Being as this has not really been addressed in this thread:
A couple things to think about, 20hp @3600 rpm is the requirement to run it at full power (7200w). As for your setup, ie. 7 to one or 3 to 1, the hp requirements will change. Because you are stepping up, with 7 to 1 you need something like 140hp PTO hp to produce 7200w (60hp at 3 to 1). Put straight. Pure and utter poppycock! Sorry but you need to go back to the text books and basic mechanics.
Hp is a rate of doing work (about 750Watts or 750 Joules of energy per second). 7 times the energy would mean that 6 times the original work done would be lost in the process? As heat? I think not. In any instance.
The torque would certainly change , which is a function of speed and power, but the rate of work will not change. The losses will be in transferring the energy from a low speed shaft to a higher speed shaft - there will always be friction losses.
Losses in tight arcs with belts are well documented - that is why they use cut-away belts (to reduce the compressive forces as the belt is turned in a tight arc). You may notice that a belt will run
hot warm due to the friction in the sheave and the flexing of the material. This is the main loss in transmission power for these systems. It does not really make a lot of difference whether the small pulley is driving or driven.
The general rule of thumb we use is 2 horse power per 1 kW electrical output. This works for the normal run-of-the-mill set-ups we find in most places.
This one recommends 20HP for 7.2kW or near 2.8Hp per kW. !
That would seem to indicate a poor efficiency performance of the generator and probably reflects high copper losses (heat lost in the windings due to I
2R and Ohms Law - due to skimping on the winding wire (cross sectional area). More heat losses then means more cooling required and so leads to a larger fan and consequent power losses along with increased noise levels. Also increases the likliehood of a burn out. It also reduces any transient overload ability that is useful for starting motor loads.
SteveU is about right re the quality and longevity.
Certainly for anyone running any appreciable hours, a 35% increase in fuel costs is a convincing argument for paying extra and recouping that extra capital cost while still having a serviceable unit at the end of the period, or the then continuing lower running costs. A balance of short term gains over long term losses. That unit appears to be little better than automotive alternator generators which are around 50% efficient (and efficiency falls as they get hotter)
Please imagine a step-up gearbox. It would not use seven times the power to achieve its rated output. It would soon glow red hot!! Many tractor driven generators use a 3:1 increase in speed and some use 6:1.
Hope this does not deter you from posting but you do need to understand the difference between torque, power, energy, etc.
Regards, RAB