Portable Power Stations for Emergencies: How to Size One for What You Actually Run

Key points

• Sizing a portable power station for emergencies is a watt-hour problem, not a watt problem. Watts tell you what it can run right now. Watt-hours tell you how long.

• Add up a real day of loads in watt-hours, then add about 15% for conversion losses. A fridge, a CPAP, a box fan, and your phones runs close to 2,500 watt-hours a day.

• Check the surge rating, not just the running rating. A fridge compressor can pull 3 to 7 times its running watts for a fraction of a second on startup.

• A power station is not a generator. It will not run central air conditioning or an electric range, and no amount of shopping will change that.

• What it will do that a generator cannot: run indoors, silently, with no carbon monoxide. The CPSC puts generators at least 20 feet from the house for a reason.

Most people buy a portable power station for emergencies by picking a number that sounds big enough. Then the power goes out for two days, and they find out the number they picked was the wrong number entirely.

The number on the front of the box is usually watts. Watts is the wrong number to shop by. The number that decides whether your fridge is still cold on day two is watt-hours, and it is usually printed smaller.

This is the arithmetic most buying guides skip. It takes about 20 minutes with a pencil, and it is the difference between a box that carries you through an outage and a box that dies at 2 in the morning.

What size portable power station do I need for emergencies?

Add up the watt-hours every device uses in one full day, then add roughly 15% for conversion losses. That total, in watt-hours, is your minimum capacity. For most households running a fridge, a CPAP, some lights, a fan, and phones, that lands between 2,000 and 3,000 watt-hours.

Here is the whole method in four steps:

1. List what you actually plan to run. Not what you might run. What you will plug in on night one.

2. Find each device's watt-hours per day. Watts multiplied by the hours it runs. For a fridge, use the yearly energy label instead (more on that below).

3. Add the list up. That is your daily energy need.

4. Divide by 0.85. Inverters and cables lose energy as heat. Roughly 15% of the rated capacity never reaches your devices.

The part most guides skip is step 4. Manufacturers rate capacity at the battery, not at the outlet. A unit labeled 2,000 watt-hours realistically delivers something closer to 1,700 watt-hours of usable AC power. If you size to exactly your daily need, you come up short on the first night.

What is the difference between watts and watt-hours?

Watts is a rate. Watt-hours is an amount. Watts is how fast the car is going. Watt-hours is how much gas is in the tank.

A power station has two separate limits and they fail in different ways:

• The inverter rating (watts). This is how much you can run at the same instant. Exceed it and the unit shuts off immediately. A 1,000 watt unit cannot run a 1,500 watt space heater for even one second.

• The battery capacity (watt-hours). This is how long you can run it. Exceed it and the unit runs fine until it simply empties.

People shop the first number and get bitten by the second. A 2,000 watt unit with only 1,000 watt-hours of battery will happily start your fridge and then be dead before lunch.

How many watt-hours does a refrigerator use in a day?

Roughly 1,000 to 1,100 watt-hours per day for a typical full size unit, and you get that number off the yellow energy label, not off the compressor.

The Department of Energy's federal purchasing guidance for residential refrigerators lists an ENERGY STAR certified model at about 363.5 kWh per year and a conventional model at about 404 kWh per year. Divide 404 by 365 days and you get about 1.1 kWh, or 1,100 watt-hours, per day.

This is the single most useful trick in the whole exercise, and almost nobody does it. Here is why it matters. Your fridge compressor might draw 120 to 150 watts while it is actually running, but it does not run continuously. It cycles. If you naively multiply 150 watts by 24 hours you get 3,600 watt-hours and you conclude you need a unit three times larger (and three times more expensive) than you do. The yearly label already has the duty cycle baked into it. Use it.

You can cut that 1,100 watt-hours nearly in half by running the fridge in bursts instead of continuously. The FDA says an unopened refrigerator holds food cold for about 4 hours, and a full freezer holds about 48 hours (24 hours if it is half full). So plug the fridge in for 30 to 45 minutes every 3 to 4 hours, keep the door shut the rest of the time, and you have bought yourself close to a free day of runtime. Keep a thermometer inside and keep the fridge at or below 40°F and the freezer at or below 0°F.

How much power does a CPAP use on battery?

Somewhere between 30 and 60 watts with the humidifier off, and 60 to 100 watts with a heated humidifier and heated tube running. Over an 8 hour night that is roughly 320 watt-hours without the humidifier and up to 800 watt-hours with it.

Those are ranges, and yours will be different. Do not take my number. Read the nameplate on the bottom of your own machine, or better, put a plug-in energy meter on it for one night and read the actual watt-hours in the morning. That single measurement is worth more than any chart.

The practical takeaway is blunt: turn the heated humidifier off when you are on battery. On many machines the humidifier and heated hose account for more than half the total draw. Losing the humidifier costs you a dry throat. Losing the machine costs you the night.

Two more things worth knowing. Many CPAP machines can run on DC directly, which skips the inverter entirely and saves you the 15% conversion loss. And most manufacturers want a pure sine wave supply, which brings us to the inverter question below.

What are surge watts and why do they matter?

Surge watts are the brief spike a motor pulls when it starts, typically 3 to 7 times its running draw, lasting a fraction of a second. If your power station cannot cover the spike, it trips and shuts down even though the running load was well within its rating.

Anything with a motor or a compressor does this:

• Refrigerator compressor: about 150 watts running, but 600 to 1,000 watts at startup

• Sump pump or well pump: often 800 to 1,200 watts running, with a much larger startup spike

• Box fan: about 50 to 100 watts running, small surge

• Anything purely electronic (phones, laptops, LED lights, most CPAP machines): essentially no surge

Power stations publish two numbers, something like "1,800 W continuous, 3,600 W surge." Make sure the surge number clears your biggest motor's startup spike with room to spare, and do not stack two motor starts at the same moment. If the fridge and the sump pump both kick on together, the unit will trip.

Should you buy LiFePO4 or NMC?

For emergency use, LiFePO4 (also written LFP) is the better buy in almost every case. It lasts far longer in cycles, it tolerates heat better, and it is more chemically stable. NMC packs are lighter and smaller for the same capacity, which matters if you are carrying it.

The number to look for on the spec sheet is cycle life, written as something like "3,000 cycles to 80% capacity." That means after 3,000 full charge and discharge cycles the pack still holds 80% of its original capacity. LFP units are commonly rated in the thousands of cycles. NMC units are commonly rated in the hundreds. For a box that sits on a shelf and gets used a few times a year, either will outlive the outage, but LFP will still be useful in 10 years.

The other thing to check, and this one is not optional: look for certification to UL 2743, the standard covering portable power packs. It is the safety baseline for a box full of lithium cells that will live in your house.

Do you need a pure sine wave inverter?

Yes, for anything with a motor or anything medical. Pure sine wave output matches what comes out of a wall outlet. Modified sine wave is a cheaper, blockier approximation.

On modified sine, motors and compressors run hotter and less efficiently, and they can buzz or fail early. Many medical device manufacturers, CPAP makers among them, specifically call for pure sine wave. Some sensitive electronics and battery chargers also misbehave on it.

Nearly all current lithium power stations are pure sine wave, but the very cheapest units and older inverters are not. Confirm it in the specs before you buy. If the listing does not say "pure sine wave," assume it is not.

How long does it take to recharge a power station with solar?

Longer than the marketing implies. Plan on a 200 watt folding panel delivering roughly 500 to 600 watt-hours on a good clear day, not the 200 watts times 8 hours (1,600 watt-hours) the arithmetic suggests.

Three things eat the difference, and this is the part most guides skip:

• Panels are rated under lab conditions. Full perpendicular sun, cool panel temperature. A real panel flat on a deck in August delivers meaningfully less.

• You only get a few strong hours. Early morning and late afternoon sun arrives at a shallow angle and produces a fraction of midday output. Most locations give you the equivalent of 3 to 5 strong hours a day, and fewer in winter or under storm clouds.

• Charge controllers lose some too. Another slice disappears between the panel and the battery.

For a site-specific estimate rather than a rule of thumb, run your address through NREL's PVWatts calculator, which uses 30 years of historical weather data for your location. It is built for rooftop arrays, but the monthly output figures tell you honestly what your sun is worth.

Practical version: if you are using about 2,000 watt-hours a day and you want solar to keep up indefinitely, you need something in the neighborhood of 600 to 800 watts of panel, not 200. Most people buy one panel, discover it refills about a quarter of what they spend, and are quietly disappointed. Buy the panel understanding it slows the drain rather than stopping it, and you will not be.

How do cold and heat change what you actually get?

Cold costs you capacity temporarily. Heat costs you lifespan permanently. Neither is a defect, and both are worth planning around.

Lithium chemistry slows down when it is cold, so a pack that delivers its full rating at 70°F delivers less at 20°F. The DOE's FuelEconomy.gov data on electric vehicles is the clearest public illustration: with cabin heating off, range at 20°F is about 12% lower than at 75°F. The same physics applies to the box in your garage. Bring it inside and let it warm up before you count on full capacity.

More important: most lithium packs will not accept a charge below freezing. The battery management system blocks it to prevent permanent damage, so a unit left in a cold garage may refuse solar input entirely on a January morning even though it will still discharge fine. Check your manual for the charging temperature range, and keep the unit indoors. Cold snaps are also when you are most likely to need it, so treat the power station as one piece of your winter blackout preparations for the house.

In heat, keep it out of direct sun and off hot pavement. Running a power station in a closed car in August is how you shorten its life by years in a single afternoon.

What will a portable power station not run?

Central air conditioning, electric ranges and ovens, electric water heaters, and electric clothes dryers. Not "not for long." Not at all, in any practical sense.

The reason is both watts and watt-hours at once. A central air conditioner draws thousands of watts continuously for hours. An electric oven element runs 2,000 to 5,000 watts. An electric water heater is typically 4,500 watts. Even if a large power station could start one of these, it would empty in minutes.

Here is the honest trade, and it is worth being clear about because a lot of marketing blurs it:

• A generator runs big loads, runs as long as you have fuel, and produces carbon monoxide that kills people every year. The CPSC safety alert is unambiguous: never run one inside a home, garage, crawlspace, or shed, even with the doors and windows open, and keep it at least 20 feet from doors, windows, and vents.

• A power station runs small loads for a day or two, silently, indoors, with zero exhaust. No fuel to store, no fuel to go stale, nothing to start in the rain.

They are not competitors. If you can only have one and your list is a fridge, a CPAP, phones, and lights, the power station is the right tool and it is the safer one. Either way it is one line item on a longer list, and the rest of that list is in the home emergency kit checklist for sheltering in place.

A worked example, start to finish

Say a two person household plans for one full day without grid power in late summer.

• Refrigerator, from the energy label: 1,100 Wh

• CPAP, 8 hours, humidifier off, about 40 watts: 320 Wh

• Box fan, 8 hours overnight at about 75 watts: 600 Wh

• Two phones and one laptop, fully recharged: 100 Wh

Daily total: 2,120 watt-hours. Divide by 0.85 for inverter and cable losses and you get about 2,500 watt-hours of rated capacity. Add a handful of LED lanterns and you are still comfortably under a 3,000 watt-hour unit.

Now check the other number. The fridge compressor's startup spike wants 600 to 1,000 watts available instantly, so an inverter rated around 1,500 to 1,800 watts continuous with a 3,000 watt surge covers it with margin. If you also want a microwave for a few minutes at a time, size the inverter to the microwave instead, since that is your biggest instantaneous load.

And if you cut the box fan and duty-cycle the fridge, that same 2,500 watt-hour unit stretches to nearly two days. The habits matter as much as the hardware.

When to get professional help

If anyone in your home depends on electricity to stay alive, a power station is one layer of a plan, not the plan. Talk to the prescribing clinician and your durable medical equipment supplier about the specific runtime and backup your device requires, and ask whether a manufacturer battery is available for it.

Call your electric utility and ask to be added to their medical baseline or critical care registry. Most utilities keep one. It does not guarantee faster restoration, but it puts you on a list and often gets you advance notice of planned outages. The CDC also recommends keeping backup batteries, a battery powered carbon monoxide alarm, and thermometers for the fridge and freezer as part of the same plan.

For refrigerated medication, follow the label and your pharmacist. The FDA notes insulin is normally stored at 36°F to 46°F, and that unopened manufacturer vials or cartridges can stay between 59°F and 86°F for up to 28 days and still work. The CDC adds the practical rule: keep it as cool as you can without freezing it, never use insulin that has frozen, monitor blood sugar closely if you have had to use insulin stored warm, and replace it once refrigeration returns.

If you are running any combustion device and someone develops headache, dizziness, nausea, or confusion, get everyone outside into fresh air and call 911. Poison Control also takes carbon monoxide questions at 1-800-222-1222.

Frequently asked questions

How many watt-hours do I need to run a refrigerator for 24 hours?

About 1,000 to 1,100 watt-hours for a typical full size refrigerator, plus roughly 15% for inverter losses, so plan on about 1,300 watt-hours of rated capacity. Get your specific number by dividing the kWh per year on the yellow energy label by 365 and multiplying by 1,000. Do not multiply the compressor's running watts by 24, which overstates it by roughly three times.

Can a portable power station run a CPAP all night?

Yes, comfortably, on almost any mid size unit. An 8 hour night with the heated humidifier off typically needs 240 to 480 watt-hours, so even a 500 watt-hour unit usually covers one night. Turn off the heated humidifier and heated tube on battery, since they can more than double the draw, and confirm your machine's actual consumption from its nameplate or a plug-in meter.

Is LiFePO4 worth the extra cost over NMC?

For a unit that lives in your house for emergencies, yes. LFP packs are typically rated for thousands of charge cycles versus hundreds for NMC, they handle heat better, and they are more chemically stable. NMC's advantage is weight and size for the same capacity, which matters mainly if you are carrying the unit any real distance.

Can I use a portable power station indoors?

Yes. That is its main advantage over a generator. It produces no exhaust and no carbon monoxide, so it is safe in a bedroom or hallway. A gasoline generator is the opposite: the CPSC says never operate one in a home, garage, basement, crawlspace, or shed, even with doors and windows open, and keep it at least 20 feet from the building with the exhaust pointed away.

How long will a 1,000 watt-hour power station last in an outage?

Roughly 850 watt-hours of that is usable after conversion losses. Running only a fridge that is about 18 to 20 hours. Add a CPAP and some lights and you are down to about 12 hours. Add a box fan and you are under 8. Runtime is entirely a function of what you plug in, which is why the load list comes before the shopping.

Will a power station run my sump pump?

Sometimes, but check the surge rating carefully. Many residential sump pumps draw 800 to 1,200 watts running with a much higher startup spike, so you need a large inverter. Watt-hours are the bigger problem: a pump cycling frequently during a storm can empty a mid size unit in a few hours. If flooding is your main risk, a dedicated battery backup sump system is the better tool.

None of this requires you to be an electrician. It requires a list, an energy label, and about 20 minutes. Do the arithmetic before you spend the money, and you will buy the right box once instead of the wrong box twice.

Then plug everything in and run it for a night while the grid is still up. A dress rehearsal costs you one evening and tells you more than any spec sheet. If hurricane season sets your calendar, run it early in the season, alongside the rest of your 72-hour hurricane preparedness checklist.

You never know, but you can always be ready.

Sources

• U.S. Department of Energy, Purchasing Energy-Efficient Residential Refrigerators

• U.S. Food and Drug Administration, Food and Water Safety During Power Outages and Floods

• U.S. Food and Drug Administration, Insulin Storage and Switching Between Products in an Emergency

• Centers for Disease Control and Prevention, Managing Insulin in an Emergency

• Centers for Disease Control and Prevention, Prepare Your Health: Power Sources

• U.S. Consumer Product Safety Commission, Generator, Carbon Monoxide and Fire Hazards Ahead of Hurricane Season

• U.S. Consumer Product Safety Commission, Portable Generator Safety Alert

• National Renewable Energy Laboratory, PVWatts Calculator

• U.S. Department of Energy and EPA, FuelEconomy.gov: Cold Weather Effects on Electric Vehicles

• UL Solutions, Portable Power Pack Testing (UL 2743)