With the weather warming up, and yes, I've given in and turned on the a/c a few times, I've been wondering how to compute cooling costs compared to a regular central air system.

I have a new 4 ton GeoMax 2, and the a/c works really well so far.  I know how to figure out both cost and KWh for cooling above and beyond normal base usage of electricity.  However, I don't know what the cooling costs were before the geothermal. 

My house had a 3 ton central a/c, installed in 2000.  Is there any way to estimate the cost of such a unit to run?  I don't have any electric bills from last year (previous owners), and we never really used the a/c when we moved in.

Did contractor give you op cost calculation print out for new geo system? If so "existing system" should be on there as well.
j

3 ton system installed in 2000 might have been a SEER 10 unit. Geo should be a good bit higher. Just based on info provided it may be that your new geo is a tad oversized for cooling, which will push its efficiency down a bit.

For a totally wild guess I'll say that geo should save 1/3 of the cost of the old system for cooling; possibly evn 1/2 if leaving water is 85 or less. That is strictly guesswork. Much depends on quality of installation, operating conditions and ductwork.

The installer didn't give me anything useful in that regard. I'm trying to work out payback on the system. The winter has been great! Far cheaper than the previously existing oil. I wasn't sure how to figure out how much I'm saving in the summer. I guess I'd rather err on the side of caution, and estimate:

Electric Bill - Base Use Cost = Geothermal Cooling Cost
Estimated Savings: 0.5* Geothermal Cooling Cost

Test Numbers:
Bill: $120 - Base Use: $105 = $15 cooling
Estimated savings: 0.5*$15 = $7.50 saved over SEER 10 Central Air

Stuart, I am a homeowner and located here in NJ. I also have 3 ton geo with approximately 2000 SF, 2 zone. So far our bills for the month of April and May are both $59 only (gas and electric combined). It hasn't been that hot so our cooling system hasn't been 100% used. In fact, the past weeks have been cold again so we're back to heating mode. I think we only used our cooling for about a week or so. But to compare when we didn't have central AC (Old system was AC window unit for summer and heating oil for winter), our bills run about $120 to $140 per month for both electric and gas. Our hot water is gas but now has desuperheater connected to it. It's probably why we have lower bill too.

We are still saving a lot compare to the old system. I can't complain.