What Size Solar Generator Do I Need? A Worked Sizing Method

Size a solar generator (a portable power station) by energy, not watts. List what must run, multiply each item’s watts by its hours of use, and add the results. Divide that total by about 0.85 to cover inverter losses. Then confirm the unit’s rated output and surge can handle your largest appliance starting up.

How do you calculate what size solar generator you need?

Five steps get you from a list of appliances to a capacity number. Keep a notepad or spreadsheet open.

Step 1: List what must run and for how long

Be specific about hours. A refrigerator runs all day, lights might run five hours, and a CPAP runs overnight. A coffee maker might run ten minutes.

Step 2: Find each appliance’s watts

Look for the nameplate on the back or bottom. A U.S. Department of Energy lesson notes that the label shows the maximum an appliance draws, not what it draws all the time. A refrigerator, for example, only uses its rated amount while the compressor is on.

For fridges, the yellow EnergyGuide label’s annual kWh figure is a better guide than the nameplate. Divide it by 365 for a daily number.

Step 3: Multiply watts by hours

The formula is energy equals power multiplied by time. In practice: watt-hours = watts x hours. A 9 W LED bulb on for 5 hours uses 45 Wh.

Step 4: Allow for inverter losses

Converting battery DC to household AC wastes some energy as heat. EcoFlow’s runtime guide recommends multiplying battery capacity by 0.80 to 0.90 for AC loads as a first estimate. We use 0.85, so divide your daily watt-hours by 0.85 to get the battery size you need.

DC loads, such as USB charging, skip the inverter. EcoFlow notes DC output typically extends runtime compared with AC.

Step 5: Check output and surge

Your largest appliance, plus anything else running at the same moment, must stay under the rated AC output. Motors and compressors need a short startup burst, which is what the surge rating covers.

A 1,500 W space heater, such as Lasko’s 754200 (listed at 900 to 1,500 W), needs at least 1,500 W of rated output. That rules it out on some small units and sits right at the limit of the Jackery Explorer 1000 v2’s 1,500 W rating.

Worked example: one outage day

This example covers essentials for a household during a 24-hour outage. Replace our figures with your own.

Load	Watts used	Hours	Wh per day	Source for the watts
Full-size refrigerator	about 52 W average	24	1,252	ENERGY STAR median, 457 kWh/yr
Four LED bulbs	9 W each (36 W)	5	180	DOE lighting guide
Modem, router and phone charging	25 W average	24	600	EcoFlow sample test log
CPAP	30 W average	8	240	EcoFlow sample test log
Total			2,272

Now apply the efficiency allowance: 2,272 Wh ÷ 0.85 = about 2,673 Wh of battery.

That is more than any single 2 kWh unit in our solar generator comparison holds. You have three options. Drop a load, add an expansion battery, or recharge during the day from solar panels. Our solar generator home backup guide covers expandable systems and transfer switches, and what size solar generator runs a refrigerator shows how many panels a fridge needs.

Where the refrigerator number comes from

We pulled all 2,963 full-size refrigerators from the ENERGY STAR certified dataset in September 2026. The median rating was 457 kWh per year. The middle 80% ranged from 297 to 685 kWh per year, and top-freezer models had a lower median of 363 kWh.

These are lab test ratings. Room temperature and door openings change real use, so treat them as a starting point.

Leave headroom for battery aging

Cycle ratings end at a capacity percentage, not at zero. EcoFlow rates the DELTA 3 for 4,000 cycles to 80%+ capacity. BLUETTI rates the AC180 for 3,500+ cycles to 80%. Jackery rates the Explorer 1000 v2 for 4,000 cycles to 70%+.

So a unit near the end of its rated life holds 70 to 80% of its original energy. Adding about 20% to your target capacity keeps the plan working for years, not just on day one. That margin is our suggestion, not a maker requirement.

Quick sizing by goal

• Camp lighting: four 9 W LED bulbs for 5 hours use 180 Wh, or about 212 Wh after losses. A 1 kWh unit covers about four nights of that, or fewer if you also charge phones.
• A CPAP for several nights: at 30 W average and 8 hours per night, about 280 Wh per night after losses. See our 1000Wh runtime guide for the full breakdown.
• A fridge and internet through a day-long outage: around 2,400 Wh by our math, so the 2 kWh class plus solar, or larger. See what a 3000 watt solar generator will run for the 3 kWh class.
• Heating or cooking loads: these need high rated output and drain batteries fast. A 1,500 W heater on high empties the roughly 850 usable Wh in a 1 kWh unit in about 34 minutes.

Solar generators by capacity on eBay

Browse current portable power stations on eBay, new condition only, from $200.

Shop portable power stations→

• 1000Wh and up
• LiFePO4
• With solar panel

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Does battery chemistry change the size you need?

Not the math, but it changes how long the capacity lasts. LiFePO4 units carry cycle ratings in the thousands, which matters if you will cycle the unit often. Our guide to LiFePO4 vs NMC solar generators explains the tradeoffs. To compare brands at your target size, see EcoFlow vs Jackery vs Bluetti.