Sizing a portable power station for a blackout is much simpler than most buying guides make it sound. You don’t need to calculate amperages, surge curves, or DC-AC conversion efficiency in your head. You need three numbers: the watts you actually plan to run, the hours you need to survive, and a 15% buffer for inverter losses. Multiply, add the buffer, and you have your target capacity in watt-hours. That’s it.
The rest of this guide walks you through real loads, realistic outage durations for your area, and the three sanity checks that prevent overbuying.
What’s on this page
Capacity (Wh) = (average load in watts × hours needed) ÷ 0.85
The 0.85 accounts for inverter losses. So if you want to run a 50 W router + laptop combo for 10 hours: 50 × 10 ÷ 0.85 = 588 Wh. Buy a 600–800 Wh unit. That’s the whole calculation. Everything below is just helping you nail the input numbers.
Step 1: List your actual loads
The single biggest sizing mistake is calculating for the maximum load you might ever run instead of the average load you’ll actually run during a blackout. The right list is what’s truly essential, not what’s nice-to-have.
Tier 1 — always on during an outage:
- Cable modem / fiber ONT
- Wi-Fi router (and mesh nodes if applicable)
- One laptop on light use
- Phone charging
Tier 2 — on for part of the outage:
- One LED lamp in the room you’re in
- CPAP machine (if applicable, overnight only)
- Mini fridge (if you have one and it’s a long outage)
Tier 3 — occasional short-burst use:
- Kettle or microwave (1–2 minutes at a time)
- Hair dryer or appliance
Size for Tier 1 + Tier 2 average load. Tier 3 only matters for the surge spec of your inverter, not for sizing capacity.
Step 2: Look up realistic device wattage
The wattage printed on a device’s power brick is the maximum draw, not the average. Real continuous draw is typically 30–60% of that number. Here are honest averages for common apartment devices:
| Device | Brick rating | Real continuous draw |
|---|---|---|
| Cable modem | ~25 W | 6–12 W |
| Fiber ONT | ~15 W | 5–10 W |
| Wi-Fi router (consumer) | ~30 W | 8–15 W |
| Mesh node (extra) | ~15 W | 6–10 W each |
| 13–16″ laptop (light use) | 65–96 W | 15–25 W |
| Phone fast-charging | 20–30 W | 15–25 W (only while charging) |
| 27″ monitor | ~80 W | 30–45 W |
| Desktop PC (mid) | 500–650 W | 80–150 W |
| LED lamp | ~10 W | 8–12 W |
| CPAP no humidifier | ~65 W | 25–35 W |
| CPAP with humidifier | ~95 W | 50–80 W |
| Mini fridge (~50L) | ~70 W | 40–60 W avg (cycles) |
| Standard fridge (~300L) | ~150 W | 80–120 W avg (cycles) |
| Starlink Standard dish | ~100 W peak | 50–75 W |
Use the right column for sizing, not the brick rating. The brick rating only matters for surge spec.
Step 3: Decide your outage duration target
Don’t size for the worst possible outage. Size for the realistic worst case based on the actual outage history of your area. Three useful reference points:
- Dense urban (NYC, SF, Paris, Tokyo): Typical outages are 1–3 hours. Size for 6–8 hours to have margin.
- Suburban (most US metros): Typical outages are 2–6 hours, with occasional storm-related multi-day events. Size for 12–18 hours.
- Rural / weather-vulnerable areas: Multi-day outages are realistic. Size for 24–48 hours or plan for solar recharge.
Check your power company’s outage history online — most utilities publish historical reliability data. Use the 95th percentile, not the average.
Step 4: Apply the 15% inverter buffer
Battery capacity is measured at DC; AC output goes through an inverter that loses 10–18% as heat. The math gets simpler if you assume 15% losses across the board — multiply your raw need by 1.18, or divide by 0.85 (same thing).
Some power stations publish their efficiency. EcoFlow and Bluetti tend to be 85–90% efficient at moderate load. Below 20 W draw, efficiency drops to 70–80% (the inverter idle overhead becomes a bigger fraction of total draw). Above 1000 W, efficiency stays around 88–92%.
Worked examples for typical setups
Example 1: studio apartment, internet-only backup
Modem (8 W) + router (12 W) + laptop (20 W) = 40 W average. Target: 8 hours of runtime.
Math: 40 × 8 ÷ 0.85 = 376 Wh. Buy a 400–500 Wh unit. The Jackery Explorer 300 v2 (288 Wh) is borderline — fine for 5–6 hours of headroom, but a stretch for 8. The EcoFlow RIVER 3 Plus (286 Wh, expandable to 572) is the smarter buy.
Example 2: 1-bedroom apartment, full home office
Modem (8 W) + router (12 W) + monitor (40 W) + desktop (120 W) + laptop (20 W) + LED lamp (10 W) = 210 W average. Target: 8 hours.
Math: 210 × 8 ÷ 0.85 = 1976 Wh. Buy a 2000 Wh unit, or a 1000 Wh unit with the extra battery option. The EcoFlow DELTA 2 (1024 Wh) + extra battery (1024 Wh) hits this perfectly, or the Bluetti AC180 (1152 Wh) + B70.
Example 3: CPAP overnight backup
CPAP no humidifier (30 W) × 9 hours = 270 W-h ÷ 0.85 = 318 Wh. The Jackery Explorer 300 v2 (288 Wh) is a tight fit; the EcoFlow RIVER 2 Pro (768 Wh) gives you margin and lets you also charge a phone overnight.
Example 4: weekend off-grid (you, two laptops, no fridge)
Two laptops (40 W combined) + router/modem if hotspot (20 W) + LED lighting (15 W) + phone charging cycles (avg 5 W) = 80 W average. Target: 48 hours.
Math: 80 × 48 ÷ 0.85 = 4517 Wh. You need solar input or you need a lot of battery. A DELTA 2 + 400 W solar setup banks ~1.6 kWh per sunny day, which makes this realistic.
Three sanity checks before you buy
- Inverter wattage ≥ your peak surge load. If you plan to run a microwave (1000 W) or kettle (1500 W), your inverter continuous spec must exceed that. The 800 W RIVER 2 Pro won’t handle a kettle even with X-Boost.
- Surge spec ≥ 1.5× your biggest motor load. Mini fridges and standard fridges have a compressor surge of 800–1500 W for ~1 second on startup. Power stations rated under 2000 W surge often can’t start a standard fridge.
- UPS-mode transfer time matches your gear. Desktops want sub-20 ms. Laptops, routers, CPAPs tolerate 30 ms easily. If you have an old desktop, prioritize a low-transfer unit like the Bluetti AC180.
Capacity-to-product map
| Your need | Capacity range | Our pick |
|---|---|---|
| Wi-Fi + laptop, 1–6 hr outage | 250–400 Wh | Jackery Explorer 300 v2 |
| Home office, 6–12 hr outage | 700–1000 Wh | EcoFlow RIVER 2 Pro / DELTA 2 |
| Full apartment, 12–24 hr outage | 1000–1200 Wh | Bluetti AC180 |
| Multi-day or off-grid | 2000+ Wh w/ solar | DELTA 2 + extra battery + 400 W solar |
Frequently asked questions
Should I size for my fridge?
For short outages (under 4 hours), no — a well-insulated fridge stays cold without power. For longer outages, only if you have a small (~50L) mini-fridge. Sizing for a full-size apartment fridge (300–400L) requires a 2000+ Wh unit and a 1800+ W inverter with high surge — expensive overkill for most renters. See our refrigerator guide.
What about cold weather? Does battery capacity drop?
LiFePO4 batteries lose roughly 10–20% of usable capacity at 0°C, more below freezing. If you’ll use the unit outdoors in winter, size up by 25%. If it stays indoors at room temperature — like most apartment use — you can ignore this.
What if I round up too far?
The penalty for overbuying is mostly weight and cost. A 2 kWh unit isn’t “worse” than a 1 kWh unit for typical use — it just sits heavier on your shelf and costs more. The penalty for under-buying is running out of power mid-outage, which is worse. When in doubt, round up by one size category.
Can I just buy the biggest one and forget about sizing?
You can, but you’ll spend twice as much and carry twice the weight as you need. The point of sizing is matching the unit to your real use — a 2 kWh unit makes no sense for a studio apartment with rare urban outages, but it’s the right answer for a suburban home with multi-day weather events.
The honest sizing answer for most renters
If you have rare urban outages, 300–500 Wh is enough. For typical apartment blackouts (3–12 hours), 700–1000 Wh is the sweet spot. Above that, you’re sizing for either multi-day outages, off-grid travel, or kitchen-load capability — in which case 1000–1200 Wh and a 1800 W inverter is the right move.