The heat of peppers are in the seeds and membranes of the pepper. Also peppers can vary in heat from pepper to pepper and depending on how much water the plant got. The more water the more dilute the peppers will get.
Serranos are more consistent than Jalapeno's which can range from med hot to not hot at all.
I put peppers in soups and stews all the time for a little extra heat. That is more or less simmering or boiling them. If they are whole, then the stew or soup is not all that hot unless I bite into one, so for more heat I would chop up the pepper with the seeds and put it in the pot. How many you want to chop depends on how hot you like it and how hot the peppers are.
We used to put three hungarian wax peppers in our soups. My husband put in 3 habaneros in one time instead and it was even too hot for him.
Happy gardening in Hawaii. Gardens are where people grow.