You are right. To a first approximation the 7805 regulator's efficiency is Vout/Vin, or in your case 5/24 = 20.8%.
You also have to be concerned with maximum current draw. The actual power dissipation of the regulator is (Vin-Vout)*Iout = (24-5)Iout = 19Iout. This total power dissipation heats up the regulator and if it gets hot enough, it shuts down. My rule of thumb is to keep power dissipation <1W for TO-220 packages. This would limit your current to about 50mA, enough for the Arduino circuitry but not much else.
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The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected