While it’s obvious that more power means greater efficiency, it also translates to a higher price. That’s why it’s important to factor in two major aspects when choosing a lawn tractor based on its horsepower — the amount of grass that you need to cut, that is, the size of the lawn and the terrain. Larger lawns require more engine power in order to save your time.
Generally, I wouldn’t recommend anything below 15 HP. This is the bare minimum, and you’ll surely need more if you have a difficult plot with lots of uneven ground as the machine will require extra power to negotiate all the bumps and pits.