Powering arduino and LED strip from 12v

Hey so I did some searching and was not able to find an answer and was hoping some of you might be able to answer a few questions. I'm relatively new to microelectronics so bear with me...

I have an arduino uno, or rather an ATMega328 on a board hooked up to a small circuit with some TIP120s. It's made for driving an led strip (SMD 5050, 3 meters long). Previously when I had this on an actual UNO I had been powering the led strip and arduino separately.

However I'm trying to make it a smaller standalone unit. I built a 5v power regulator as described here (Beginning Embedded Electronics - 1 - SparkFun Electronics) and it seems to be doing the job just great. The only thing I'm worried about is powering the LEDs off the 12v (before it goes to the LM7805). While I'm pretty sure it will work as the LEDs will rarely be all full white, I'm wondering how I would even begin calculating this? How would I calculate the power usage so I could find the optimal power source to drive both?

Thanks for reading hope that makes sense!

Hi, how many rgb leds on your 3m strip?

The leds could require up to around 60mA each. However, each group of 3 leds probably share the same current. You must post a link to any component for which you need advice rather than our guesses.

Compared to that, the current requirements of the mega328 will be very small. If you assume 100mA that will be more than enough.

PWM-ing 3m of those leds will be switching a lot of current, so I would recommend a large cap, e.g. 1000uF, accross the 12V supply.