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@AndreasKvist you are going through the correct exercise around PoE budgetary planning, I just wanted to add the reminder that the MR42E 20W figure is definitely a maximum, and on average might only need half that much power.
Stealing from an old post I made, go to the Switch > Switches page and click into the switch and click on the power tab. You will see a "Consumption" section and a "Budgeted" section. The number to worry about is the consumption. The budgeted number is just reflecting the total amount of theoretical power that could be consumed, as if every device pulled the full 15 or 30 watts of power it advertised via CDP/LLDP when it first negotiated.
See screen shot below. I have a compact MS220-8P switch running 7 PoE devices all the time (MV cameras and MR APs). Looks at my "Budgeted" section, it says 140W/124W. That's showing that the switch max budget is 124W of PoE power, and those 7 devices (at max power draw) would take 140W. That's because 2 of my devices are an MV72 and an MR52 both of which negotiated 30W, and the other 5 devices negotiated 16W, so there's the 140W number. It rounds 15.4W to 16W when doing this to err on the conservative side.
But notice the consumption tends to hover around 41 to 45 watts. Will that fluctuate? Sure. I don't watch it that closely, but I've never seen it over 60 watts, not even halfway to the max the switch can deliver.
