When testing power supplies, there are a couple of different loads that can be used for this process.
There are benchtop electronic load testers, such as the B&K Precision part shown above, that can be dialed into different load sizes to accommodate testing on a number of different devices. Another benefit of the electronic load testers is that they have built-in protections, which a standard power resistor does not have. The user is also able to program in test sequences to emulate various conditions that the power supply will see throughout its lifetime such as inrush current spike at startup. The user is also able to set up a test that will give a pass/fail result as well. Another benefit is many of these devices are able to be connected to a computer to record results of the testing. Digi-Key currently has a few different options in stock for electronic loads, and they are listed here https://www.digikey.com/short/zhfrdh
If you know the exact load you are looking at testing at a certain voltage, you may be able to get away with using a large resistor. Say you would like to test a 5V power supply at a current draw of 2 Amps. Using Ohm’s law you would be able to calculate that you can use a 2.5 Ohm 10W resistor to simulate the correct load. To be on the safe side I would recommend that you go with a bit higher power rating than the 10W so that the resistor doesn’t get to warm and overheat.
Being this resistor in the above example only requires a 10 watt power rating, we have some resistors in our through-hole section that will meet the requirements. Bourns part PWR220T-20-2R50F would fit the application in this application. The user will have to include a heatsink into the equation, however. As this resistor heats up it will be derated and at a case temperature of 85 degrees Celcius (185 degrees Farenheight) that it is only able to handle about 11 watts of power. As long as the case is kept cool, it would be able to handle up to 20 Watts continuously.
This section for the through-hole resistors is located here https://www.digikey.com/short/zhdh57
If we did not have any resistors with ratings that meet the requirements needed in the through-hole section, the next place I would look is our chassis mount resistor section. The chassis mount resistors are located here https://www.digikey.com/short/zhdhfp In this section, we have more of a selection of higher rated power resistors. These resistors range from under a watt up to 2500 watts for the power ratings. Above part, UAL50-2R5F8 fits the specifications listed in the application above. One thing to note on this part is that without a heatsink it is derated to only 20W of power. In order to accomplish the 50W rating, a heatsink 0.059" thick with an area of 291sq inches is required. This would be roughly a 17" x 17" plate.
If the designer did not want to use a heatsink, there is a type of tubular power resistor that does not require a heatsink for its full power. These types of resistors are located here https://www.digikey.com/short/z9mnf3 Looking into the example above, we were looking for a 2.5ohm resistor. Currently, at the time that I write this post, Digi-Key does not carry a 2.5ohm tubular style resistor. The designer however could use two 5 ohm resistors placed in parallel to get the 2.5ohm rating needed. For this, I would choose Ohmite part number L25J5R0E This is a 5ohm 25W tubular style resistor that would work perfectly for the application.
When testing the power supply, the electronic load is connected to the power supply and then a multimeter is connected. The user is then able to take the measurements from the multimeter with the supply loaded down to make sure it is putting out the correct voltage and is able to handle the amount of current that is needed.