I'm building a fairly simple solar-powered lighting system to use at home, which charges up a battery by day and then the battery powers a few bulbs at night, switching automatically. So far, I got that switching mechanism working with an LDR and a relay.
Now I am planning my circuit to look like this (a few things omitted for simplicity): 
I have a few questions:
1)Does the circuit make sense and do you see it working?
2)I know I need a 'common ground' between all voltage sources, so I simply connect all the grounds (negative battery terminals) to a single node?
3)Each of my bulbs draw 333 mA (4W / 12V). If I put many of them in parallel my total current draw will be very high. Are transistors the best option here, or am I better off with relays?
Thank you
Best Answer
The circuit is rubbish (I'm not noted for my human skills BTW) because the top NPN will only ever produce about 8.2 volts at the emitter to charge the battery - you need to consider using a PNP transistor wired differently.
Also the two voltage sources are floating i.e. their negative connections are floating and please don't whine saying that I should presume they are connected to ground.
Accuracy is something that needs to be a commitment in electronics.
Also, the LED shown in red isn't described as being a 12V type and therefore it might need a current limiter BUT why is it there at all when you have the other LED in parallel?
I also read that you are using lamps but, please get used to the fact that a circuit diagram (and what it contains) is the gospel and, contradicting it with statements about lamps is really just a sign of laziness when using the circuit diagram editor. Get it right in the editor.
In other words, draw an accurate circuit to the best of your ability and let that speak for itself. Transistors all round are probably the best option when it comes to Q3.