Human energy consumption has always gone up when the supply got more efficient, from coal to oil to fission. While here, the power generation itself won't have such a drastic impact (unless humanity overdoes it, though, we seem to always do that) but maybe more power means more consumption and demand for other goods that get used up while using things that consume power. And with enough power you can think about making things that consume more or even build more things that consume more power. Lots of processes that were considered too wasteful suddenly become feasible.
Just like with digital documents, when people predicted that paper wouldn't be needed in the modern economy and the demand for paper would decline, it has actually skyrocketed - because a paper document isn't important anymore. You throw it away and print a new one when it has a crease.
Historically, we've been very bad at predicting such side effects. So I won't buy in a 'fusion solves everything' mindset. Fusion power can be a boon for humanity on a global scale but it could also make some problems worse.