The most common mistake people make when buying a portable power station is getting one that's too small. The second most common mistake is overspending on one that's way too big.
Here's how to figure out the right size for your specific needs.
The Quick Answer
Add up your daily watt-hours, then multiply by your backup days and divide by 0.8.For example: if you use 500 Wh per day and want 2 days of backup:
500 Wh × 2 days ÷ 0.8 (depth of discharge) = 1,250 Wh minimum
Want an instant answer? Use our free calculator — it does this math for all your appliances automatically.
Step-by-Step Sizing Method
Step 1: List Your Appliances
Write down everything you plan to power. For each device, note:
- •Wattage (check the label or manual)
- •Hours per day you'll actually use it
Common devices and their typical wattage:
| Device | Watts | Typical Hours/Day | Daily Wh |
|---|---|---|---|
| LED Light | 10W | 6h | 60 Wh |
| Laptop | 65W | 8h | 520 Wh |
| Phone Charger | 20W | 2h | 40 Wh |
| Mini Fridge | 60W | 24h (cycling) | 720 Wh* |
| CPAP Machine | 50W | 8h | 400 Wh |
| Electric Fan | 45W | 8h | 360 Wh |
| Wi-Fi Router | 12W | 24h | 288 Wh |
*Fridges cycle on and off — actual runtime is about 50% of the time, so real consumption is closer to 360 Wh.
Step 2: Calculate Your Daily Energy Need
Add up all the daily Wh values:
Daily Energy (Wh) = Σ (Watts × Hours per day)
Step 3: Account for Battery Depth of Discharge
You should never drain a battery to 0%. The safe discharge depth depends on chemistry:
- •LiFePO4 batteries: 80% usable (multiply by ÷ 0.80)
- •Li-ion (NMC) batteries: 70% usable (multiply by ÷ 0.70)
Step 4: Multiply by Backup Days
How many days do you need to run without recharging?
- •Camping weekend: 1–2 days
- •RV with solar: 2 days (solar tops up daily)
- •Home emergency backup: 3–5 days
- •Off-grid with no solar: 5–7 days
Final Formula
Required Capacity = Daily Wh × Backup Days ÷ DOD
Size Recommendations by Scenario
| Scenario | Typical Daily Use | Recommended Size |
|---|---|---|
| Phone + laptop only | 100–200 Wh | 250–500 Wh |
| Weekend camping | 300–500 Wh | 500–1,000 Wh |
| RV/Van full-time | 800–1,500 Wh | 1,500–3,000 Wh |
| Home backup (essentials) | 1,000–2,000 Wh | 2,000–4,000 Wh |
| Off-grid cabin | 1,500–3,000 Wh | 3,000–6,000 Wh |
Don't Forget: Inverter Wattage Matters Too
Battery capacity (Wh) tells you how long you can run. But inverter wattage (W) tells you what you can run simultaneously.
- •Running a laptop (65W)? Any power station works.
- •Running a microwave (1000W)? You need at least 1000W inverter.
- •Running a fridge + space heater? Add up the watts and check surge ratings.
Use Our Free Calculator
Don't want to do the math by hand? Our Power Calculator does all of this automatically:
- Select your appliances from 48 pre-loaded options
- Set your location (for solar panel sizing)
- Get instant battery, solar, and inverter recommendations
It's free, no signup required, and generates a shareable report you can discuss with your electrician or share in online communities.
→ Open the Power Calculator