In nearly everyone's mind the word "service" is followed by "to someone" or something. It is a voluntary act, in the sense of "helping", "doing a favour", "being useful". It usually supposes a return, as in "something for something", even if that is implicite, or an exchange of good services. 

Unmotivated service is something else.  It's an act of kindness only meant to please someone, or to promote a noble cause out of the goodness of your heart. It is done without any idea of receiving an advantage in return other than the satisfaction to do good. This one is known as selfless service and it is rare.

There is yet another kind - service to God. This one may be subject to caution because it is more elusive, more easily abused. Bhakti is the sanskrit word for Devotional Service. In the timeless Bhagavad-gita, the blessed Lord is said to teach mankind :

"Whatever you do, whatever you eat, whatever you  give away,

and whatever trouble you take – do it as an offering to Me." (9.7)

Bhakti is devotion of an active kind, not just a sentiment. If you hear me say, "I love you", you have every right to expect some some action behind the declaration. And without displaying highest regards, best behavior, unfailing care and kindness, how will anyone believe in "I love you"? Neither will the Supreme Being. Let's say I only imagine my work to be pleasing to God. According the the above quote, whatever work you do can be offered. Your wages to can be offered, as work allows you to maintain body and soul together. That can be counted as a level of bhakti.  Whatever you cook and eat can also be sanctified as an offering to the Divine, as even ordinary people do in India. Christians sanctify food by thanking God for their meals, Hindus place a sample of the meal on their altar. Each to their own, the result is always devotion.

Personal or impersonal doesn't matter here. Would Divine Energy be less likely to respond to bhakti than a personal Deity?  You'd have to believe that Divine Energy is so beyond care that pleasing Is just doesn't come into the equation. Whatever the case may be, the intention of behaving in harmony with the universe is what counts.

Is service to others devotion? No religious belief is required to do good to others. The only difference with devotion is consciousness. Service to humanity with the awareness of its connection to the larger universal reality is best. Just imagine the feeling of being connected to a conscious universe.