What Size Portable Power Station Do You Need for a Power Outage? | How Many Days a Fridge, Electric Blanket and Phones Last, and How to Choose the Capacity

The short answer: phones and lights get 3+ days from 1,000 Wh; add a fridge and even 2,000 Wh lasts a day and a half

The capacity you need for a blackout depends entirely on what you want to run. Phones, lights and a Wi-Fi router use about 230 Wh a day, but adding a fridge pushes it past 1,200 Wh — more than five times as much. Here are the results for the four setups in this guide (AC output efficiency 85%, keeping 10% in reserve).

• For basic contact and light, 500–1,000 Wh covers 2–3 days
• If you do not want the fridge to stop, plan on 2,000 Wh or more, or daily top-ups from solar panels (around 500 W on a sunny day)
• Needs change with the season — electric blankets in winter, fans in summer. Cold also reduces how much of the battery you can use, so choose a size 20–30% above the calculation

Below we cover how many days to prepare for, the calculation behind each setup, the output a fridge needs, recharging during an outage, and storage and safety. All figures were checked with the same calculation as the Portable Power Station Runtime Calculator.

How long do blackouts last? Using past major outages to decide how many days to cover

Before choosing a capacity, decide how many days you want to cover. Most outages in Japan are fixed within hours, but major disasters have caused outages lasting days or even weeks.

For food and water, the usual advice is to stock at least 3 days and ideally a week. Electricity works the same way: make 3 days your baseline, and if you live where restoration may be slow — on a typhoon path or in the mountains — plan to get through a week by topping up with solar panels. Even once the grid is back, remember that staying at home rather than going to a shelter (sheltering in place) can last a long time if your house is safe.

By setup: daily energy use and the capacity you need

Daily energy use (Wh) is the sum of “power draw (W) × hours used per day × number of units” for each appliance. The capacity you need is that figure times the number of days, divided by the conversion efficiency and the share you keep in reserve.

Fridges and electric blankets switch on and off to hold a temperature, so we use their average power, not the power while running. The fridge’s 35 W average is an annual consumption of about 300 kWh divided by the 8,760 hours in a year. If you know your own fridge’s annual figure from its spec sheet, you can substitute it.

• Phones, lights, Wi-Fi: 2 phones (60 Wh) + 1 LED bulb (48 Wh) + router for 12 h (120 Wh) = 228 Wh. The surprise is the router — left on 24 hours it uses 240 Wh, more than half of this on its own
• Winter at home: 2 electric blankets (400 Wh) + 2 LED bulbs (96 Wh) + 2 phones (60 Wh) + router for 12 h (120 Wh) = 676 Wh
• Keep the fridge running: fridge (840 Wh) + 2 LED bulbs (96 Wh) + 2 phones (60 Wh) + router for 12 h (120 Wh) + DC fan (120 Wh) = 1,236 Wh
• CPAP: CPAP for 7 h (280 Wh) + 1 phone (30 Wh) = 310 Wh

These are the days until the battery is empty, calculated as capacity × 0.85 ÷ daily energy. They get shorter if you open the fridge often on hot days or keep the unit in a cold room where the battery chills. How to read an appliance’s power rating, starting from the difference between W and Wh, is explained in how to calculate electricity costs.

Running a fridge: rated output, surge output, and how long food lasts with the fridge off

The fridge is both the appliance you miss most in a blackout and the one that works a portable power station hardest. Besides capacity, check two output conditions.

Most current 1,000 Wh units have a rated output of 1,000 W or more and a pure sine wave, so running a fridge itself is not the problem. Capacity is: even the fridge alone lasts about 24 hours on 1,000 Wh and about 48 hours on 2,000 Wh.

On the other hand, a fridge does not warm up the moment the power goes off. According to the U.S. Department of Agriculture (USDA), with the door kept closed the fridge compartment keeps food at a safe temperature for about 4 hours, and a full freezer for about 48 hours (about 24 hours if half full).

• If the outage lasts only a few hours, just keep the door shut. Start using the power station once the outage looks set to last more than half a day
• If it looks long, move perishables from the fridge compartment to the freezer or a cool box with ice packs, set the fridge to its lowest cooling setting, then power it
• Keeping frozen water bottles or ice packs in the freezer gives you ice packs for an outage and helps the freezer stay cold longer
• Use an extension cord rated for the fridge’s power (for example 1,500 W), and keep it short, close to the power station

Air conditioner, electric kettle, microwave: appliances limited by rated output

For high-power appliances, the rated output decides whether they work at all, before capacity even matters. If the power draw exceeds the rated output, the power station shuts its output off for safety.

Boiling water and reheating take only minutes, so they use little energy and work as long as the rated output is high enough. Still, a portable gas stove does the same job without electricity, and saving the power station for the fridge and lights makes it last longer.

Recharging during an outage: how many watts of solar panel, and how long from a car socket?

When an outage drags on, you need to put back what you use each day. With no grid power, your options are solar panels and a car.

The table shows the panel wattage needed to cover each setup from solar alone every day (energy drawn from the battery is daily use divided by the 0.85 conversion efficiency). A single 100–200 W panel covers phones and lights, but covering a fridge takes 400 W or more, and cloudy or rainy days will not keep up. The realistic plan is to get close to full on sunny days and ride out bad weather on the battery.

• Car accessory socket (around 8 A at 12 V ≈ 96 W): about 9 hours to charge 1,000 Wh from 20% to full, about 12 hours from empty. Keep the engine running while charging
• Running the engine in snow or inside a garage can let exhaust into the car or house and cause carbon monoxide poisoning. Clear snow from around the exhaust pipe and do it outdoors with good airflow
• Once the power is back, charge quickly from an AC outlet. A unit that takes 600 W charges 1,000 Wh from 20% to full in about 1.5 hours, and a full charge of 1,000 Wh costs about 34 yen in electricity

Day-to-day storage, and what to watch out for during an outage

A power station bought and put away may not have enough charge when you need it. Batteries slowly lose charge even unused, so manage an emergency unit like this.

1. 1

   Check the charge every 3 months

   Tying it to the change of seasons or your emergency-kit check day makes it easy to remember. The recommended charge for long storage differs by model (often around 50–80%), so follow the manual.

2. 2

   Charge it fully when a typhoon forecast or warning is issued

   A typhoon is an outage you can see coming, so charge fully the day before. Charge your phones and power banks at the same time.

3. 3

   Once a year, actually plug in your appliances

   Check that the fridge or air conditioner really runs, that your extension cords are long enough, and that everyone in the family knows how to set up the solar panel.

4. 4

   Keep it out of hot places

   A car in summer or a spot in direct sun damages the battery. Keep it indoors somewhere cool and dry.

• Never try to feed the whole house by plugging the power station into a wall outlet. A cord with plugs on both ends can cause electric shock or fire, and endangers workers repairing the lines
• A portable power station produces no exhaust, so it can be used indoors. Gasoline or gas-canister generators produce carbon monoxide and must never be used indoors or in a garage
• If you leave home to evacuate, switch off the main breaker. This prevents fires when power returns — for example from a heater that fell over — known in Japan as “power-restoration fires”
• For medical devices such as CPAP or home oxygen, confirm with the device maker or your doctor which power stations are suitable (output, waveform, UPS function)

Battery life also matters when choosing. Lithium iron phosphate (LiFePO4), the mainstream chemistry today, only drops to about 70–80% capacity after 3,000–4,000 charges, so for an emergency unit used a few times a year, you are more likely to replace it for other reasons before the battery wears out. NCM (ternary) batteries, common in older models, reach about 80% after 500–1,000 charges.

Calculate it for your own home with the Portable Power Station Runtime Calculator

The figures in this guide use typical appliance power ratings. To match your own fridge and habits, recalculating with the Portable Power Station Runtime Calculator is the reliable way.

1. 1

   Choose “Power cut (fridge, lights, phones)”

   This adds a fridge, two LED bulbs, two phones, a Wi-Fi router and a TV. Remove what you will not use and add an electric blanket or a fan.

2. 2

   Adjust power draw and hours to your home

   For the fridge, convert the annual consumption on its spec sheet (kWh) to average watts with ÷ 8,760 × 1,000. If you switch the router off at night, set it to 12 hours.

3. 3

   Enter the number of days under “What capacity do you need?”

   Enter 3 days and keep 10% in reserve, for example, to see the capacity you need and the rated output for the appliances you use at the same time.

4. 4

   Check solar panels on the “Charging time (AC, car, solar)” tab

   Choose the panel wattage and the season to see the energy per day and the panel wattage needed to cover your daily use.

All calculations run on your device, and nothing you enter is sent anywhere. The same tool’s presets also cover winter car camping and outdoor camping.

FAQ

How many Wh of portable power station do I need for a power outage?

For phones, lights and Wi-Fi only, you use about 230 Wh a day, so 1,000 Wh lasts more than 3 days. With an electric blanket in winter, 1,000 Wh lasts a little over a day. Keeping the fridge running uses about 1,240 Wh a day, so even 2,000 Wh lasts only about a day and a half. Aim for 3 days, and if you want to run the fridge, plan on combining it with solar panels.

How long will a 1,000 Wh power station run a fridge?

A fridge of around 400 L (35 W average) alone runs about 24 hours. Add lights, phones, a router and a fan and it drops to about 16 hours. In summer, with the door opened often, it gets shorter still.

How long does food in the fridge last in a power outage?

According to the USDA, with the door kept closed the fridge compartment lasts about 4 hours, and a full freezer about 48 hours (about 24 hours if half full). Keep the door shut during short outages, and start the power station only once the outage looks long — that saves battery.

Can a portable power station run an air conditioner?

A unit with enough rated output can usually run a small 100 V room air conditioner, but cooling averages around 400 W, so it lasts about 2 hours on 1,000 Wh and about 4 hours on 2,000 Wh. Most units cannot run 200 V air conditioners. Realistically, use it at the hottest time or before bed and rely on a fan the rest of the time.

What size solar panel do I need to recharge during an outage?

Energy per day is roughly panel W × 0.7 × peak sun hours on a sunny day. For phones and lights only, around 100 W is enough on a sunny spring or autumn day; to cover a fridge you need around 500 W. Cloudy days deliver only about a third to a quarter of a sunny day.

How should I store a portable power station kept for emergencies?

Keep it indoors somewhere cool and dry, and check the charge every 3 months. The recommended charge for long storage varies by model, so follow the manual, and charge fully when a typhoon is forecast. Plugging in your fridge or other appliances once a year to confirm they run gives extra peace of mind.

Can I plug a power station into a wall outlet to power the whole house?

No. Connecting to a wall outlet with a double-plug cord can cause electric shock or fire and endangers people working on the power lines. Plug the appliances you need into the power station’s outlets directly or through an extension cord.