What Size Power Station Do I Need?
Two numbers decide it. Add the watts of everything you want running at once: that is the output the station needs, and anything with a motor wants roughly three times its running figure for the instant it starts. Multiply those watts by the hours you want and add about fifteen per cent for losses: that is the capacity. Most households land between 1,000 and 3,000 Wh.
The short version
- Watts (W)
- what the station can deliver at once. This decides what will run at all.
- Watt-hours (Wh)
- how much it holds. This decides for how long.
- The sizing sum
- total watts × hours wanted ÷ 0.85 = watt-hours needed
- Motors need headroom
- a fridge, a freezer, a pump or an air conditioner wants about 3× its running watts to start
- Heat is the enemy
- a kettle, a microwave or a space heater will flatten any of these in about an hour
- Where most people land
- 1,000 to 3,000 Wh, once the list is honest
Everybody buys on one number, and it is usually the wrong one
The figure on the front of the box is capacity, in watt-hours. It is the one people compare, and on its own it tells you nothing about whether the thing will work. Output, in watts, is what decides whether the appliance runs; capacity only decides how long it runs once it is running.
That produces the two ways a purchase goes wrong. Someone buys a large battery with a small inverter, plugs in the refrigerator, and the station shuts down the instant the compressor tries to start. Or someone buys a big output with a modest battery, runs the fridge beautifully, and watches it empty before the outage is over.
There is a third mistake, and it is the expensive one: sizing for a list nobody will actually plug in. Write down what genuinely has to keep running — usually the fridge, the router, some phones and essential devices — and size for that. Then check what happens to the sum if you add the kettle, and decide whether you were ever going to.
The method, then the calculator
Three steps. One: list what must run at the same time and add up their running watts. Two: take the largest motor on that list and multiply its running watts by three — if the station's output is below that figure, nothing else matters. Three: multiply the total running watts by the hours you want, divide by 0.85 for inverter losses, and you have the watt-hours to shop for.
| What is running | Watts | For | Watt-hours |
|---|---|---|---|
| Refrigerator, averaged over its cycling | 150 | 12 h | 1,800 |
| Router and modem | 20 | 12 h | 240 |
| Two phones and a laptop, topped up | 60 | 3 h | 180 |
| A lamp | 10 | 5 h | 50 |
| Energy needed, before losses | 2,270 Wh | ||
| Divided by 0.85 for inverter losses | about 2,670 Wh | ||
The fridge is ninety per cent of that bill, which is the general lesson: one appliance almost always dominates, and the rest of the list is noise. The surge check is separate — the compressor wants roughly 450 W for an instant, so any output above about 1,000 W has margin.
And to skip the arithmetic, the same sum against published capacities:
How long will one of these run your things?
Pick what you need to keep alive and for how long. The answer is an estimate, not a measurement: that model’s watt-hours, minus 15% for inverter losses, divided by the draw we assume. The arithmetic is printed with every result so you can put your own numbers through it.
The label is usually on the back or inside the door. If it gives amps, multiply by 120 in the US.
Read these as estimates. The draw is a typical figure, not your appliance: a large American refrigerator and a small European one differ by a factor of two, and a compressor cycles on and off, so its average depends on the room temperature, how often the door opens and how old the seals are. We remove 15% for inverter losses; the real figure varies with load. And matching the running watts is not enough to start a motor: a fridge, a freezer, a sump pump or an air conditioner pulls roughly three times its running draw for the first instant, which is what the surge column is about. Check the label on your own appliance before you rely on any of this.
Size by what you are protecting
Find the row that describes your list.
- Phones, a router and a laptop — nothing with a motor
- 500 Wh is generous. This is the cheapest problem on the page to solve.
- A CPAP overnight
- About 40 W without the humidifier and roughly double with it, so 500 to 1,000 Wh. Check your own machine: the humidifier is most of the draw.
- A refrigerator through a working day
- 1,000 to 1,500 Wh, and an output above 1,000 W for the compressor.
- A refrigerator overnight, plus the network
- About 2,000 Wh.
- A fridge and a chest freezer
- Assume 250 W together, which is 3,000 Wh for twelve hours. Consider an expandable unit rather than one large fixed one.
- A sump pump in a wet basement
- Size on the surge, not the capacity: a pump drawing 800 W wants about 2,400 W available to start. It cycles, so the running cost is lower than it looks.
- Anything with a heating element — kettle, microwave, space heater
- Accept that it is minutes, not hours. A 1,500 W heater flattens 2,000 Wh in a bit over an hour, and no portable battery changes that arithmetic.
The size that fits the most common list
If your list is the fridge, the router and the phones, overnight, indoors, then the sum lands at roughly 2,000 to 2,700 Wh with output to spare for the compressor. That is a specific enough shape to name one.
- 2,073.6 Wh, which covers the worked example above with the fridge running the whole night and the door mostly shut.
- 2,600 W continuous, so every motor on the household list starts without the inverter arguing.
- Solar input, which is what changes the answer for outages that run past a day.
- It runs indoors, which for an overnight outage is not a detail: the alternative is opening a door at three in the morning.
Where this is the wrong answer. If your honest list includes a chest freezer as well, size up or buy expandable. And if it includes central air or an electric water heater, no portable station in this class is the answer.
The full BLUETTI page — the whole range, who should skip it, and what a return costs
Other ways to answer the same question
Two smaller stations instead of one large
Cheaper to buy in stages, and one can charge while the other works. The catch is that neither one alone starts your largest motor, so the surge check has to pass on the smaller of the two.
A station plus a solar panel
Changes the question from how much you can carry to how much you can collect each day. It is the only way a battery answers a multi-day outage.
A fuel generator
The right answer for whole-home, multi-day and heavy motor loads, with everything that comes with an engine. Battery versus generator, side by side.
Questions people ask
Do I size on watts or watt-hours?
Both, and in that order. Watts first, because if the output cannot start the appliance the capacity is irrelevant. Watt-hours second, because that is what decides how long it lasts.
How much should I add for inverter losses?
About fifteen per cent is a reasonable working assumption. Divide the energy you need by 0.85 rather than multiplying by 1.15 if you want to be strict about it; the difference is small at this scale.
Is the surge figure really three times?
It is a rule of thumb for induction motors, not a specification. Some compressors are gentler and some soft-start. If a specific appliance is the reason you are buying, read the surge figure from its own label rather than trusting a multiplier.
Can I run my whole house from one?
No, and anything selling you that is selling you something else. These run a list of appliances through an extension lead. Whole-house backup means a transfer switch, an electrician and a different class of equipment.
What about running a space heater?
Arithmetically possible and practically pointless. Heat is the one load that empties any battery in this class inside two hours.
If you have decided
Everything above is the reasoning. If it landed where we think it does, this is the model it points at — and the full BLUETTI page has the whole range, the return terms and the people who should skip it.
Sources and method
The method on this page is arithmetic. The capacities and outputs in the calculator are read from the merchant's own product pages on the date below; the appliance draws are our working assumptions and are labelled as such wherever they appear.
- Capacity, continuous output and surge for every model shown
- bluettipower.com, the product page for each model
- Appliance watt figures used in the worked example and the calculator
- Our own working assumptions, printed alongside every result
- The fifteen per cent inverter loss and the three-times surge rule
- Working assumptions, not manufacturer figures
Checked on the merchant’s own pages: 10 September 2026.