What are you running?
Set a quantity and the hours per day you need it. Every watt figure is editable - yours will differ.
| Appliance | Watts | Qty | Hours |
|---|---|---|---|
| Refrigeration | |||
| Fridge (full size) 1000W surge · 33% duty | |||
| Chest freezer 800W surge · 30% duty | |||
| Mini fridge 450W surge · 30% duty | |||
| Essentials | |||
| LED light bulb constant | |||
| Phone charging constant | |||
| Laptop constant | |||
| Wi-Fi router + modem constant | |||
| TV (50 inch LED) 150W surge · constant | |||
| Box fan 200W surge · constant | |||
| CPAP (no humidifier) constant | |||
| CPAP (with humidifier) constant | |||
| Motors and pumps | |||
| Sump pump (1/3 hp) 1800W surge · 20% duty | |||
| Furnace blower 1200W surge · 40% duty | |||
| Well pump (1/2 hp) 2500W surge · 10% duty | |||
| Anything that makes heat | |||
| Microwave 1500W surge · constant | |||
| Coffee maker constant | |||
| Electric kettle constant | |||
| Toaster constant | |||
| Hair dryer constant | |||
| Space heater constant | |||
What you need
Capacity includes a 15% allowance for inverter losses.
Set a quantity above to see a recommendation.
The three numbers
Surge (W) = biggest single surge + running watts of everything else
Capacity (Wh) = ∑ (watts × duty × hours) × days × 1.15
People size on capacity alone and get caught by the second line. An inverter that trips the moment the compressor kicks in is useless no matter how many watt-hours sit behind it, which is why this tool reports all three separately rather than collapsing them into one reassuring number.
Duty cycle is the number everyone drops
This is where most sizing advice online quietly goes wrong. A fridge is rated at roughly 150W, so the tempting arithmetic is 1000Wh divided by 150W, giving about seven hours, and that figure gets repeated constantly. It is wrong, because a compressor does not run continuously. It cycles to hold temperature, running roughly a third of the time in a normal room, so the fridge that draws 150W while running averages closer to 50W across the day.
That is the difference between "your 1kWh unit dies before morning" and "your 1kWh unit very nearly does a full day". Same fridge, same station, and the only difference is whether the person doing the arithmetic accounted for the compressor switching off. This tool models duty cycle per appliance, which is why its fridge answers are longer than the ones you will find elsewhere - and closer to what people actually report.
Duty cycle is not fixed, though. A fridge in a hot garage, packed full, opened every ten minutes, runs a much higher fraction of the time. Treat a third as a fair-weather default, not a promise.
Surge, and why motors are different
A motor pulls a large gulp of current for the fraction of a second it takes to get spinning. A fridge running at 150W can spike near 1000W; a sump pump at 800W can hit 1800W or more. The surge line above adds the biggest single spike to whatever else is already running, on the reasoning that two motors starting in the same instant is bad luck rather than a design case. If two of your loads genuinely start together, size for both.
Resistive loads - kettles, heaters, toasters - have no meaningful surge. They just pull their full rating from the first millisecond to the last. Which brings us to the thing this category cannot do.
Why the 15%
The capacity printed on the box is what the cells hold, not what reaches your appliance. The inverter that converts DC to AC runs at roughly 85-90% efficiency and the remainder leaves as heat and fan noise. Cold takes another bite, and most units keep a little reserve rather than running to true zero. 15% is the floor. Push it to 20% if you expect to be cold, or if the consequences of guessing low are worse than spending a bit more.
Common questions
What size power station do I need?
It depends on three numbers, not one. Your continuous watts is everything running at once, and it sets the minimum inverter rating. Your surge watts is the biggest motor starting up while the rest is already running, and a fridge can pull roughly 1000W for a second or two. Your watt-hours is the running watts multiplied by the hours you need them, plus about 15% for inverter losses. For most people wanting the essentials through an outage, that lands in the 1kWh to 2kWh class.
How long will a 1000Wh power station run a fridge?
Roughly a day, not the eight hours you will see quoted elsewhere. The common mistake is dividing 1000Wh by the fridge running wattage of about 150W and getting seven hours. A fridge compressor does not run constantly - it cycles on and off to hold temperature, typically running about a third of the time, so a fridge that draws 150W while running averages closer to 50W over 24 hours. That is roughly 1200Wh a day, so a 1kWh unit gets you close to a full day and a 2kWh unit comfortably clears it. Duty cycle rises in a hot room and with a full fridge that gets opened often.
Why does my power station need a surge rating?
Anything with a motor - a fridge compressor, a sump pump, a well pump, a furnace blower - draws several times its running wattage for the moment it starts. A fridge running at 150W can spike to around 1000W on startup. If the inverter cannot supply that spike it shuts down, and it does not matter that the running wattage was well within range. This is why buying on continuous watts alone leaves people with a unit that trips every time the compressor kicks in.
Can a power station run a space heater or a kettle?
Briefly, and it is almost always the wrong tool. Anything that makes heat converts electricity straight into it at 1000W to 1500W with no duty cycle to save you. A 1500W space heater flattens a 1kWh unit in about 35 minutes. Power stations are for the fridge, the router, the lights, the CPAP and the laptop. If the plan is heat, the plan is a generator or a fuel stove.
Why add 15% on top of the watt-hours?
Because the number on the box is the cell capacity, not what reaches your appliance. The inverter that turns DC into AC is roughly 85% to 90% efficient, and the rest becomes heat and fan noise. Cold weather takes another bite, and most units hold a little reserve rather than discharging to absolute zero. Adding 15% is a floor, not a luxury. Adding 20% if you expect cold is defensible.
Are these appliance wattages accurate for my appliances?
They are ballpark planning figures, aggregated from plate ratings and published measurements rather than measured by us. Real draw varies a lot by model, age and setting - fridges especially, where a new inverter-compressor model can average half what an old one does. If a number matters to your decision, measure your actual appliance with a plug-in energy meter for a day. Every figure in this tool is editable for exactly that reason.
Got a number and want the shortlist? Our portable power station guide has the picks by class. To convert a cell or power bank instead, use the mAh to Wh converter.