How to Size a Power Station: Watts, Wh and Surge
How to size a power station in four steps: list running watts, check start-up surge, total daily watt-hours and allow for losses, with worked examples.
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To size a power station, add up the watt-hours your appliances use per day and divide by 0.85 to get the rated capacity you need. Then check two inverter numbers: continuous watts must cover everything running at once, and surge watts must cover the biggest motor start. A CPAP night needs about 528 Wh; a day of outage essentials about 3,150 Wh.
Three specs decide every sizing question, and any one of them can rule a station out:
| Spec | What it limits | Where it fails you |
|---|---|---|
| Battery capacity (Wh) | How long things run | You run out before morning |
| Continuous AC output (W) | What can run at all, together | The station shuts off the moment you add a load |
| Surge AC output (W) | Whether motors and compressors start | The pump or fridge never kicks on |
Our watts vs watt-hours explainer covers the vocabulary. Here’s the method.
How big a power station do I need?
Most buyers land in one of three bands. Use this to get close, then run the four steps to confirm.
| Use | Typical daily load | Rated capacity to look for | Inverter |
|---|---|---|---|
| Phones, laptop, lights, 12V fridge | 300-600 Wh | 500-800 Wh | 300-1,000 W |
| CPAP for one or two nights, or an evening outage | 500-1,100 Wh | 1,000-1,500 Wh | 1,000-2,000 W |
| A full day of home essentials | 2,500-4,000 Wh | 3,000-5,000 Wh | 2,000-4,000 W |
Those bands map onto our rankings of small, mid-size and home backup stations.
Step 1: list your appliances and their running watts
Write down every device you want powered, its running watts and how many hours a day it’ll actually run. Running watts come from the appliance label, the manual or a sourced table. Every number below comes from our appliance wattage table, which cites EWEB, the EPA, manufacturer manuals and generator-sizing charts for each entry.
| Appliance | Running watts | Notes |
|---|---|---|
| Smartphone charger | 10 W | |
| Wi-Fi router | 6.5 W | Always on |
| LED bulb (60 W equivalent) | 9 W | |
| Laptop | 50 W | A 16-inch MacBook Pro adapter is rated 140 W |
| ResMed AirSense 11 CPAP | 56.1 W | ResMed’s typical figure |
| 12V fridge (Dometic CFX5 45) | 58.2 W cooling | Averages about 7 W once cold |
| ENERGY STAR full-size fridge | 160 W | Runs about 32% of the time |
| 45-inch LED TV | 162 W | |
| Drip coffee maker | 1,300 W | While brewing |
| Full-size microwave | 1,500 W | |
| 1/2 HP sump pump | 1,000 W | 3,000 W to start |
Two traps at this stage. Don’t use the “watts” on a charger brick as the device’s draw: it’s the maximum, and most devices draw less most of the time. And don’t forget duty cycles. Fridges, freezers and furnace fans cycle on a thermostat, so their average draw is a fraction of their running watts. Our data carries a duty cycle for each, such as 0.32 for an ENERGY STAR fridge, derived from the source’s monthly run hours.
Step 2: check startup surge against inverter output
Now look at what runs at the same time. Add the running watts of everything that could be on together; that total has to fit the station’s continuous rating. Then take the single biggest start-up spike and add it on top; that peak has to fit the surge rating. Our calculator assumes only one motor starts at a time, which is how most real loads behave.
Here’s a worked example: an ENERGY STAR fridge, a 1/2 HP sump pump, a router, a modem and three LED bulbs.
- Running total: 160 + 1,000 + 6.5 + 6 + 27 = 1,199.5 W
- The pump’s start adds 2,000 W above its running draw (3,000 W start minus 1,000 W running)
- Peak: 1,199.5 + 2,000 = 3,199.5 W
| Station | Continuous / surge | Runs the load? |
|---|---|---|
| EcoFlow DELTA 3 Plus | 1,800 W / 3,600 W | Yes |
| DJI Power 2000 | 3,000 W / not published | No: peak exceeds its 3,000 W continuous rating |
| EcoFlow DELTA Pro 3 | 4,000 W / 8,000 W | Yes |
The DELTA 3 Plus, with less than half the battery of the DELTA Pro 3, still passes because its published 3,600 W surge covers the start. The bigger DJI fails on paper only because DJI doesn’t publish a surge figure, so we hold it to its continuous rating. When a brand doesn’t publish surge, assume nothing above continuous. Our running vs starting watts guide covers which appliances surge and by how much.
Step 3: add up daily watt-hours
Watt-hours are watts multiplied by hours. For each appliance, multiply its average draw by the hours it’ll run in a day, then add them up.
- An LED bulb at 9 W for 5 hours: 45 Wh
- A laptop at 50 W for 3 hours: 150 Wh
- An ENERGY STAR fridge at 160 W × 0.32 duty cycle × 24 hours: about 1,229 Wh
- A microwave at 1,500 W for 15 minutes: 375 Wh
That last line is the one people get wrong. High-wattage appliances that run briefly can be cheap in watt-hours. Fifteen minutes of microwave uses less energy than a day of a cycling fridge. Low-wattage loads that run all day add up fast.
Step 4: allow for inverter losses and reserve
A station’s rated capacity isn’t what reaches your appliances. Converting battery DC to 120 V AC, and stepping voltages for USB and 12 V outputs, costs energy. We count 85% of rated capacity as usable, which is the figure behind every runtime on this site. Our runtime formula guide explains where it comes from.
So divide your daily total by 0.85:
Rated capacity needed = daily Wh ÷ 0.85
Then consider a reserve. LiFePO4 stations are rated for a number of cycles to 80% of original capacity, typically 3,000-6,000 in our data. A station sized exactly to today’s need will fall short as it ages. Adding 20% covers that, and leaves room for a cold night or an extra device.
Worked examples: camping, CPAP and a 24-hour outage
A camping weekend
| Load | Average W | Hours/day | Wh/day |
|---|---|---|---|
| 12V fridge (Dometic CFX5 45) | 7.0 | 24 | 168 |
| Laptop | 50 | 3 | 150 |
| Starlink Mini | 33 | 4 | 132 |
| Two LED bulbs | 18 | 5 | 90 |
| Two phone chargers | 20 | 2 | 40 |
| Total | 580 |
Rated capacity: 580 ÷ 0.85 = 682 Wh per day. Peak running load with everything on, using the fridge’s 58.2 W cooling draw, is about 179 W, so any inverter of 300 W or more works.
A 768 Wh station like the BLUETTI AC70 covers a full day with no sun (653 Wh usable). For a two-day weekend without recharging you’d want about 1,360 Wh, or add a 200 W panel: at 75% of its rating it delivers about 150 W, which puts back roughly 750 Wh of capacity in five hours of good sun. That’s more than a day’s use.
CPAP for two nights
This is power-draw math only. Ask your equipment supplier or clinician about running a medical device from battery power.
ResMed’s AirSense 11 user guide lists a typical consumption of 56.1 W. Over an 8-hour night that’s 449 Wh, and 449 ÷ 0.85 = 528 Wh of rated capacity per night. Two nights need about 1,056 Wh, which happens to be exactly the capacity of the Anker SOLIX C1000. The 1,024 Wh DJI Power 1000 V2 gives about 15.5 hours, just under two full nights.
Your own machine may draw more or less depending on pressure, humidifier and heated-tube settings, so check your manual. Running from a DC adapter instead of the AC outlet avoids inverter losses and usually stretches runtime beyond our 85% assumption.
A 24-hour summer outage
| Load | Average W | Hours | Wh |
|---|---|---|---|
| ENERGY STAR fridge (cycling) | 51.2 | 24 | 1,229 |
| Router and modem | 12.5 | 24 | 300 |
| 45-inch TV | 162 | 4 | 648 |
| CPAP | 56.1 | 8 | 449 |
| Microwave | 1,500 | 0.25 | 375 |
| Four LED bulbs | 36 | 6 | 216 |
| Two phone chargers | 20 | 3 | 60 |
| Total | 3,277 |
Rated capacity: 3,277 ÷ 0.85 = 3,855 Wh. Peak running load, if the microwave runs while everything else is on, is about 1,947 W. The EcoFlow DELTA Pro 3 fits: 4,096 Wh rated (3,482 Wh usable) and a 4,000 W inverter.
Make it a winter outage and add an efficient furnace fan (about 48 W average, 1,152 Wh a day) and the total jumps to 4,429 Wh, or 5,211 Wh rated. Now even the 5,040 Wh Jackery Explorer 5000 Plus falls just short, and you’re looking at an expansion battery or a daily solar top-up. That’s why heating season is where outage plans most often come up short, and why we don’t count on a power station running a space heater for long.
Try your own numbers
The runtime calculator does Steps 2 through 4 for you. Pick a station, add appliances and quantities, and it shows running watts, peak surge, average draw and hours, plus the smallest stations that can carry the load.
What you'll plug in
- Full-size refrigerator (ENERGY STAR)160 W, cycles 32% of the time
- Wi-Fi router6.5 W
- LED bulb (60 W equivalent)9 W
- Smartphone charger10 W
Runs all of it for about 8.3 h on a full charge.
- Runtime
- 8.3 h
- Running watts
- 214of 1,800 W inverter
- Start-up peak
- 214of 3,600 W surge
- Average draw
- 105W after cycling
Runtime = 85% of the battery's rated Wh (usable after inverter and DC losses) ÷ the average draw. The start-up peak assumes one motor starts at a time.
Smallest stations that run this load
| Station | Battery | Inverter | Runtime | Buy |
|---|---|---|---|---|
| EcoFlow RIVER 3 | 245 Wh | 300 W | 2.0 h | Amazon |
| EcoFlow RIVER 3 Plus | 286 Wh | 600 W | 2.3 h | EcoFlow |
| Anker SOLIX C300 | 288 Wh | 300 W | 2.3 h | Anker SOLIX |
| BLUETTI Elite 30 V2 | 288 Wh | 600 W | 2.3 h | BLUETTI |
| Jackery Explorer 300 Plus | 288 Wh | 300 W | 2.3 h | Amazon |
| BLUETTI AC50B | 448 Wh | 700 W | 3.6 h | BLUETTI |
| Goal Zero Yeti 500 | 499 Wh | 500 W | 4.1 h | Goal Zero |
| DJI Power 500 | 512 Wh | 1,000 W | 4.2 h | DJI |
Once you have a target, filter the power station rankings by capacity and check our scoring method to see how we weigh the specs that don’t show up in a sizing calculation, such as cycle life, recharge speed and UPS switchover.
Frequently asked questions
How do I know what size power station I need?
List each appliance's running watts and hours of use, multiply to get watt-hours, add them up and divide by 0.85. Then make sure the inverter's continuous rating covers everything you'll run at once and its surge rating covers the biggest motor start.
What size power station do I need to run a refrigerator?
An ENERGY STAR fridge averages about 51 W over its compressor cycle, about 1,229 Wh a day. That needs roughly 1,450 Wh of rated capacity per day, plus an inverter comfortably above its 160 W running draw.
Is 1000Wh enough for a power outage?
For an evening, yes. A 1,024 Wh station runs a fridge, router, three LED bulbs and two phone chargers for about 8.3 hours. A full day of essentials usually needs 3,000 Wh or more.
Should I buy a bigger power station than I need?
A 20% margin is sensible. LiFePO4 cycle ratings are quoted to 80% of original capacity, so a station sized with no headroom will fall short late in its life.
What's the difference between running watts and surge watts?
Running watts are what an appliance draws once it's going. Surge watts are the brief spike when a motor or compressor starts, often two to three times higher. A station needs enough of both.