Power Station Sizing Guide: How Big a Unit Do You Need for an Outage?

Sizing a power station comes down to two separate questions. People often mix them up:

1. Power (watts): can it run everything you plug in at the same time, including the startup surge from motors?
2. Energy (watt-hours): can it keep doing that for as long as the outage lasts?

A unit can pass one test and fail the other. A 1 kWh station may start a sump pump easily but run out in a few hours, and a big battery with a small inverter can’t start a well pump. This guide walks through both tests. The runtime calculator does the same math for you.

Step 1: List what actually needs power

Ready.gov’s first piece of outage-prep advice is to take an inventory of everything you rely on that needs electricity (Ready.gov). For most homes, the realistic “essentials” list is short:

• Refrigerator and/or freezer
• Sump pump (if you have a basement)
• Furnace blower in winter (gas furnaces still need electricity for the fan and controls)
• Medical devices such as a CPAP
• Internet router/modem, phone and laptop chargers, a few LED lights

Things that are usually out of reach for a portable power station: central air conditioning, electric water heaters, electric ranges, and electric dryers. Champion lists these at 2,000–6,000 W of running power each (Champion).

Step 2: Add up running watts, then add the biggest surge

Use the nameplate on each appliance if you can. Otherwise, start from published typical ranges. These come from Champion’s generator wattage chart:

Appliance	Typical running watts	Typical starting watts
Refrigerator	150–400 W	800–1,200 W
Freezer	100–500 W	500–1,000 W
Sump pump, 1/3 HP	500–800 W	1,000–1,600 W
Sump pump, 1/2 HP	800–1,050 W	1,300–2,150 W
Furnace fan, 1/2 HP	300–800 W	800–1,600 W
Well pump, 1/2 HP	750–1,000 W	1,500–2,000 W
CPAP (no heat / heated humidifier)	30–60 W / 60–120 W	no extra
Internet router/modem	10–20 W	no extra
LED bulb (each)	8–15 W	no extra

Source: Champion Power Equipment, checked October 7, 2026. These are typical estimates, not measurements of your equipment.

Generator makers size with a simple rule: total running watts + the single largest extra starting surge (Honda). Honda also points out that you can avoid stacking surges by starting big motors one at a time. Then compare:

• Your running total against the power station’s rated (continuous) AC output
• Your peak against its surge rating

Step 3: Convert to watt-hours, then add inverter losses

For each appliance:

Fridges, freezers, pumps, and blowers cycle on and off, so their duty cycle is well below 100%. For a fridge, the EnergyGuide label is better: kWh per year ÷ 365 gives kWh per day (see our refrigerator guide).

Then account for the inverter. Converting the battery’s DC to AC wastes some energy. StormWatts assumes 85% efficiency across the site (a stated assumption; real values vary by model and load):

Step 4: Match to a capacity tier

Tier	Examples in our dataset	Rated capacity
~1 kWh	BLUETTI AC180, Jackery Explorer 1000 v2, EcoFlow DELTA 3 Plus, Anker SOLIX C1000 Gen 2	1,024–1,152 Wh
~2 kWh	Jackery Explorer 2000 v2	2,042 Wh
~3.6–4 kWh	Jackery HomePower 3600 Plus, Anker SOLIX F3800 Plus, EcoFlow DELTA Pro 3	3,584–4,096 Wh
Expanded systems	Add-on batteries for the DELTA 3 Plus, DELTA Pro 3, HomePower 3600 Plus, or F3800 Plus	Up to 12 kWh per DELTA Pro 3 unit; the F3800 Plus takes up to 6 expansion batteries (manufacturer figures)

Leave some headroom. Batteries lose capacity as they age (manufacturers rate cycle life “to 70%” or “to 80%” of original capacity), and cold weather can reduce what you get.

Worked scenario A: overnight essentials

A fridge rated at 600 kWh/yr, a router, 4 LED bulbs for 5 hours, phone/laptop charging for 3 hours, and a CPAP (no heat) for 8 hours:

Load	Running W	Wh per day
Refrigerator (600 kWh/yr label)	~275 (typical estimate)	~1,644
Router/modem, 24 h	15	360
4 LED bulbs × 12 W × 5 h	48	240
Chargers, 60 W × 3 h	60	180
CPAP, 45 W × 8 h	45	360
Total	~443 W running	~2,784 Wh

• Peak: 443 W + the fridge’s extra start (1,200 − 275 = 925 W) ≈ 1,368 W. Every model in our dataset is rated well above that.
• Battery needed: 2,784 ÷ 0.85 ≈ 3,275 Wh for a full day, or about 1,640 Wh for 12 hours.
• Verdict: a ~2 kWh unit covers a 12-hour night with room to spare. A full day without recharging points to the ~3.6–4 kWh tier, or to a smaller unit plus daytime solar charging.

Worked scenario B: winter storm with a basement

Fridge (600 kWh/yr), a 1/2 HP furnace blower running half the time, a 1/2 HP sump pump running 10% of the time, a router, and 4 LED bulbs:

• Running: 275 + 550 + 925 + 15 + 48 ≈ 1,813 W
• Peak: + the sump pump’s extra start (2,150 − 925 = 1,225 W) ≈ 3,038 W
• Energy: fridge ~1,644 + blower 550 × 24 × 0.5 = 6,600 + sump 925 × 24 × 0.1 = 2,220 + router 360 + lights 240 ≈ 11,064 Wh per day
• Battery needed: ≈ 13,016 Wh per day
• Verdict: no single portable unit in our dataset covers a full day of this on its own. The blower and the sump dominate. Realistic options are an expandable system (several kWh of add-on batteries), recharging from a generator or solar, or rationing (running the blower in shifts). The blower and many sump and well pumps are hardwired or 240V, which means a transfer switch or inlet installed by a licensed electrician.

Safety notes

• Run plug-in loads from the power station’s own outlets. Don’t backfeed your house by plugging a power source into a wall outlet. Use a properly installed transfer switch or inlet.
• If you also use a gas generator, Ready.gov says to run it outdoors only, at least 20 feet from windows, doors, and attached garages, and to install battery-backed CO detectors (Ready.gov).
• For medical devices, Ready.gov recommends talking with your provider about an outage plan.

Specs used in this guide

Sources

• Champion Power Equipment, Generator Wattage Chart (checked Oct 7, 2026)
• Honda Power Equipment, What Size Generator Do I Need? (checked Oct 7, 2026)
• Ready.gov, Power Outages (checked Oct 7, 2026)
• Manufacturer spec pages for each model, linked in the table above (checked Oct 7, 2026)