In a circuit, how much wattage resistor should I use? I'm using a 400 mAh 9V battery so should I use a resistor of wattage more than 9x0.4=3.6 watts!!! Coz I always used the 1/4 watt one that came with my kit. Does a circuit use up all the current and voltage or should my v and i values be lesser in the VxI=P equation?

 

Two in an hour today!
https://forum.allaboutcircuits.com/threads/ohms-law-for-noobies-or-the-amp-hour-fallacy.69757/

You don't calculate wattage based on current that is not going to flow. You use the current that will flow for the math.

 

9V x 0.4A = 3.6W

Is there a single resistor that is consuming this wattage?
Work out the power dissipated by each resistor separately, then double it for the actual resistor rating you want to install in the circuit.

See this:
https://forum.allaboutcircuits.com/threads/resistor-wattage-help.133880/#post-1114963

 

400 mAh is the capacity of a 9V battery. A 22.5 ohm resistor across the terminals would draw 400 mA (and in theory last an hour). You'd want that resistor to be 5W (standard size with some headroom) as that wouldbe the only thing consuming current. Note, the 22.5 ohm resistor would not actually draw 400 mA because batteries have internal resistance that needs to be factored in. There are techniques for measuring IR, if you care.