hot water not lasting
With an avg GPM of about 3 GPM, you'll exhaust the 30 gallon tank in 10 minutes. If your shower head is rated at 2GPM, then you'll get about 15 minutes.
As was previously stated, recovery time is much slower on electric WH. This is why one typically puts in larger electric units to ensure ample supply.
Your other option, is to increase the temp up from 120 to maybe 125. Then as you are showering, you'll mix more cold water in with the hot water to get longer run time. If you have kids, this is NOT recommended due to the potential for them to be scalded.