I'm in central WI, have almost 2000 sq ft conditioned and had 1489 hdd last month. I used 120 therms and 639 kwh heated by natural gas forced air in a house built in 1905. There's myself and my wife and a fair amount of electronics, aquarium and my detached workshop (saws/lights/dust cleaners) on the electric bill.
What's eating your electricity?

Assuming the bulk of the electricity use was within conditioned space, converting that to it's thermal-equivalent adds another 77 therms of "fuel" input to the house, so call it ~0.09 therms/HDD, which is about half what my ~2000 square foot 1923 antique of a house currently uses, heating with a mid-efficiency gas boiler.

The COP of your geo is probably about 3, so I expect with other power uses subtracted, that 77 "therms" of electricity is applying bout 150-180 therms of heat into the house & pool. You pay a bit up front for it, but the efficiency of geothermal is for-real.

greentree the geothermal heat pump is the big energy user and the pool circulation / filter pump runs off peak as well, 350w for 14 hours a day. I am guessing my increased electrical consumption is making up for my lower gas usage. The geothermal runs between 50-60 hours a week right now and about 35 in the spring summer and fall. In the summer I heat the pool and cool the house and ignore the field.

Dana so your say a total of 137.5 therms? Or do you mean we consumed 137.5 therms of energy. But then roughly adjusting for the geo we actually "put out" about 210 therms of heat in to the space. That would be .138 therms/hdd? I have no idea how that might compare to a similar sized newer house. I do like comparing the energy consumed / hdd.

Is that typically how it is calculated? I keep hearing mention of millions of BTU per year.

MMBTU/annum is kicked around when trying to determine what's sustainable for the planet as whole, but is independent of climate differences.  BTU/HDD gives a measure of the combined efficiency of the building envelope and heating system independent of climate.  Others (like the PassivHaus folks) also put in a per-square-foot factor.

A home in Seattle or Vancouver can have nearly twice the BTU/HDD number of a house in Green Bay, yet have the same MMBTU/annum number, since Green Bay has about an 8K-HDD climate to about a ~4.6K-HDD climate for the ocean-tempered PNW on the western slope of the mountains.

The exercise where you guesstimate the geo output is useful for comparing the performance of the building envelope, ignoring the efficiency factors.  If you do that, you also need to factor in the efficiency of the natural gas burner. 100 therms burned in an 83% efficient boiler/furnace is only delivering 83 therms as heat to the building- the rest went up the flue.  So factor that into your calc from gas use if you're going to use the COP of the geo for a fudge-factor. Unless you know otherwise, assume all kwh consumption is inside the building envelope, at a conversion of 29.3kwh/therm, and use your lowest-month electricity use as the "non heating/cooling" baseline to subtract before applying the COP of the geo.  That way you'll end up with gas-burner output + geo-output + other power dissipation for your total thermal energy applied.

In pretty-good new construction built to code in my neighborhood (~7K-HDD climate) tends to run 0.12-.15therms/HDD (12K-15KBTU/HDD) in actual thermal load (efficiency of systems factored) but better ones I've looked at run 7-10KBTU/HDD.  My antique is currently running ~14KBTU/HDD, down from ~22-24K less than a decade ago, with known thermal leaks left to fix. (It'll likely be under 13K once I get through hacking on the heating system, so the air handler rarely runs, since air handlers drive a significant amount of infiltration.  TBD.)

Good point on the furnace; it is rated at 93%. The problem with our geo setup is the pool requires heating all year round. I do have an hour meter on the geo unit that I record weekly and I know the running consumption is close to 4.5kw so I could go at it from that direction as well. The problem really is trying to pull the pool out, I am sure I get a decent amount of heat from the pool in the house even with the room being insulated all the way around. In summer our cooling load is likely more that it would be without the pool, but then we heat the pool and dump the cooling in to the house if it needs cooling.

I guess bottom line I am somewhere between .09 and .13 therms / hdd and that isn't way off, considering the indoor pool.

"... and had 1489 hdd last month."  I'm curious as to where one obtains up-to-date HDD data for various locations. I live in Colorado, west of Denver, and would like to do  some similar comparisons to Brock's, Greeentree's, etc.  Also, I assume that the base temperature used for HDD is 65 F.  Is that correct?

Thanks for the help.

PMH

My utility provider has this information on my online account.

http://www.degreedays.net/

How perfectly does heating energy usage track HDD? Obviously windy days, sunny days, etc have some effect.
http://www.energylens.com/articles/degree-days

That's a decent article on how NOT to use degree day data. But given sufficiently long sample periods and sufficiently good correllation much can be surmised about heating system or envelope efficiency. Monthly is usually good enough for 2.5significant digit accuracy.  Weekly or daily, no way.

While outdoor temp is a woefully inadequate model of instantaneous heat load, average outdoor temp tends to be a pretty good model of average heat load for most buildings. But buildings designed for high solar gain will have significant error using a temperature model alone.

I got ours off our utility bill. I have found it on weather underground as well, but the advantage of using the uitlity one is their monthly HDD's matches the billing period.