I personally use Sildenafil 25mg (bite off half of a 50mg tablet). In my case, it ensures that I get a rock hard erection, last longer (around 40 minutes for 1st shot) and recover between shots very quickly (typically around 10 minutes). My energy level is also very high for 2 shots. Note that sometimes I have trouble "cumming" during the 2nd shot. I would like to emphasize that although this medicine does raise my energy level during the session (and between shots), the delay in ejaculation does mean that I get physically tired during the 2nd shot.... and so does the girl. No wonder Sildenafil ("Viagra") is known as the magic pill.
Note of caution: improving your physical conditioning via workouts and proper diet is preferable to using Sildenafil or other medicines/enhancers.... especially if you have other underlying medical issues that could cause complications (like low blood pressure). In that case, it is better to see a doctor before trying an enhancer. If you are in good physical shape, then Sildenafil can help improve the quality of your erection and also the recovery time between shots.
Another variable is alcohol consumption before or between sessions. That might impact your energy levels too. Alcohol may also impede the benefits of performance enhancers.