How long will a portable power station actually last?

The box says "1000Wh," you do the mental math against your mini fridge's wattage, and you figure you're covered for a couple of days during a power outage. Then the power station taps out well short of that, and the fridge's compressor kicks on with a click and a brief shudder that makes you wonder if you're overloading the thing. Both surprises trace back to the same gap: the number printed on the box isn't the number you actually get to use, and average wattage isn't the peak wattage a compressor pulls at the instant it starts.

Rated capacity vs. usable capacity

A power station's rated watt-hours describes the total energy chemically stored in the battery. You never actually get to draw all of it, for two separate reasons:

• Depth of discharge (DoD). Lithium batteries last far longer if they aren't run down to absolute zero or charged to absolute 100% every cycle, so most units — and most sensible usage patterns — reserve some percentage of rated capacity rather than draining to the cell's true empty point. A typical usable DoD lands somewhere around 90%.
• Inverter losses. The battery stores DC power, but most of what you plug in — a fridge, a lamp, a laptop charger — runs on AC. The inverter that makes that conversion isn't lossless; a chunk of every watt-hour is lost as heat in the process, commonly landing the inverter somewhere around 85–90% efficient.

Stack those together and a "1000Wh" power station might only deliver somewhere around 750–800Wh to your actual devices — a meaningful haircut before you've plugged in a single thing. This is exactly why the box number and the real-world runtime never quite match.

How solar panel wattage relates to recharge time

Recharge time works the other direction: how long it takes a solar panel to refill the battery back to its full rated capacity. The panel's nameplate wattage is a best-case number measured under ideal, direct, perpendicular sunlight — real-world output is lower due to panel angle, cloud cover, dust, and losses in the charge controller (MPPT losses), so a "200W panel" more realistically delivers something like 70% of that nameplate figure across an average day.

That effective wattage is what actually determines recharge time: rated battery capacity divided by the panel's effective output gives hours of sun needed for a full recharge. Note that recharge time is measured against the full rated capacity, not the smaller usable capacity — you're refilling the whole tank, even though you only get to draw from most of it.

Worked example

Take a 1000Wh power station with 90% usable depth of discharge and an 88% efficient inverter, running a 60W mini fridge continuously (24 hours/day) plus a 65W laptop for 4 hours/day, recharged by a 200W solar panel at 70% real-world efficiency:

• Usable capacity: 1000Wh × 0.90 × 0.88 = 792Wh.
• Daily load: (60W × 24h) + (65W × 4h) = 1,440Wh + 260Wh = 1,700Wh/day.
• Runtime: 792Wh ÷ 1,700Wh/day ≈ 0.47 days — roughly 11 hours before the battery is spent.
• Effective panel output: 200W × 0.70 = 140W.
• Recharge time: 1000Wh ÷ 140W ≈ 7.1 hours of usable sun for a full recharge.

That 11-hour runtime against a fridge running continuously is a much shorter window than the "1000Wh should last a day or two" intuition suggests — because a compressor fridge running 24/7 is a genuinely demanding, continuous load, not an occasional draw. Drop the fridge to a more realistic duty cycle (compressors cycle on and off, not run flat-out all day) or add a second panel to shorten the recharge window, and the picture improves considerably; the point of running the numbers is seeing which lever actually moves the outcome.

The startup-surge caveat for compressor devices

The wattage figure you'd enter for a fridge, freezer, or window air conditioner is its steady running wattage — what it draws once the compressor motor is already spinning. But at the instant a compressor starts, it briefly pulls several times that figure, sometimes 3–7x the running wattage for a fraction of a second, as the motor overcomes its own inertia. This surge is sometimes listed as "starting watts" or "LRA" (locked rotor amps) on an appliance's nameplate.

Average-wattage math like the runtime calculation above says nothing about whether the power station can survive that instantaneous spike — that's a separate spec, usually listed as the unit's surge or peak output rating, and it's worth checking against a compressor appliance's starting wattage specifically, not just its running wattage. A power station with plenty of average capacity can still trip its own overload protection on a fridge's startup surge if its instantaneous peak rating is lower than the appliance needs at that one moment.

Getting a realistic number before an outage

A few habits make the runtime estimate closer to what you'll actually experience:

• Use nameplate or measured running wattage for each device, not a guess — a cheap plug-in watt meter pays for itself the first time you use it to size backup power.
• For compressor devices, separately confirm the power station's surge rating covers the appliance's starting watts, not just its running watts.
• Plan solar recharge time around realistic daily sun hours in your location and season, not the panel's best-case nameplate output.

Our solar generator runtime calculator takes your power station's rated capacity, depth of discharge, and inverter efficiency alongside up to two connected devices and their daily hours, and returns usable capacity, daily load, expected runtime, and recharge time for a given solar panel wattage. Run your actual fridge, lights, and electronics through it before the next outage, so you know the real number instead of the one printed on the box.