I see many people struggling with their energy, so here are a couple of approaches to never run out of it.
First of All
You'd have to spend something to get anything in return. (and it's not limited to RSG either) It's kinda funny how some refrain from spending anything but expect to make more than others. Here we'll be discussing the Energy recovery items available.
Cold Pizza Slice
These are the absolute bad boys of energy recovery. They fill up your entire energy bar upon consumption and then go into a 24hr cooldown. I like them since they are not "chance-based" - you spend 10K STARBITS (similar to investment) and are guaranteed a daily refill to 100%. They slowly, but surely cover their own cost over time, and even if you are going to stop using them, you can easily list them for 9K STARBITS and have them instantly sold.
I like them best and I haven't been bothered to deal with energy calculations since gathering 21 of them. 21 Pizzas are too many for day-to-day use, but would make sense if you're planning on spamming event missions - as I was doing back then.
Pizza Box
Pizza Boxes are only advised if you're going for a more cost-effective approach and instead of buying a ton of Pizza Slices, you wanna factor in the Pizzas you find during mission.
There's an 18% chance of finding a Pizza during a mission. You can significantly decrease the number of Cold Pizzas you need per day if you can find a balance between these two, but your energy's "availability" becomes a bit more chance-based as you might not find any pizzas for an entire day.
A Pizza Box for every 2 Cold Pizza Slices seems decent. (at least theoretically)
Also these boys cost 7500 SB in the marketplace and have no cooldown. (but you need to find the Pizza in the mission to be able to use them first)
Coffee
You can't buy Coffee directly in the marketplace. Instead, you can use Coffee Cups that store the Coffee you find in your runs. They recover 3% on consumption, double your energy recovery rate for 30mins, and every time you finish a mission during this 30min duration, you'd recover an additional 3%.
You can use them for an even more cost-efficient approach to energy recovery. People usually use them when they want to spam 5-min missions as the duration of the coffee itself is so short.
You can get them for 5000 SB in the game's marketplace.
Birthday Cakes
These are limited-edition NFTs and work the same as Cold Pizza slices but are worth twice more - as the cooldown duration is only 12hrs after each use. You can still get them now (until the 30th of April) by paying 30K SB and doing the event's mission in the "Special Missions" section of the game. They'd most definitely gain value after the event's expiration.
The Bottom Line
I prefer the Cold Pizza Slices, as they are a guaranteed resource for energy. I paid the price a long time ago and they've covered their cost many times over. But if you're struggling to get all of them in one go, like I did, put your investment into energy recovery over a span of time instead of one go.
Cakes are also a solid investment as they most probably increase in value after some time, and even if they don't, you'd have some "fast pizzas" for sure.