OK, explosions may be a problem. One could then go for 0 HTK but also 0 explosion with really small power plants.
Explosions aside, with multiple HS1 power plants I can lose some, one with each hit to power plants area, and am at partial power generation. If I have one big power plant instead it can get only damaged for few hits but with more of them it will be destroyed completely. With smaller power plants I will have still some power generation after the same amount of damage. And as they are smaller and cheaper to repair, I could get power generation for my beam weapons more gradually and thus faster up.
The basic explosion chance is just 5% and forth low energy tech the size of explosion could be just 1-3. With basic Pebble Reactor tech you get double HTK for using HS1 power plants and 5% chance to lose one HTK when destroyed so on that level you always get +1 HTK and have low chance to back lose -1 HTK, so HS1 power plants should be always better than bigger ones. For higher energy tech levels, with explosions counted in, it should be statistically better to use HS1 power plants up to 19 explosion size rather than bigger ones, shouldn't it?