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 sure, and to me it's definitely worth getting the nicer drivers with efficient idling and interpolated 256-microstepping.
Conceptually though, ignoring any power saving idling features or other fancy algorithms, stepper drivers basically want to drive a constant amount of current through the active phase (which is what you're adjusting with the potentiometer). In order to do so, it needs to overcome the voltage drop of the resistance of the windings, and the induced voltage of any changing magnetic fields. The resistive voltage drop is roughly constant, while the induced voltage is proportional to the speed of the motor. Power is equal to current times voltage. The current is constant, and the voltage increases proportionally with velocity, and so power should go up proportionally with velocity. The motor gets hot regardless of what it's doing because of the constant resistive losses, but the total power goes up the faster the motor is moving.
Motors are a lot more complicated than that in reality, and higher end stepper drivers don't actually drive constant current at all times. Semantically, a stepper driver's promise is to move the motor one step "quickly enough" after each pulse on its step input. A clever stepper driver will only apply current when it's actually needed to produce useful torque. Torque is only needed when the motor/load needs to accelerate to get to its target position. Ignoring the rotational aspect, force equals mass times acceleration, and power equals force times velocity. For a maximally clever stepper driver, power is therefore proportional to the product of acceleration and velocity. The motor only gets hot when accelerating regardless of velocity, while the total power goes up when accelerating at higher velocities.
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.
Indeed. Depending on the intended use of the UPS though, the energy capacity might not matter all that much. For a 3D printer, it seems like an UPS would be in place to weather the occasional hiccup in utility power. Utility power interruptions seem inherently bimodal. Either it glitches out for a few seconds and comes back, or it's out for hours. Pretty much any UPS will have enough energy to run a 3D printer for a few seconds or minutes, but very few UPSes exist that would run one for hours on end.
Taking that use case to the extreme, maybe someone lives in an area with very unreliable utility power. They might absolutely need to have enough energy stored to run their 3D printer for an entire print. Maybe they solve that with lots of batteries, but in many places with unreliable utility power, it's more likely they're just going to fire up a gas powered generator whenever the power goes out. Once again, the UPS only needs to last long enough for someone to notice and fire up the generator.
Folk certainly might have more exotic use cases. Maybe someone lives completely off-grid and powers their 3D printer off of solar panels, and a generator isn't a sustainable option. They'll definitely be more interested in energy capacity.
The power capacity seems important no matter what, since exceeding it will either result in a voltage brownout, tripping of a protection circuit, or melting something.
