Starting watts explained
Updated
Running watts and starting watts
Running watts are what an appliance draws once it is going. Starting watts are the peak in the first moment, while a motor gets up to speed or a compressor kicks on. Heaters, kettles and toasters have no such peak, and electronics such as routers and laptops have none worth sizing for.
The peak lasts a moment, so it barely adds to the watt hours of the day. It decides the power check instead: a station that cannot supply the peak cannot start the appliance, however large its battery.
How big the jump is
Of the 79 appliance types in the data set, 24 have published starting watts. Their starting watts run from 1.2× the running watts for the countertop blender to 10.2× for the portable compressor fridge (12 V). For 13 of the 23 motors and compressors among them, the start needs 2 to 3 times the running watts.
Small compressors show the largest ratios. A portable compressor fridge (12 V) runs at 49 W and needs 500 W to start, so a station with a modest surge rating can fail it even though the running watts are tiny.
Surge ratings on a power station
Makers publish two output figures: the continuous output, which the station can hold, and a surge or peak figure for short bursts. 47 of the 54 stations in the data set publish a surge rating. Where one is missing, the calculator checks the start against the continuous output and says so on the result.
The data set records the surge figure only, not how long a station can hold it. A start lasts a moment, so the check compares the peak with the surge figure and nothing more.
One motor starts at a time
The calculator assumes everything on the list runs at the same moment, then lets one motor start on top. For that motor it swaps its running watts for its starting watts. With several motors, it takes the one whose start adds the most.
- Running at once
- 52 + 150 = 202 Wsmall loads plus the 150 W refrigerator
- Refrigerator starts
- 202 - 150 + 600 = 652 W
- Station
- 300 W continuous, 600 W surge
- Result
- runs at 300 ≥ 202, fails the start at 600 < 652
That station goes under Cannot power it, with both numbers printed. More battery would not help: the limit is power, not energy.
Two motors can try to start in the same moment, for example a refrigerator and a pump when power returns after an outage. The calculator does not model that case. Switch one of them off and start them one at a time, or leave room above the surge figure the result shows.
When starting watts are not published
14 of the 37 motor and compressor types in the data set have no published starting figure. For those, the calculator checks the start at running watts, so it never rejects a station on a guess, and it shows this note on the load: "Starting watts not published; motors often need 2 to 3 times their running watts to start."
With that note on your list, look for a starting figure on the label or in the manual, and enter it as your own appliance. Without one, pick a station whose surge rating clears 3 times the running watts of the largest motor.
Ways to lower the peak
- Start the motor with the rest of the list switched off. The peak drops by the running watts of everything else: 600 W instead of 652 W in the example above.
- Use the label figure for your own appliance. A typical value covers a range of models, and yours may start lower.
- Check the appliance pages, which list typical starting watts and show which stations start each appliance on its own.
Sources
- Power station and appliance figures: the Enough Watts data set, each record with its own sources and dates.