I was curious about that myself. Did some snooping and came across this.

[ (your party's overall force) / (10 * your party's number of compositions [1 ~ 4]) - (enemy's defense / 4) ] * [1.01 + 0.01 * (your party's number of compositions [1 ~ 4]) ] * (ability multiplier)

The power of the Solar Drive is not affected by your helper's overall force, but their team's ability multipliers will go along with the above formula.

Basically, if I'm understanding this right, it's based on the average, total power of all the squads you bring into a campaign, minus the helper squad.