You don't need to be an electrician. You need a list of what you'll run, for how long, and a few safety margins.
1. List what you'll actually run
Write down every device and its wattage. The best source is the label or manual. Better still, plug it into an inexpensive plug-in electricity usage monitor (a "watt meter") for a day. That's the only way to get an accurate number for anything that cycles on and off, like a refrigerator. Typical wattages on the internet, including the presets in our calculator, are rough starting points.
2. Convert to watt-hours per day
Watts × hours used = watt-hours (Wh). A 60 W laptop for 4 hours is 240 Wh. A 20 W router for 24 hours is 480 Wh. People often underestimate that one because it never turns off.
For cycling appliances (fridges, freezers), don't multiply the running watts by 24. Use the yellow EnergyGuide label: annual kWh ÷ 365 × 1,000 gives average Wh per day. A fridge rated 550 kWh/year averages about 1,500 Wh/day. Real use goes up in a hot room and down in a cold one.
3. Add conversion losses and a reserve
Not all of the rated capacity reaches your devices. Converting battery DC to 120 V AC wastes some energy, and the battery management system keeps a little in reserve. BLUETTI's own runtime formula for the AC180 uses 90% depth of discharge and 85% inverter efficiency, plus about 15 W of self-consumption (AC180 FAQ). That works out to roughly 75–80% of the label figure when running small AC loads. Devices on the USB or 12 V ports lose less.
A practical rule: divide your daily Wh by 0.85, then add 10–20% reserve so you're not running the battery flat every day. In freezing temperatures, plan for less usable capacity (see cold weather).
4. Check continuous AC output
Add up everything that could be on at the same time. That total must be under the unit's rated continuous output. A coffee maker (≈900 W) and a microwave (≈1,500 W draw for a "1,000 W" model) together need about 2,400 W, which is more than most 1 kWh-class units. Running them one at a time is fine.
Some makers advertise a "lifting" or "boost" mode (BLUETTI "Power Lifting", EcoFlow "X-Boost"). These run resistive heating loads like kettles or hair dryers by lowering the voltage. BLUETTI says not to use them for compressors such as air conditioners or washers. Don't count boost wattage as real continuous output.
5. Check surge
Motors and compressors briefly draw a multiple of their running watts when starting. BLUETTI's AC70 FAQ cites 2–4× for built-in motors and compressors. Take the biggest motor load, multiply its running watts by 3–4, and add everything else that is running. That peak must be under the unit's surge rating. If a maker doesn't publish a surge figure, treat that as a reason to leave extra headroom.
Worked examples (illustrations, not measurements)
Weekend tent camping
Phones 2 × 15 Wh, LED lights 20 W × 5 h, 12 V cooler averaging 20 W × 24 h: about 30 + 100 + 480 = 610 Wh/day. ÷ 0.85 ≈ 720 Wh, plus 15% reserve ≈ 830 Wh for one day. For two days without recharging, that's a ~1,000 Wh-class unit, or a smaller one with a solar panel.
Outage: fridge + Wi-Fi + lights + CPAP
Fridge ≈1,500 Wh/day (from its label), router 480 Wh, lights 100 Wh, CPAP 40 W × 8 h = 320 Wh: about 2,400 Wh/day. ÷ 0.85 ≈ 2,800 Wh. That's more than a single 2 kWh unit holds, so plan on a larger or expandable system, or recharging during the day, or unplugging the fridge for stretches.
Common questions
How do I calculate how long a power station will run my device?
Divide usable battery energy by the device's power draw. Usable energy is roughly the rated Wh times 0.8 to 0.9, after inverter and conversion losses. For example, a 1,000 Wh unit running a 100 W load lasts roughly 8 to 9 hours.
What is the difference between watts and watt-hours?
Watts are the rate of power use at a moment; watt-hours are energy over time. A 100 W device running 5 hours uses 500 Wh.
Why does my fridge need more than its rated watts?
Compressor motors draw a brief starting surge, often several times their running watts. The power station's surge rating must cover that burst.