I am going to assume the 13 Seer unit is the builder's grade unit of the Goodman line. This is not a good choice.

Has your contractor to verified with a load calculation you need a 2.5 ton condenser? A 1200 square foot house with new windows and insulation would probably would be fine with a 2 ton unit. You could put the saved money towards a better unit.

These are AC or Heat Pump condensers?

Post models. Both condensers include matching evap coil?

Furnace is conventional model or var speed model? If paired with air handler, is it conventional blower or var speed blower.

I would say a range of 7-10% savings. If you use $300 of cooling in summer, you could save $21-$30. As you can readily see, a very long payback based on the $800 upgrade price.

I do not care for Goodman though and would suggest spending a bit more on a better condenser. You need to understand the warranty though.

IMO

my question is why the lowest efficiency is consitered?
meeting code only means that you have met the
lowest standard allowable by law..its the least you can do.

I'd look at 14SEER & 15SEER prices as compared
to 13SEER. and make sure that new ducts are
mastic sealed. not foil tape, not duct tape but
a mastic that will provide a long life seal.

best of luck.

I don't want to get off subject and certainly not disagreeing with my forum friend energy, but both dealers and manufacturers must do a better job in keeping upgrade prices in line with a reasonable payback.

What is a reasonable payback may differ from customer to customer. My idea at a minimum it should be less than 10 years.

I would say from the OP that this dealer failed to do this.

IMO

Can you explain the difference in the warranty in the two Goodman units? It sounds like you are getting a better condenser for the extra $800. Is the air handler also being upgraded?

When you wrote "going solar", what do you mean, PV, thermal for DHW, thermal for space heat? If PV, are you staying on the grid?

The 15 Seer will probably come with a txv valve and the 13 seer a piston orifice. The 15 seer will do much better removing humidity in my opinion. If I went with a 13 I would get a coil with a txv. So with the 15 and lower humidity levels a higher temp setting might allow the same comfort level which would save money also. I would say the 15 will be quieter also inside and outside since I doubt it is a builders grade unit.

Minimum 13 seer with matching coil that is a txv valve and if it is a heat pump add demand defrost to the list.

For country

Be careful assuming anything with Goodman...

LOL...

IMO