panel cable through a cracked window or door with a towel stuffed in the gap. Panels don't mind the cold. Batteries do.

Before you rely on it, check the station against recall lists and store it the way the maker says. See battery pack recalls and storing lithium safely.

09What it costs (as of October 2026)

Brand-name 400-watt folding panels run about $600, and third-party 400-watt folding panels list for roughly $175 to $370. You'll also need a power station or battery with enough capacity to ride through a night and a couple of gray days. We compare them in power station vs. generator. Panels and a battery cost money up front, but they run quietly indoors with no fuel and no carbon monoxide.

10Do this before the first ice storm

• Find the sunniest south-facing spot in your yard at noon this week, before the leaves are all down and the shadows get long.
• Put a watt meter on the fridge for 24 hours and write the number on the fridge door.
• Run the formula with 2 sun hours, not 5. If the December number covers your must-run list, you're set. If it doesn't, cut the list or add panels.
• Do a test day: unplug the fridge or CPAP from the wall and run it off the panels and battery on a cloudy weekend.

The main thing to remember: size it for December, not June.

Related: Midwest Power Outages: How Long They Really Last (NIPSCO's August Storm)

Sources (6)

• NREL PVWatts Version 5 Manual (14% default system losses)
• Unbound Solar: Peak sun hours by state (NREL/NSRDB data)
• Solar Size Calculator: Peak sun hours and seasonal variation
• SolarReviews: How many watts does a refrigerator use
• Sleep Backup Lab: CPAP battery sizing
• EcoFlow 400W Portable Solar Panel

Photo: Defense.gov News Photo 110906-N-YO394-014 - Explosive Ordnance Disposal Technici by Petty Officer 2nd Class Paul D. Williams, U.S. Navy, Public domain.