Jalapenos are significantly hotter than poblanos -- so no substituting!
They're also quite different in size, with poblanos typically being much larger. Here's a look at them side by side along with their respective Scoville units:
The Scoville Scale is a quick way to gauge the heat of a chili pepper. Below you can see where the two peppers land in the scale -- note that it heats up as you go higher on the scale:
Growing conditions can affect the heat of a chile, and that's why you'll see a Scoville range instead of an exact number. So using the above table, it's fair to say that jalapenos have a capsaicin concentration that is roughly 2-5 times higher than poblanos. That's some heat!
And their flavor?
Poblanos have a unique flavor that you won't get from other chiles -- a rich, earthy flavor that works wonders in a variety of dishes, like this Roasted Poblano Soup.
Note that using 4-5 poblanos still gives this soup plenty of zip! Just because poblanos are towards the bottom of the Scoville scale doesn't mean they won't bring the heat.
And keep in mind that poblanos are roasted in most recipes -- this really enhances their flavor so it comes highly recommended if you're new to poblanos. For example, in the above soup the poblanos are roasted for 25-30 mintues before being pureed.
Jalapenos have a sharper flavor than poblanos and can be used either raw or roasted. They are my default chile for a simple but delicious Tomato Jalapeno Salsa:
I like to test jalapenos by sticking the tip of my tongue here:
The heat can vary amongst similar looking peppers so it's an easy way to get a quick gauge of their heat. But I wouldn't recommend this if you are sensitive to spicy foods. More tips on handling hot chiles.
Okay, keep in mind that jalapenos are hotter than poblanos and all will be well in your kitchen!
Let me know if you have any other questions about these chiles! For reference, here are some other articles on my site to fill in any gaps: