I have lived off grid since 2008, and we built our house with solar. It is an awesome way to power things if the system is sized correctly. for what it sounds like you are looking for, lights and tunes should be no problem--your only issue will be the coffee maker. any electric heating element pulls a lot of power; most coffee makers are in the 1200-1500 watt range. this is a lot of power for a single battery, deep cycle or not. if you just get a drip cone for the top of your cup and cook water on the stove top you would be better off...even if your battery can handle the coffee maker occasionally, it will cycle your battery pretty darn hard. the equations for figuring out how much power you use are pretty straightforward, watts=amps*volts. that would be more useful for the inverter side like your coffee maker. there is a device called a "kill-a-watt" that is pretty handy, you just plug your device into it and it will tell you amps watts and volts. on the DC side, you can look at most anything in an RV and it will tell you amps at 12 VDC... add up your amperage, multiply by time they are in use, and you will get amp hours. compare this with both the amp hours on your battery and the output you think you will get from your 40 w panel (so that would be watts*sun hours divided by volts to give you amp hours). hope this is not too much information, feel free to pm me or post if this does not make sense.