UPS for a 3D printer: size and runtime

A 3D printer needs an uninterruptible power supply (UPS) rated in watts, not just volt-amperes (VA), above the printer’s rated maximum. That means at least 350 W for a Bambu Lab P1S or A1 and 150 W for an A1 mini. A 1500 VA / 900 W unit clears both and keeps a P1S printing PLA running for roughly 50 to 75 minutes.

What size UPS does a 3D printer need?

A 3D printer needs a UPS whose watt rating exceeds the printer’s rated maximum draw, not its printing average. A Bambu Lab P1S or A1 is rated at 350 W on a 110 V supply, so it needs at least 350 W. The VA rating must also cover that draw divided by the printer’s power factor.

Size for the maximum because the heated bed can switch on at full power at any moment, and it pulls the most while heating from cold. Bambu Lab publishes no measured warm-up peak for these printers, so the rated maximum is the safe figure. If your printer lists no rating, the power supply calculator adds up its bed, hotend, motors and fans and gives the draw at the wall.

VA vs watts

A UPS carries two ratings, and Schneider Electric (APC’s parent) states that “neither the watt nor the VA rating of a UPS may be exceeded by the attached equipment.” Watts are the real power the printer converts to heat and motion. VA is volts times amps, which runs higher when the printer’s power factor is below 1. The power factor is watts divided by VA.

printer VA  = printer watts ÷ power factor
UPS check:  printer watts ≤ UPS watts  AND  printer VA ≤ UPS VA

Bambu Lab lists printing power factors of 0.41 to 0.65 for its A1-series and P1S printers. If that power factor held at full draw, the printers would need this much VA:

Printer	Rated maximum (110 V)	Power factor while printing	VA at maximum
Bambu Lab A1 mini	150 W	0.60 to 0.65	231 to 250 VA
Bambu Lab A1	350 W	0.41 to 0.64	547 to 854 VA
Bambu Lab P1S	350 W	0.45 to 0.64	547 to 778 VA

A UPS’s watt rating is never higher than its VA rating, and often much lower. CyberPower’s CP1500AVRLCD3, for example, is rated 1500 VA but only 900 W, a power factor of 0.6. Read the watt figure on the spec sheet before buying.

Simulated sine vs pure sine wave

On battery, a line-interactive UPS outputs either a simulated (stepped) sine wave or a pure sine wave. CyberPower warns that equipment with an Active PFC (power factor correction) power supply “may unexpectedly shut down or crash entirely” on a simulated sine wave, because that waveform “produces a zero output state during the phase change cycle.”

Check your printer’s power supply before you choose:

• Active PFC supply. Use a pure sine wave UPS. Mean Well sells its UHP-350 as a supply “with PFC”; CyberPower lists its pure sine CP1500PFCLCD as “Active PFC Compatible”.
• No PFC stage. CyberPower’s warning covers Active PFC supplies only. Datasheets show which you have: Mean Well’s UHP-350 lists a power factor, while the LRS-350 lists none and sets its input range with a manual switch.
• Unknown. A pure sine wave UPS removes the question.

How long can a UPS keep a 3D printer running?

A 1500 VA / 900 W UPS with two 12 V, 9 Ah batteries runs a Bambu Lab P1S printing PLA at its 105 W average for roughly 50 to 75 minutes. If the printer pulls its full 350 W, the same unit lasts about 15 minutes. Runtime is delivered battery energy divided by average watts.

runtime (min) = usable battery Wh × inverter efficiency ÷ average load W × 60

Makers rarely publish the two middle terms, but their half-load runtime gives you the product. CyberPower lists its 1500 VA / 900 W CP1500AVRLCD3 with two 12 V, 9 Ah sealed lead-acid batteries and 12 minutes at half load:

nameplate battery:     2 × 12 V × 9 Ah = 216 Wh
delivered at 450 W:    450 W × 12 min ÷ 60 = 90 Wh   (42% of nameplate)
P1S printing PLA:      90 Wh ÷ 105 W × 60 = 51 min

That 51 minutes is a floor. Lead-acid batteries deliver more energy when drained slowly. CSB’s datasheet for a 12 V UPS battery shows it giving 206 W for 15 minutes (51.5 Wh) but 68.9 W for 60 minutes and 50 W for 90 minutes (68.9 and 75 Wh). That is 34 to 46 percent more energy at printer-sized loads, which stretches the P1S estimate to about 69 to 75 minutes.

Printer and material	Average watts	Floor (90 Wh)	Likely, new batteries
A1 mini, PLA	80 W	68 min	90 to 98 min
A1, PLA	95 W	57 min	76 to 83 min
P1S, PLA	105 W	51 min	69 to 75 min
P1S, ABS	140 W	39 min	52 to 56 min
Any of the 350 W printers at maximum	350 W	15 min	about 15 min

Averages are Bambu Lab’s measurements. To get your own printer’s average, read a smart plug over a whole print, as the electricity cost calculator explains.

Why runtime collapses when the bed reheats

The bed sets the draw. Printing PLA, a P1S averages 105 W because the bed only tops up heat it already has. A print that starts during the outage, or a bed heating back up after cooling, pulls toward the 350 W maximum. That cuts runtime from about an hour to about 15 minutes. At its full 900 W load, CyberPower lists 3 minutes. That is 45 Wh, about a fifth of the 216 Wh nameplate.

Runtime also falls as the battery ages. CyberPower’s runtimes assume new, fully charged batteries, and CSB rates its battery for up to 5 years of standby service at 25 °C (77 °F). After an outage, CyberPower’s typical recharge time is 8 hours.

Is a UPS enough 3D printer power backup?

For blips and short outages, yes. CyberPower lists a typical transfer time of 4 ms, the gap before the battery takes over. The U.S. Energy Information Administration (EIA) counts 1.5 interruptions per customer in 2024, and outside major events they total about two hours a year. For long storm outages, pair the UPS with the printer’s power-loss recovery feature.

The UPS plugs into the same outlet the printer would, so the circuit load does not drop. Count the printer as before in the circuit capacity calculator.