Far too easy.
You have a great art style and there is nothing wrong with the concept at all.
But the game is so damn easy that, as long as you're able to survive long enough to get a couple of battery upgrades for the larger enemies, you would probably not be able to die in normal, "I am actually trying to win" gameplay unless you suddenly suffered a massive heart attack at the keyboard, and even THAT may not end your game if you have a mini-coil. Really, as long as you're able to get over the early hump of having no money and a battery that drains too quickly, you'll both overcome the 'low battery' problem, and have so much money that you can heal yourself to full for a mere pittance within a couple of stages. And even the battery isn't a problem when you realize that for low-level enemies, you actually kill them faster and for less of a battery drain if you click them a few times instead of holding the button down.
The fact that the game is so easy really hurts it; every enemy feels exactly the same, give or take a larger health bar, when I can easily kill just about everything in the game before it makes it halfway across the screen- and it makes the game feel VERY repetitive. Maybe if you gave enemies weak points you had to hit, maybe if you made enemies shoot projectiles that had to be blasted out of the air (if there ARE any that do this, they didn't live long enough to notice), maybe if you did something to the upgrades to make you less horrifyingly overpowered, this wouldn't be the case.
I know I gave you a low rating, but this isn't a BAD game- you just need to tweak the gameplay so that victory isn't an utter guarantee.