Hi! There is a relation between voltage, current, and power. It's very simple, too! It's voltage multiplied times current.
[tex]P=IV[/tex]
or [tex]VI=P[/tex] might be easy to remember because V.I.P. (like V.I.P. stadium seating)
Then you just have to work with units to get watts. Watts is volt-amps.
You can convert at any point to get watts as the unit. I'll turn the current to amps first.
[tex]40\text {mA}\cdot\frac{1\text A}{1000\text{mA}}=0.040\text A[/tex]
Then, multiply [tex]VI=P[/tex]
[tex]24\text V\cdot0.040\text A=0.96\text V\cdot\text A = 0.96\text W[/tex]
