I dealt with this subject years ago in my preps. I prepared for no power, therefore no fridge. Powdered butter is too expensive and it isn't solid like butter, so I didn't store any. For baked goods, I would use, which I have stored, Butter Flavored Crisco, to substitute for butter. Why spend gobs of money for powder when you can use butter Crisco and get the same results? Don't want to dig out a can but I recall the substitution instructions are on the Butter Flavored Crisco can.
To put on food to eat right then, I stored bottles of Molly McButter sprinkles. It has an intense butter flavor and you will definitely taste butter on your potatoes or whatever you put it on. I haven't tried sprinkling it on toast but it that didn't work well, I'd use a flavored syrup on toast and eat it with a fork.
So, my opinion is, which I did - is, the cheapest method with reliable results is, Butter Flavored Crisco and Molly McButter sprinkles.
I would guess the powdered butter will have a better shelf life than Crisco.