Enter your battery capacity (Wh or mAh) and pick the devices you want to power. See the runtime in hours, how many days an outage plan lasts, the Wh you need, and the charge or solar time. It also checks rated output and startup surge.
Battery capacity
Enter your own battery
Rated output (W)
Surge output (W)
Battery level
100%Usable energy: about 425Wh (after 15% conversion loss)
Runtime with everything running nonstop
At your daily hours of use, it should last about 7.1 days
Days of outage backup
About 7.1 days
Based on daily use
Lasts a full day?
Yes
Uses about 60Wh per day
Phone charges only
About 27
Startup peak
About 15W
Surge output 1,000W
Tip
This setup should last a full day or more. If the outage runs longer, charging from solar panels during the day can add more days.
Devices to power
Phones & communication
Phone charging
Wi-Fi router
Emergency radio
Laptop
TV (32-inch)
Lighting
LED lantern / flashlight
LED ceiling light
Heating & cooling
Fan (medium)
Small room A/C (steady running)
Electric blanket
Space heater (low setting)
Kitchen & home
Full-size refrigerator
Mini fridge
12V car fridge / cooler
Rice cooker (while cooking)
Electric kettle
Microwave
Hair dryer (high)
Medical
CPAP machine
Add a device that is not listed
Find the watts on the device's label or in the manual. If it only lists volts and amps, multiply them (120V × 0.5A = 60W). After adding, tap the watts in the list to set hours per day.
How Long Will a Portable Power Station Last? The Formula and Real-World Losses
The basic estimate is battery capacity (Wh) ÷ total load (W) = hours. In practice you also need to subtract the energy lost when the unit converts battery power to AC, and account for appliances such as refrigerators that cycle on and off. Skip those and the result can be far from what you actually get.
This calculator takes your battery size (Wh or mAh) and the devices you choose, then shows the runtime if everything runs nonstop and how many days it lasts at each device's daily hours of use. It can also work backward to the Wh you need, and estimate charge time from wall power, a car outlet, or solar. It checks two things that are easy to miss: a load above the rated output, or a startup surge above the surge output, can stop an appliance even when the battery has plenty of energy.
Use it to size a backup for storm and winter outages, to keep a refrigerator and router running, or to plan power for a camping trip or van. Everything runs in your browser, so nothing you enter is sent anywhere.
When to Use It: Outages, Camping, Remote Work, Buying a Unit
Planning for a multi-day outage
Enter your refrigerator, router, lights, and phones with realistic daily hours. You see how many days a battery covers and which devices to turn off first.
Comparing sizes before you buy
Compare 300Wh, 500Wh, 1,000Wh, and larger units against your devices. The calculator also flags a rated output or surge output that is too low for your load.
Camping and van life power
Estimate a night or a weekend with a 12V cooler, fan, lights, and phones. Add solar to see how many days it takes to refill the battery.
Keeping a laptop and router online
See how long a laptop and Wi-Fi router can run during an outage so you know how much work or video-call time you have.
Calculate Your Backup Time in 4 Steps
- Choose a mode: calculate runtime, find the Wh you need, or calculate charge time.
- Pick a preset battery, or enter your own capacity in Wh or mAh along with its rated output and surge output.
- Select your devices and set the quantity. Tap the watts under a device to enter the number from its label and set its hours per day.
- Read the runtime, days of backup, and any warnings about rated output, startup surge, or missing AC outlets.
Note: Use a preset such as Outage basics, Winter outage, or Camping / van life to select a typical set of devices in one tap.
Accuracy and Assumptions
- Results are estimates. Real power use changes with the model, settings, and temperature.
- Usable energy = battery Wh × battery level × inverter efficiency. Efficiency defaults to 85% and can be set to 80% or 90%. The same factor is applied to every output type, including USB.
- Refrigerators, A/C units, space heaters, and electric blankets use an effective wattage (rated watts × duty cycle). Untick the duty cycle option to use the full rated watts instead.
- The rated output check compares the sum of the devices' rated watts (not the effective watts) with the unit's rated output.
- The startup peak is the total rated watts plus the largest single startup surge. It assumes only one appliance starts at a time. The surge multipliers are the calculator's estimates, not measured values.
- Default wattages and daily hours are typical examples. Check your own device labels and edit the numbers.
- Charge time uses the battery's full capacity (not the usable energy) and a 90% charging efficiency.
- Everything runs in your browser. Nothing you enter is sent to a server.
Appliance Watts and Runtime on 300Wh, 500Wh, and 1,000Wh Batteries
Runtime for each appliance running alone and nonstop. The numbers include 15% conversion loss and use effective watts for refrigerators, A/C, space heaters, and electric blankets.
| Device | Rated watts | 300Wh unit | 500Wh unit | 1,000Wh unit |
|---|---|---|---|---|
| Phone charging | 10W | About 25.5 hr | About 42.5 hr | About 85 hr |
| LED lantern / flashlight | 5W | About 51 hr | About 85 hr | About 170 hr |
| Wi-Fi router | 15W | About 17 hr | About 28.3 hr | About 56.7 hr |
| Fan (medium) | 45W | About 5.7 hr | About 9.4 hr | About 18.9 hr |
| Laptop | 60W | About 4.2 hr | About 7.1 hr | About 14.2 hr |
| Mini fridge (28W effective) | 70W, 40% duty | About 9.1 hr | About 15.2 hr | About 30.4 hr |
| Full-size refrigerator (60W effective) | 150W, 40% duty | About 4.2 hr | About 7.1 hr | About 14.2 hr |
| Electric blanket (25W effective) | 50W, 50% duty | About 10.2 hr | About 17 hr | About 34 hr |
| Space heater, low (480W effective) | 600W, 80% duty | About 32 min | About 53 min | About 1.8 hr |
| Small room A/C (300W effective) | 500W, 60% duty | About 51 min | About 1.4 hr | About 2.8 hr |
| Rice cooker (while cooking) | 500W | About 31 min | About 51 min | About 1.7 hr |
| Electric kettle | 1,200W | About 13 min | About 21 min | About 42 min |
Calculated at 85% inverter efficiency. Kettles, microwaves, and space heaters draw 1,000W or more, so a unit with a low rated output cannot run them even if the battery is full. Refrigerators and A/C units also need surge headroom at startup. For several devices together, or for days of backup, use the calculator above.
Wh, Watts, mAh, and Surge: What the Numbers on a Power Station Mean
Spec sheets and real-world runtime often disagree. These are the formulas and the assumptions behind them.
Runtime = Wh × efficiency ÷ watts
The basic formula is battery Wh ÷ load W, but that is the best case.
The unit converts DC battery power to 120V AC, and some energy is lost in the process. This calculator assumes about 15% loss by default, so a 500Wh unit running a 50W device lasts about 8.5 hours, not 10.
Days of backup depend on daily Wh, not nonstop watts
During an outage you do not run everything all day. Multiply each device's watts by its hours per day and add them up to get Wh per day.
A phone (10W × 3 hr), lantern (5W × 6 hr), radio (3W × 6 hr), and router (15W × 24 hr) total about 438Wh per day. A 1,000Wh unit (850Wh usable) lasts about 1.9 days.
Add a mini fridge at 28W effective for 24 hours (672Wh) and daily use rises to about 1,110Wh, which a 1,000Wh unit cannot cover for a full day.
Wh, rated output, and surge output are three different limits
Capacity in Wh is how long the unit can run. Rated output in W is how much load it can run at once. Surge output is how much it can supply for a moment at startup.
A 1,000Wh unit rated at 300W cannot run a 1,200W kettle, and a refrigerator or A/C that needs two to three times its running watts at startup needs surge headroom. Check output before capacity if you plan to run heating or motor appliances.
How to find the watts for your appliance
The most reliable source is the rating label on the appliance or the manual.
If it lists amps, multiply by the voltage for a rough wattage: 120V × 0.5A = 60W.
For appliances whose draw changes, such as refrigerators and A/C units, a plug-in watt meter shows the real number. In this calculator you can edit the watts and daily hours of every device.
Converting mAh to Wh
mAh measures charge, so it cannot be compared across batteries with different voltages. Use Wh = mAh ÷ 1000 × volts.
Power banks are typically rated at 3.7V, so 10,000mAh is about 37Wh and 20,000mAh is about 74Wh. A power bank also loses some energy boosting to 5V, so you cannot use the full labeled capacity.
Sizing for a target runtime, and planning charge time and solar
Work backward with Wh needed = load W × hours ÷ efficiency. A mini fridge (28W effective) and a router (15W) for 24 hours need about 1,215Wh.
Charge time is missing Wh ÷ (input W × charging efficiency). A 500Wh unit from empty on a 100W input takes more than 5 hours.
For solar, divide the charge hours by your peak sun hours. A 100W panel with 4 peak sun hours refills an empty 500Wh unit in about 1.4 days. For a long outage, plan both the Wh you use and the Wh you can recover each day.
Sizing Mistakes That Leave You in the Dark
Buying on capacity alone, then finding the rated output is too low
A 1,000Wh unit with a 300W rated output cannot start a 1,200W kettle or a 1,000W microwave. Capacity and rated output are separate specs, and it is easy to look at only one.
💡 See the solution▼
Rated output looked fine, but the refrigerator would not start
A refrigerator may be rated around 150W, but its compressor draws several times that for a moment at startup. With other devices running, the total went over the surge output and the unit shut down.
💡 See the solution▼
Planning to run AC appliances from a USB-only power bank
A large power bank with only USB ports cannot run a fan, router, or mini fridge that plugs into a wall outlet.
💡 See the solution▼
Sizing for 24 hours of everything, then buying far more than needed
Calculating as if the TV, lights, and laptop run all day pushes the required size up and can make backup look unaffordable.
💡 See the solution▼
Using a refrigerator's rated watts as if it ran nonstop
Treating a refrigerator as a constant load can make you buy a unit that is larger than necessary. The opposite error also happens: frequent door openings and a warm room raise the duty cycle and drain the battery faster than the estimate.
💡 See the solution▼
Forgetting how long it takes to recharge
After an outage or a trip, a unit with a 100W input may need five hours or more to refill, which is a problem if you need it again the next morning.
💡 See the solution▼
Mixing up mAh and Wh
A 20,000mAh power bank is about 74Wh, not 20,000Wh. Treating mAh as Wh makes the battery look hundreds of times bigger than it is, and it runs out in hours.
💡 See the solution▼
Leaving the unit charged in a closet and never checking it
A unit stored for months may not hold the charge you expect when an outage arrives. Lithium-ion batteries can also age faster when stored at full charge for long periods.
💡 See the solution▼
Frequently Asked Questions About Battery Backup
Q.How do I calculate how long a portable power station will run?
A. Divide the usable battery energy by the load: runtime in hours = battery Wh × inverter efficiency ÷ load in watts. The efficiency factor matters because the unit loses some energy converting battery power to AC. This calculator uses 85% by default (you can switch to 80% or 90%). For example, a 500Wh unit running a 50W device lasts about 500 × 0.85 ÷ 50 = 8.5 hours.
Q.How long will a power station last in a power outage?
A. It depends on how many hours per day you run each device. A phone (10W for 3 hours), an LED lantern (5W for 6 hours), a radio (3W for 6 hours), and a Wi-Fi router (15W for 24 hours) add up to about 438Wh per day. A 1,000Wh unit gives about 1.9 days of that routine, and a 500Wh unit gives about 23 hours. Set daily hours for each device in the calculator to see your own number.
Q.Where do I find an appliance's wattage?
A. Look for the rating label on the back or bottom of the appliance, or check the manual. If the label lists only volts and amps, multiply them for a rough wattage: 120V × 0.5A = 60W. In this calculator you can tap the watts under any device to enter the number from your own label, and you can add devices that are not on the list.
Q.How long will a power bank run a 10W device?
A. A 10,000mAh power bank holds about 37Wh (10,000 ÷ 1000 × 3.7V). Running a 10W device gives about 37 × 0.85 ÷ 10 = 3.1 hours. A 20,000mAh bank (about 74Wh) gives about 6.3 hours. USB output skips the AC conversion, so real runtime can be somewhat longer; try the 90% efficiency setting to see that case.
Q.How do I calculate power station charge time?
A. Charge time in hours = missing Wh ÷ (input watts × charging efficiency). The calculator uses 90% charging efficiency. If a 500Wh unit is at 20%, it is missing 400Wh. With a 100W input that takes about 400 ÷ (100 × 0.9) = 4.4 hours. Switch to the charge time mode to compare USB-C PD, AC, car outlet, and solar inputs.
Q.How many days does it take to charge a power station with solar panels?
A. Divide the charging hours by your peak sun hours per day. A 100W panel charging an empty 500Wh unit needs 500 ÷ (100 × 0.9) = 5.6 hours of full-sun output. With 4 peak sun hours per day, that is about 1.4 days. Clouds and low winter sun cut output, so use fewer sun hours if you are planning for an outage. The calculator lets you set the sun hours with a slider.
Q.What size power station do I need to run a refrigerator and Wi-Fi?
A. A mini fridge (70W rated, about 28W effective at a 40% duty cycle) plus a 15W router averages about 43W, which needs roughly 43 × 24 ÷ 0.85 = 1,215Wh for 24 hours. A full-size refrigerator (about 150W rated, 60W effective at a 40% duty cycle) plus the router averages about 75W and needs roughly 2,100Wh for 24 hours. Check your own refrigerator's label for running watts, then use the Find Wh needed mode.
Q.Why won't my appliance run when the battery has enough Wh?
A. Every power station has a rated output in watts, separate from its capacity in Wh. If the total watts of your devices is higher than the rated output, the overload protection may shut the unit off. A 300W unit cannot run a 1,200W kettle, however large its battery. Enter your unit's rated output and the calculator warns you when your load goes over it.
Q.What is surge output, and why does it matter for a refrigerator?
A. Motors and compressors draw extra power for a moment when they start. Refrigerator compressors commonly need two to three times their running watts at startup. Surge output is how much the unit can supply for that moment. The calculator estimates a startup peak (total watts plus the largest single startup surge) and warns you if it is above the surge output you entered.
Q.What is the difference between Wh, mAh, and watts?
A. Wh (watt-hours) measures stored energy. mAh (milliamp-hours) measures charge and only becomes energy once you multiply by voltage: Wh = mAh ÷ 1000 × volts. Most power banks are rated at 3.7V, so 10,000mAh is about 37Wh. Watts measure how fast a device uses energy. Compare batteries in Wh to avoid mistakes.
Q.Why does my fridge run for less time than the calculator shows?
A. Refrigerators cycle on and off, so their average draw is below the rated watts. The calculator applies a 40% duty cycle to refrigerators by default. Frequent door openings, warm rooms, and a full load of warm groceries make the compressor run more often, which lowers the real runtime. Untick the duty cycle option to calculate at full rated watts instead.
Q.How much battery do I need for camping or van life?
A. For one phone and one LED lantern, usage is about 60Wh per day, so a 300Wh unit covers roughly four days of that routine. Adding a 12V car fridge, a fan, a second phone, and a second lantern raises usage to about 912Wh per day, so you need roughly 1,000Wh or more per day without recharging. The Camping / van life preset shows this setup, and you can edit every wattage and hour.
Q.Can I run a CPAP machine on a power station?
A. Many people do, but check your own machine first. A CPAP rated at 40W running 8 hours uses about 320Wh per night, so a 500Wh unit gives about 1.3 nights at 85% efficiency. Features such as a heated humidifier can raise the draw, so enter the watts from your device's label. Do not rely on a calculator alone for medical equipment; ask your equipment supplier or doctor about backup power.
Q.Can I fly with a power bank?
A. FAA rules limit lithium-ion power banks to carry-on bags only. A power bank of 100Wh or less needs no airline approval. One between 101Wh and 160Wh needs airline approval, with a limit of two per passenger. Above 160Wh is not allowed. A 20,000mAh power bank is about 74Wh, which is under the 100Wh limit. Policies can differ by airline, so check yours before you fly.
Q.Is the information I enter sent to a server?
A. No. All calculations run in your browser. Nothing you enter is sent to or stored on our servers, and you do not need an account.
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