Disclosure: NightLightBackup is reader-supported. If you buy through links on this page, we may earn a commission at no extra cost to you. This never changes which products we recommend.
To size a portable power station, you need two numbers: how many watts your devices draw at the same time (this decides the output you need) and how many watt-hours they use over the time you want them running (this decides the battery size you need). As a rule of thumb: 300 Wh covers phones, laptops and lights, 500 to 800 Wh covers a CPAP machine and a router overnight, 1,000 Wh gets a refrigerator through a night, and 2,000 Wh or more handles a full day of essentials. Below is the simple method, with real examples you can copy.
Watts vs Watt-Hours: The Only Two Numbers That Matter
These two numbers are easy to mix up, but they answer different questions:
- Watts (W) measure power: how much a device pulls at one moment. A station’s output rating says how many watts it can deliver at once. If your devices add up to more than that, the station shuts off.
- Watt-hours (Wh) measure energy: how much is stored. A station’s capacity says how long it can keep supplying power. A 1,000 Wh station can deliver 100 W for about 10 hours, or 500 W for about 2 hours (before losses).
Think of it like a car: watts are the speed, and watt-hours are the size of the fuel tank.
The 5-Step Sizing Method
- List every device you want to run, and find its wattage on the label, the charger or the manual.
- Add up the watts of everything that will run at the same time. This is the output your station must cover.
- Check the startup surge of anything with a motor, such as a refrigerator or pump. The station’s surge rating must cover it.
- Multiply each device’s watts by the hours you will use it and add them up. This gives the watt-hours you need.
- Add a safety margin. Divide by 0.85 to cover conversion losses, then add 10 to 20% so you never run it to zero.
The formula:
Wh needed = (watts \times hours) \div 0.85To estimate runtime from a station you already own: runtime in hours = capacity in Wh × 0.85 ÷ watts.
Typical Wattage of Common Devices
These are typical ranges. Your own devices may differ, so check the label or measure them (see the tip below).
| Device | Typical running watts | Notes |
|---|---|---|
| LED light bulb | 8 to 10 W | Very low draw |
| Phone charging | 10 to 20 W | About 15 Wh per full charge |
| Wi-Fi router and modem | 15 to 25 W | Keeps you online |
| Laptop | 45 to 90 W | Higher when charging fast |
| TV (LED) | 50 to 120 W | Depends on screen size |
| CPAP machine | 30 to 60 W | Higher with a heated humidifier |
| Refrigerator | 100 to 200 W running (50 to 100 W average) | Startup surge can be several times higher |
| Microwave | 700 to 1,200 W | Short bursts |
| Coffee maker | 600 to 1,200 W | Short use |
| Sump pump | 400 to 1,100 W | High startup surge |
| Space heater | 750 to 1,500 W | Drains batteries fast |
| Window air conditioner | 500 to 1,200 W | High startup surge |
| Hair dryer or electric kettle | 1,000 to 1,800 W | Very high draw |
Tip: for exact numbers, plug your devices into an inexpensive plug-in watt meter. It shows real watts and total watt-hours used.
Sizing Table by Use Case
| What you want to power | Suggested capacity | Suggested output |
|---|---|---|
| Phones, tablets, lights, a laptop | 300 Wh | 300 W |
| A CPAP machine and router overnight | 500 to 800 Wh | 500 W |
| A refrigerator for a night, plus a router and lights | 1,000 Wh | 1,000 to 1,500 W with a surge rating that covers the fridge |
| A refrigerator plus a laptop workday | 1,500 to 2,000 Wh | 1,500 to 2,000 W |
| A full day of home essentials or a long weekend off-grid | 2,000 Wh or more | 2,000 W or more |
| Multi-day outages | 3,000 Wh or more, plus solar panels | 2,000 W or more |
| Heating, air conditioning or cooking | Very large (several kWh) | Very high output |
Heating, cooling and cooking are the hardest loads. If they are your priority, plan for a large expandable system or a different backup source.
Worked Examples
These use typical numbers. Replace them with your own.
Example 1: Overnight CPAP and Wi-Fi (8 hours)
- CPAP: 40 to 60 W for 8 hours = 320 to 480 Wh
- Router and modem: 20 W for 8 hours = 160 Wh
- Total: 480 to 640 Wh, divided by 0.85 = about 565 to 750 Wh
- Choose: a station in the 600 to 800 Wh range, with output of at least 200 W.
(For medical devices, always check the manufacturer’s guidance and your doctor’s advice about backup power.)
Example 2: Working from Home During an Outage (8 hours)
- Laptop: 60 W for 8 hours = 480 Wh
- Monitor: 30 W for 8 hours = 240 Wh
- Router and modem: 20 W for 8 hours = 160 Wh
- Total: 880 Wh, divided by 0.85 = about 1,035 Wh
- Choose: a 1,000 Wh station is the minimum. A bit more is more comfortable.
Example 3: Home Essentials for 12 Hours
- Refrigerator: 75 W average for 12 hours = 900 Wh
- Router and modem: 20 W for 12 hours = 240 Wh
- LED lights: 30 W for 5 hours = 150 Wh
- Laptop: 60 W for 4 hours = 240 Wh
- Phone charging: 4 charges = 60 Wh
- Total: 1,590 Wh, divided by 0.85 = about 1,870 Wh
- Output: the running load is about 280 W, but the refrigerator can surge to several times its running watts, so pick a station with a surge rating of at least 1,500 W.
- Choose: a 2,000 Wh class station.
Example 4: Weekend Camping (2 nights)
- 12V camping fridge: roughly 300 Wh per day, so about 600 Wh (this varies a lot by model and weather)
- Laptop: 60 W for 3 hours = 180 Wh
- LED lights: 10 W for 4 hours a night = 80 Wh
- Phones, camera and small gear: about 160 Wh
- Total: about 1,020 Wh, divided by 0.85 = about 1,200 Wh
- Choose: a 1,000 to 1,500 Wh station, or a smaller one plus a solar panel.
Common Sizing Mistakes
- Looking only at capacity. A 1,000 Wh battery with a 300 W output cannot run a 1,000 W appliance.
- Forgetting the startup surge. Motors (refrigerators, pumps, compressors) need extra power for a moment when they start.
- Ignoring conversion losses. Plan on about 85% of rated capacity being usable from the AC outlets.
- Planning on heaters and air conditioners. They drain even large batteries quickly.
- Not planning how you will recharge. In a multi-day outage, solar panels matter as much as capacity. See our guide on How Many Solar Panels Do I Need to Charge a Power Station?
Tips to Make Your Power Last Longer
- Use USB and DC ports for phones and laptops when you can. They are more efficient than running a charger through the AC inverter.
- Limit the refrigerator openings and keep it full so it holds cold longer.
- Run big loads in short bursts (for example, the microwave) instead of continuously.
- Use LED lights and turn off devices you do not need.
- Keep the station charged before storm season.
Which Models Fit Each Size?
Once you know your size, you can compare models:
- Around 300 to 800 Wh: see our guide to the Best Portable Power Station Under $500.
- Around 1,000 to 2,000 Wh: see our guide to the Best Portable Power Station Under $1,000.
- Whole-home and multi-day needs: see our roundup of the Best Portable Power Stations for Home Backup.
Frequently Asked Questions
How many watt-hours do I need for a power outage?
It depends on what you run and for how long. For a refrigerator, router, lights and phones for 12 hours, plan for about 2,000 Wh. For only a router, phones and lights overnight, 300 to 500 Wh is enough.
Is a 1,000 Wh power station enough for a refrigerator?
Usually for about half a day to a day, depending on the model and how often the compressor runs. It is enough to get through a night, but not through a multi-day outage without solar.
What is the difference between running watts and surge watts?
Running watts are what a device uses continuously. Surge watts are the short burst some devices need to start, especially motors. Your station’s surge rating must cover the biggest startup spike.
Should I buy a bigger power station than I need?
A little extra capacity is wise, because real-world losses and cold weather reduce what you get. But a much larger station costs more and weighs more, so size it to your actual list rather than guessing.
Can I power a whole house with a portable power station?
Not with a small unit. Large expandable systems can run several circuits through a transfer switch, but heavy loads like electric heating or central air conditioning need very large capacity. Any home panel connection should be installed by a licensed electrician.
Final Verdict
Sizing comes down to a short list: write down your devices, add the watts for output, multiply watts by hours for capacity, divide by 0.85 and add a margin. For most households, 1,000 Wh covers the essentials for a night, and 2,000 Wh covers a full day. Start with your own list rather than the biggest battery on the shelf, and you will spend less and be better prepared.