Interesting concept, well executed overall. Graphics and movements were all really smooth - gun movement that smooth is really rare for a project developed on a timescale this short. Game fit the "running out of power" theme well. Various cool enemies with unique attack patterns.

Things to improve:

It seemed like there was nothing past the initial level in terms of content - it just keeps scrolling right with no enemies. Given how polished and complex the rest of the game was, this was surprising to me. Maybe I'm not playing it correctly which brings me to my next point - this game needs some instructions. I recommend posting instructions and some gameplay screenshots on the page to make the listing more complete.

Nice game, I like the upgrades and overall mechanics. But I'm not sure if there were supposed to be any other enemies besides the initial two, I wish there were more.
It took me some time to figure out the gun-shield switch thing and that to use a shield I need an upgrade.
On the graphics note, if you make pixel art game, it is generally advised to use pixels of the same size, as this improves the overall looks.