How to Size a Power Station for Your Needs (Calculator Guide)
Buying a portable power station without knowing what size you need is like buying shoes without knowing your foot size — you might get lucky, but you’ll probably end up with something that doesn’t fit.
Too small, and you’ll run out of juice when you need it most. Too big, and you’ve wasted money on capacity you’ll never use (while lugging around extra weight). The sweet spot? Understanding exactly how much power your devices consume and matching that to a power station’s capacity.
This guide will walk you through the exact calculations, with real-world examples from campers, RV owners, and home backup users who’ve tested these numbers in the field. If you’re looking for quick recommendations, check out our best portable power stations roundup.
The Only Formula You Need to Remember
Here’s the core calculation:
Runtime (hours) = Power Station Capacity (Wh) × 0.85 ÷ Device Wattage (W)
The 0.85 accounts for energy lost through the inverter and power station’s own electronics. In real-world testing, most power stations deliver about 85% of their rated capacity.
Let’s break this down with a practical example. If you have a 1000Wh power station running a 100W device:
1000 Wh × 0.85 ÷ 100W = 8.5 hours of runtime
Simple enough. But the real challenge is figuring out how much power your devices actually consume — and that’s where most people get confused.
Step 1: Find Your Device Wattages
Every device that plugs into an outlet has a wattage rating. You’ll find this on:
- The label on the device itself (usually near the power cord)
- The power adapter (for laptops and chargers)
- The product manual or manufacturer’s website
If you can only find amps and volts, multiply them together: Watts = Volts × Amps
For example, a 7-amp refrigerator running on a standard 120V outlet uses: 120V × 7A = 840 watts.
Here’s a quick reference for common devices:
| Device | Typical Wattage |
|---|---|
| Smartphone charger | 5-25W |
| Laptop | 50-100W |
| LED light bulb | 5-15W |
| CPAP machine | 30-60W (without humidifier) |
| Mini fridge | 50-80W (running) |
| Full-size refrigerator | 100-400W (running) |
| 32” TV | 30-55W |
| 55” TV | 80-150W |
| Coffee maker | 800-1500W |
| Electric heater | 1000-1500W |
| Window AC (5,000 BTU) | 450-550W |
| Well pump | 750-1500W |
Step 2: Account for Startup Surge
Here’s a trap that catches many first-time buyers: some devices use significantly more power when they first turn on.
Compressor-based appliances — refrigerators, air conditioners, and freezers — can draw 2-3x their running wattage for the first few seconds. A fridge that runs at 150W might surge to 400W+ when the compressor kicks in.
Real user experience from an r/preppers member: “My mini fridge kept tripping the overload protection on my 500W power station even though it only draws 45W running. Had to upgrade to a 1000W unit with 1500W surge to handle the startup.”
Your power station’s continuous output must exceed your device’s running wattage, and its surge capacity must handle startup spikes. Most quality power stations list both specs. If you’re planning to run refrigerators or AC units, look for our home backup power stations guide for models with adequate surge handling.
Step 3: Calculate Your Total Daily Usage
Most people don’t run all their devices 24/7. To get an accurate picture, multiply each device’s wattage by the hours you’ll actually use it.
Here’s an example for a weekend camping trip:
| Device | Watts | Hours/Day | Daily Wh |
|---|---|---|---|
| CPAP (no humidifier) | 40W | 8 | 320 Wh |
| Phone charging (2 phones) | 20W | 4 | 80 Wh |
| LED lantern | 10W | 4 | 40 Wh |
| Laptop | 60W | 3 | 180 Wh |
| Total | 620 Wh |
For one day, you’d need at least 620 Wh of usable capacity. With the 85% efficiency factor, that means a power station rated at about 730 Wh or higher.
For a two-day trip without recharging? Double it: you’d need roughly 1,460 Wh rated capacity.
Real-World Runtime Examples
Theory is nice, but what actually happens when you plug things in? Here’s what real users report:
CPAP Machines
CPAP users are one of the largest groups buying portable power stations, and there’s good data on real-world performance.
Actual camping test from a Pacific Northwest camper: “After using my Jackery 300 Plus with my CPAP for three nights with a DC adapter and no humidifier, it had 84% power remaining after the first night, 68% power remaining after the second night, and 52% power remaining after three nights.”
That works out to roughly 50 Wh per night — meaning a 300 Wh power station can run a CPAP for 5+ nights under optimal conditions.
Key tips for CPAP users:
- Use a DC adapter instead of AC when possible (skips inverter losses)
- Turn off the humidifier — it’s the biggest power draw
- Use standard tubing instead of heated hoses
- A 300-500 Wh power station handles 3-7 nights for most users
For dedicated CPAP recommendations, our upcoming guide on portable power stations for CPAP will cover this in depth.
Refrigerators and Coolers
Refrigerators are tricky because they cycle on and off. A fridge rated at 150W doesn’t run at 150W constantly — the compressor kicks on when needed, then shuts off.
From solar lab testing: “A standard fridge running for 12 hours will use around 650Wh of stored power” — meaning roughly 55 Wh per hour on average.
Overlanding user on Reddit: “Without permanent solar, I’d recommend 750+ Wh. Permanent solar and a fridge work really well together.”
Here’s a rough guide for fridge runtimes:
| Power Station | Mini Fridge (50W avg) | Full Fridge (80W avg) |
|---|---|---|
| 500 Wh | ~8.5 hours | ~5 hours |
| 1000 Wh | ~17 hours | ~10 hours |
| 2000 Wh | ~34 hours | ~21 hours |
Important note: These assume the fridge is already cold. Starting with a warm fridge drastically increases initial power draw. If you’re comparing options, our EcoFlow vs Jackery comparison covers units suitable for running refrigerators.
Multiple Devices (Camping/RV Setup)
Here’s a real-world example from Popular Mechanics testing:
“Laptop that uses 80 watts for four hours (320 Wh), refrigerator that consumes 80 watts per hour for the entire day (1,920 Wh), charging four smartphones (100 Wh), three lamps for six hours (180 Wh) — total daily consumption of about 2,520 Wh. A 5,000 Wh power station would have enough capacity to power these devices for nearly two days.”
For typical camping setups, users on r/overlanding report: “1 hour driving does 4 hours fridge assuming 50% duty cycle” — meaning car charging while driving extends your runtime significantly.
Sizing Guide by Use Case
Based on real user experiences and our testing, here’s what we recommend:
Light Use (300-500 Wh)
Best for: Phone/laptop charging, LED lights, fans, CPAP
- Charges smartphones 20-30 times
- Powers a laptop for 5-8 hours
- Runs a CPAP for 3-7 nights
- Weight: 7-15 lbs
- Price range: $200-$400
Good picks: Jackery 300 Plus, EcoFlow River 3, Bluetti EB3A
Weekend Camping (500-1000 Wh)
Best for: Short trips with phones, laptops, small cooler, lights
- Runs a mini fridge for 8-16 hours
- Powers a CPAP with humidifier for 2-3 nights
- Multiple devices simultaneously
- Weight: 15-25 lbs
- Price range: $400-$800
Good picks: EcoFlow River 2 Max, Jackery 1000 v2, Bluetti AC70
For camping-specific options, see our best power stations for camping guide.
Extended Trips / Home Backup (1000-2000 Wh)
Best for: Week-long camping, RV boondocking, power outage backup
- Runs a full-size fridge for 10-20 hours
- Powers multiple devices for days
- Can handle moderate power tools
- Weight: 25-45 lbs
- Price range: $800-$1,500
Good picks: EcoFlow Delta 2, Jackery 1500, Bluetti AC200L
Heavy-Duty / Whole Home (2000+ Wh)
Best for: Full-time RV, extended outages, running large appliances
- Powers fridges for multiple days
- Can run AC units and heaters
- Powers entire home setups during outages
- Weight: 50-100+ lbs
- Price range: $1,500-$4,000+
For off-grid and extended power needs, check our solar generator guide.
Common Sizing Mistakes to Avoid
Mistake 1: Only looking at capacity, ignoring output
A 1000 Wh power station with only 600W output can’t run your 800W coffee maker — even though it has plenty of stored energy. Always check that continuous output exceeds your highest-draw device.
Mistake 2: Forgetting inverter efficiency
Going from DC battery to AC outlet loses 10-15% of energy. If you can use DC outputs (USB, 12V car ports) directly, you’ll get more runtime.
Mistake 3: Not accounting for cycling appliances
Refrigerators don’t run constantly. Their “rated wattage” is the running power when the compressor is on. Actual daily consumption is typically 30-50% less because they cycle off.
Mistake 4: Buying for peak instead of typical use
If you occasionally need 2000W but typically use 500W, consider a smaller daily driver and a separate solution for rare high-power needs. The difference between a 500 Wh and 2000 Wh unit is often 30+ pounds and $800+.
The Quick Sizing Calculator
Don’t want to do the math? Here’s a simplified approach:
- Light users (phones, laptops, lights only): 300-500 Wh
- Moderate users (add a fan, mini-cooler, or CPAP): 500-1000 Wh
- Heavy users (fridge, multiple devices, longer trips): 1000-2000 Wh
- Maximum power (home backup, AC, power tools): 2000+ Wh
Then add 20-30% buffer for peace of mind and cold weather (batteries perform worse in cold).
Should You Add Solar?
Solar panels extend your runtime indefinitely — as long as the sun shines. But there’s a catch: most portable solar panels only deliver their rated wattage under perfect conditions (direct sun, optimal angle, no clouds).
Real-world expectations:
- A 100W panel typically produces 60-80W in good conditions
- Expect 4-6 hours of meaningful charging per day
- That’s 240-480 Wh of daily solar input
If you’re using 500 Wh daily and generating 400 Wh from solar, you’ll slowly drain your battery. Size your solar capacity at 20-30% above your daily usage for sustainable off-grid operation.
For van life and extended travel, combining solar with a properly-sized power station is the key to true independence. See our best power station for van life guide for solar-compatible recommendations.
FAQ: Power Station Sizing
How many watts do I need to run a refrigerator?
A mini fridge needs a power station with at least 500W continuous output to handle startup surge. A full-size fridge typically needs 1000W+ continuous and 1500W+ surge capacity. For runtime, expect 500-1000 Wh capacity to run a mini fridge for 8-16 hours.
Can a portable power station run a window AC?
A small 5,000 BTU window AC draws 450-550W running with a 1000W+ startup surge. You’d need a power station with at least 1500W continuous and 2500W+ surge. But runtime would be limited — expect 2-4 hours from a 2000 Wh unit.
What size power station for a 3-day camping trip?
For basic camping (phones, lights, small fan): 300-500 Wh. Add a CPAP: 500-800 Wh. Add a mini fridge: 1000-1500 Wh. These assume no solar charging. With a 100W solar panel, you can often get by with a smaller unit.
Is it better to have one large power station or two smaller ones?
Two smaller units offer redundancy (if one fails, you’re not powerless) and flexibility (bring what you need per trip). One large unit is often more cost-effective per Wh and has fewer things to manage. For home backup, go large. For camping, smaller and modular often wins.
How long will a power station last before needing replacement?
Quality lithium-ion batteries last 500-1000 charge cycles before dropping to 80% capacity. LiFePO4 batteries last 2000-3500+ cycles. At one cycle per day, that’s 2-3 years for standard lithium and 5-10 years for LiFePO4. Most users cycle far less frequently.
Bottom Line
The “right” size power station is the one that handles your actual needs with 20-30% headroom. Don’t pay for capacity you won’t use, but don’t get caught short either.
Quick rules:
- Calculate your daily Wh usage (device watts × hours of use)
- Divide by 0.85 for real-world capacity needs
- Add 20-30% buffer
- Match output wattage to your highest-draw device
- Consider surge requirements for compressor-based appliances
Start with your highest-priority devices (medical equipment, refrigerator, communication) and work down. For most campers and home backup users, something in the 1000-2000 Wh range hits the sweet spot of capability versus cost and portability.
For specific recommendations based on your use case, explore our guides on budget power stations under $500 or see how portable power stations compare to gas generators to make the best choice for your situation.