OP: "We use about 300 gallons per year with burning 2-3 cords. I would want to size the new system as if we were not wood burners though."
The KEY factor in all this is your winter BTU demand, both for sizing the HP compressor tonnage AND figuring out how big strips you need (and thus if your panel can carry it). And this info from the OP is key.
Let's say a normal winter is 2.5 cords and 300 gallons total per year. If we figure each cord displaced 150 gallons of oil, then this is 150*2.5 = 375 + 300 = 675 gallons of oil/yr if OP never burned a twig. If this figure includes the boiler is running all summer and it has a standby oil consumption of at least 0.75 gal/day, or 100 gal during the non heating season, we would discount this. But it sounds like the OP shuts it down, so I'll run with the 675 gal/yr figure for heating. Summer shutdown also gives a higher seasonal eff rating (no parasitics).
675 gal * 138,000 BTUs/gal * 0.75 (true eff) = 70 MMBTU/year. DEMAND
If we assume that OP has an annual HDD of 6000, then we can figure: 70 MM/ 6000 = 11,600 BTU/HDD. This is ~500 BTU/hr°F for his house envelope.
In MA, I would design strip heat to carry a -10°F temp with no heat pump, to 70°F, which is +80°F dT. Given that loads are slightly quadratic (due to stack driven losses going like dT^2, while conduction losses are linear) I'd round up to 90 or 100°F dT. This implies that heating with strips during a polar vortex event would be 45-50,000 BTU/hr. Since strips are 100% efficient, this is 13-14.5 kW of strip heat. 14.4 kW of strip heat pull 60A at 240V.
So, if he rips out the boiler, OP needs 15 kW strips
, wired with 75A wiring and breaker (80% service rating).
CHECK: I have an older, 2340 sq ft house with a HP, it would call ~500 gallons per year, and I have 15 kW strips. I have estimated that my demand at 0°F is about 12 kW, based on timing the thermostat duty cycle. So 15 kW is sensible.
OP question: the strips themselves are just small coils like in a toaster that live in the air handler, and cost like $200, and are easy to swap out. The cost is in the heavy wiring run to the air handler. You are looking at 75A wiring and breaker. IF you decided to install undersize strips (while the boiler remains as a second stage), you would still install 75A wiring, but say put in a smaller coil and breaker to match. At a later date (when your service is increased, say), the coil and breaker can get swapped out to 15 kW for a few hundred dollars of parts.
Severe cold weather may mean you need a good heat strip for your Goodman air handler or package unit. The Goodman HKSC15XB SmartFrame 15k 240V Heat Strip might be exactly what you've been Googling to find. It features rust-resistant nickel chromium heat elements, plug-in wiring harness for air...
A heat pump split system is an effective way to keep your home warm, but low temperature efficiency loss is a concern on those really cold winter days. Rely on a supplemental heat strip like the Goodman HKSX Series Electric Heat Kit to alleviate this problem entirely. This upgrade is built to...
The other decision is tonnage. If you get two units/zones, total tonnage. I'm on a 4 ton system, and based upon OPs colder climate and 15-20% higher estimated demand, he needs 5 tons minimum, which is the max size for a single compressor unit. My system carries me to about 24°F outdoor temp. I run the HP down to 5°F, and the second stage makes up the difference.
The issue to come up is the installers. They are all gonna spitball the heating load, as I have above. Some might say OP needs 6 tons (like two 3 ton units) and 20 kW strips (like 10 kW in each air handler). Others (fearing a call back) might spec in 8 or 10 tons (two 4 or 5 ton units) and 30 kW of strips (two 15 kW strips, 150A).
I"m saying the 6-7 tons (two 3 ton units, or a 3 upstairs and a 4 downstairs) and 15 kW total strips is probably fine. Higher tons will get you a lower balance point, and save operating cost (less strip calling).
OP...what is your AC tonnage (its on the compressor label) Its probably 3 tons. This tons heat versus tons AC is why you need a variable speed compressor and more ducting. Its (possible) that you wire the two units so the upstairs unit is covering most of the AC (hot humid air enters at the top, and cool dry air falls) and get away with single or dual speed units. Maybe.
Don't worry about ducts in the attic...get them **masticked** so they don't leak, wrapped in code-level insulation, and bury them. I had to tear mine upon to mastic them after install (seriously), but then found I was losing <2% heat on my longest run. If your installer doesn't mastic ducts in the attic...look for a new installer.
You still need to call an electrician to confirm your service wiring is 100A, not just your box. Or you can read the gauge off the wiring and post it here. If you are limited to 100A service, then you are looking at fixing the boiler, wiring it as a second stage, and putting in heavy wires to the air handlers, but no or dinky strips (to take the chill off). And as said, burning 100 gallons during the summer to keep the boiler from leaking? Or you can bite the bullet, get a new service run and a 200 A panel, and be ready for the future.
When you get quotes... share the tonnage and total kW strip they say you need...