What Size Power Station Do I Need

There are two numbers that decide what power station you actually need, and most buyers only look at one of them. Capacity — measured in watt-hours — tells you how long your devices can run. Continuous output — measured in watts — tells you whether they’ll turn on at all. A station with a huge battery and a modest inverter will sit there doing nothing when you plug in a microwave at startup. And even when both numbers work, the label on the box isn’t the number you’ll actually get. Conflate these two things, skip the surge question, or trust the name-plate rating, and you’ll size wrong every time.

Here’s how to think through all three, in the right order.

Output Watts First: The Gate Your Devices Have to Clear

Before you think about capacity at all, you need to know whether your intended devices can even run on a given station. Every power station has a maximum continuous AC output, and that ceiling is absolute — more battery behind it changes nothing.

The clearest illustration: the Jackery 300 is rated at 300W continuous output. A space heater draws somewhere in the 750–1500W range depending on its setting. It doesn’t matter how much energy is stored — the station physically cannot deliver what the heater needs. It won’t run. Not slowly, not poorly. It won’t start.

Independent lab testing confirms the output numbers on real units: the Anker C1000 caps at 1800W, the Jackery 2000 v2 at 2200W, the EcoFlow DELTA Pro 3 at 4000W. These are the gates your appliances have to clear before capacity matters at all.

The category of devices that hits this gate hardest is resistive heating loads — space heaters, microwaves, coffee makers, electric kettles. Seller reference ranges put space heaters at 750–1500W and microwaves at 800–1500W. These figures come without a stated measurement method, so treat them as directional rather than exact — but directionally, they’re in the range that a 1800W station handles tightly and a 1000W station can’t handle at all.

There’s a second layer the reference tables routinely leave out: startup surge. Motors and compressors — fridges, pumps, some tools — briefly spike well above their running wattage when they first start. A device that runs comfortably at 120W can spike two or three times that on compressor startup. The tables list a single running-watt figure and say nothing about this. It’s why something that “fits on paper” can still trip an undersized inverter the moment the compressor kicks on.

The practical check: find the wattage of your highest-draw device, add real margin for startup if it has a motor or compressor, and confirm your station’s continuous output clears it. If it doesn’t, no amount of capacity fixes it.

The Resistive-Load Problem Hiding in Your Gear List

Not all devices are equal, and the difference is dramatic. Small electronics barely register:

• Phones: roughly 15–30W
• Tablets: roughly 20–40W
• Laptops: roughly 30–200W depending on the machine
• LED lighting, routers: roughly 10–30W

The jump to heating loads is an order of magnitude. A coffee maker (seller ranges: 600–1200W) draws as much as 30–40 phones simultaneously. A space heater on high approaches what a 1800W inverter can sustain — and running a station at or near its output ceiling drains capacity fast. Lab testing found a 1800W-rated station running a space heater lasted roughly 30 minutes before the battery was spent. That’s not a defect; it’s physics. A station that size carries enough energy for a short burst of high heat, not a whole evening of it.

Fridges sit in their own category. A 12V compressor fridge is genuinely efficient — field-measured draw while the compressor runs came in around 36–38W in one tested setup. A household AC mini-fridge running through an inverter is a different beast, with seller estimates in the 40–100W range. Neither of those figures is wrong; they’re different appliance classes. What matters for both is that the compressor cycles — it runs for a while, stops, runs again. The fridge is never drawing continuously, which changes the sizing math considerably.

How to Actually Calculate the Capacity You Need

Once you know your devices can run on a given station’s output, you size for how long. The working approach: estimate your total running watts, multiply by hours of use, then account for the gap between what’s stored and what you actually get out.

That gap is real and consistent. AC conversion losses eat into every watt-hour you draw through the inverter. A factor of roughly 0.85 — meaning you recover about 85% of stored energy as usable AC output — is a common planning convention. Treat it as a reasonable rule of thumb, not a certified spec. Whether the true figure for your unit is 82% or 88% matters less than knowing it isn’t 100%.

A 20% output buffer on top of your summed running watts is similarly conventional — it accounts for the load variation the formula can’t see. Again, directional, not authoritative.

Worked through some seller-provided examples (same source, same caveats about precision): a laptop, router, and lights summing to roughly 100W running for five hours works out to around 588Wh needed. A CPAP drawing roughly 40W overnight for eight hours comes to around 376Wh. These are illustrative — your gear will draw differently — but they show how quickly the numbers land in the 500–1000Wh range for modest overnight use.

A fridge across ten hours is where the formula misleads most. If you take a 120W mini-fridge figure and multiply by ten hours with a conversion factor, you get something like 1,400Wh. But a compressor fridge doesn’t draw 120W for ten straight hours. Field-tested runtimes are far longer than that math predicts, because the compressor cycles off.

Fridges: Why Tested Runtimes Destroy the Formula

This is where trusting real-world data over spec-sheet arithmetic pays off most. Testing tells a different story than the formula, and it’s more useful.

One tested setup ran a 12V compressor fridge on a small portable station — with a couple of ice packs — for about 36.5 hours before the battery hit 10%. Another setup ran an Iceco JP40 fridge on a larger station: roughly 2.5 days in high-summer heat, and nearly 4 days in cooler weather. Same fridge, same station. The difference is ambient temperature — when it’s hot outside, the compressor runs more often and runtime roughly halves.

This is the variable the sizing tables never show you. No formula that takes a fridge’s rated watts and multiplies by hours will capture this. The single biggest lever on fridge runtime isn’t the station’s capacity — it’s the temperature outside. A station that gets you four days of cold storage in autumn might get you barely two in August, in the same setup, with the same fridge.

The practical takeaways from real testing:

• 12V compressor fridges are dramatically more efficient than household mini-fridges running through an inverter — the right fridge matters as much as the right station
• Pre-cooling with ice packs before you rely on the station extends runtime meaningfully
• Hot ambient conditions can cut runtime roughly in half compared to cooler conditions
• Opening the fridge frequently costs more than people expect

The Label Isn’t the Number You’ll Get

There’s one more correction to build in before you commit to a size: the rated capacity on the box is not what you’ll actually draw out. This isn’t fine print — independent lab measurement across multiple units found a consistent gap:

The Anker C1000 delivered 860Wh — about 14% under the implied 1000Wh of the model name. The Jackery 300 delivered 260Wh against its 300 model number. The pattern holds: the name and the actual deliverable aren’t the same thing. BMS reserves, inverter conversion losses, and the gap between what cells store and what comes out the AC port all contribute.

The rule of thumb: plan on roughly 85% of the label number as what you’ll actually use. If your calculation says you need 1000Wh, size to a unit that nominally carries more like 1150–1200Wh. Don’t treat this as a precise correction — treat it as a structural habit. The label is a ceiling, not a promise.

It also follows that model numbers are marketing, not specifications. A station called the “2000” may deliver 1710Wh. That’s still excellent — but it’s not 2000.

Putting It Together: The Right Sizing Order

The mistake is treating capacity as the primary decision and everything else as detail. The right order is the reverse:

1. List your highest-draw device. That’s your output floor. Your station’s continuous watt rating has to clear it — with room for startup surge if the device has a motor or compressor.
2. Sum your running load and multiply by hours. Apply roughly 15% for AC conversion loss. Add a buffer for the variability the formula can’t see.
3. Correct for the label gap. Plan on needing about 15–20% more rated capacity than your Wh math suggests.
4. If a fridge is involved, don’t use continuous-draw math. Use tested runtimes as reference and budget for ambient heat cutting those numbers.

Get the output watt rating wrong and nothing else matters — the device won’t turn on. Get it right and the capacity math is straightforward. The label on the box is where you start; it’s not where you finish.