How to find the wattage of a power relay?
The marking on the relay tells you it takes 5v.

Apply 5v, and measure the current. Multiply the current by 5v, to get watts.

The temperature coefficient of resistance for copper is about +10% per 25 degrees C rise. If the relay is going to be used in a hot environment, you will find that it takes less power when hot. It will also be taking slightly less power after being on for several minutes, due to self heating. Expect that drift, don't let it confuse you into thinking something mysterious is happening. In fact, I often measure winding resistance to estimate how hot copper coils have become in use.