My solar experience is with roof-top residential panels and batteries. So, I am not able to exactly answer your questions, but maybe I can help guide you. And you aren't going to get past 25% efficiency, not at this time. Anything past that level is solely in the lab stage right now. They might be available in a year or two. Look for Perovskite Tandem solar. Some thoughts / questions:
1) What is it that you want to do with the power? That will drastically impact the best choice. What exactly do you want to power and for how long?
2) Can you go without power and if so, for how long? Solar is great but unreliable. That is where batteries come in.
3) What will you do in bad weather? Those small portable solar panels tend to be not weather-worthy. Meaning they can fly away in high winds, be destroyed by rain / humidity, or have their output permanently lowered by hail.
4) Big brand names of portable solar panels to look for include Jackery, Bluetti, Goal Zero, Anker, Ecoflow, and Renology.
5) Wattage ratings are typical maximum ratings*. Meaning, unless things are perfect, you will get less than the rated power. So, if you need 40 W of power, a 40 W panel won't cut it even on a blue, sunny day. Things you might not even think about will lower output just enough to notice. For example, you might not see any clouds but you might be underneath a plume of small wildfire particles. Here is the current map.
* On cold days, the cloud edge effect can get you slightly above the rated power as the clouds cool your panel and as soon as the cloud moves away the silver lining gets you a short focus of more intense light. In that moment, you can briefly exceed the rated power.