Out of curiosity, did you measure the prusa's load on a kill-a-watt or similar while printing, before you put it on the UPS? I wonder what the load during printing is.
IIRC it's around 120W printing something like PLA. Obviously higher when heating (pretty much maxes out the PSU), and higher steady state when printing other polymers.
It was a measured amount at the outlet with a Kill a Watt style device, so including electronics, stepper motors, and PSU inefficiency. That said, I did it a year ago so I could easily be misremembering. I'm pretty confident it was < 150W printing PLA ~215°C with a 60°C bed.
Can't you just look at the power supply ratings? For example, 20A*12V would be 240W. Assume 80% efficiency, so 300W on the 120V side. Seems reasonable for an UPS designed to power a desktop.
You can if you want an absolute maximum of what the thing can draw as a load, it'll be labeled, but very often the real world use of a thing with an AC to DC power supply is a very different wattage figure from what the power supply is theoretically capable of.
Such as having an ATX midtower size 'gaming' desktop PC with an 850W power supply, that might be measured at 350W at the wall under full CPU+GPU benchmark load.
Right, so if your max rated load is below your UPS's capability, it should work with plenty of margin and you can stop worrying about it. If your max rated load is higher than your UPS's capability, then you can either analyze the actual load more to see if it still works, or just buy a beefier UPS.
There's a dramatic difference between max draw and how much energy it takes to keep everything at a given temperature. Particularly so if heating the bed to a high temperature for ABS or PC or whatever. Looking at the power supply rating you could easily be 2-3 times the average real power consumption, which is a big deal if you're trying to use battery back up.
Yes I agree with you? You need to size your battery supply to meet both peak power and overall energy needs.
Generally when folk express concern about running random equipment on an UPS, they are concerned about peak power rather than energy capacity.
I'll also make the claim that most hobby grade 3D printers come with power supplies that are barely adequately rated for the printer's peak power, so using the supply's rating is probably in the right ballpark.
Edit: ah, I interpreted the thread a bit differently. Ignoring the UPS rating aspect, yeah the average power draw will likely be a fraction of the peak. My guess would be around 100W total for most Prusa style printers printing PLA. The stepper motor draw would be highly dependent on the speed of the print and the shape of the part, since power draw will be highest when accelerating at higher speeds.
For a UPS you want to know both the max wattage of the load, as what % of the total output that the UPS's inverter can put out at any given time, and also the nominal steady-state watts so that you can calculate the Watt-hours, which is what will determine runtime on a given battery size.
If you have a UPS with, for example, 4 x 12V 8Ah AGM batteries, it has a certain amount of Wh you can realistically use before you deep-discharge the batteries into severe damage.
I measured this previously on a Kill-a-Watt with a Prusa clona (Monoprice Maker Select v2). Heat-up sequence draws between 120-130W (heating both bed and hotend at the same time). Once at temp for PLA, it only draws around 80-90 average depending how many steppers are moving.
