The source for the MMT number is a spreadsheet I devised that compares various scenarios based on available data and references, and help from others such as Mike, Steve & Pat.  I include a small extract below.

Its actually 2.3 MMT, and it represents the REDUCTION associated with using "cold climate" heat pumps for 21% of residential units (800K units) after accounting for the electricity they use ("newly generated" cell below), which for this purpose was modeled on the 2030 NJ RPS electricity standard as the assumption for clean energy at that point.   Thus we get 16% (rounded) actual GHG reduction.

Electric panel discussion: in general no electric panel change is required if the consumer is just changing from existing central AC to a heat pump using the same electrical wiring in a retrofit/replacement situation.  This would be true in many cases, e.g. most houses in our neighborhood already now have central AC, though almost none did when they were built.   For retrofits, the potential panel impact comes about where the consumer does not already have central AC and/or also wants to account for other appliances not already accounted for such as 220V electric vehicle charging, water heating heat pump, dryer heat  pump, and/or induction cooktop (or I suppose the case where upon arrival an electrician determines that the electrical situation is not up to code), but not for each of these if already electric, e.g. an existing operating electric dryer, stove, or water heater where the existing wiring will probably suffice.  I have not read Section 3.3, so if it only deals with new construction, then my comment might not be that relevant, but you still might want to list these other appliance needs along with heat pumps.

I recommend adding "cold climate" wording to heat pumps where ever possible.  I think we need to try to avoid non cold climate heat pumps despite somewhat lower appliance (though not installation) cost as that will likely leave some embedded carbon consumption for another 20 to 30 years.  But I recognize that there could be and are hybrid solutions (Steve has one) using non cold climate heat pump solutions that might be cost effective leaving some carbon on the table but still with large reductions (Steve indicated an impressive 90% reduction with his custom solution).

Another concern: any legislation needs to prevent municipalities and counties from assessing additional real estate tax for any capital improvements associated with clean energy retrofits, e.g. adding "cold climate" mini-splits in addition to retaining the existing HVAC system, because at a 2% of capital investment rate per year, one can see that over 15 years for example, this is a 30% government tax on clean energy improvements.    The same problem occurs if one was going to install a cold climate heat pump where no central AC exists today in a retrofit, as this would definitely be treated as a capital improvement by the tax assessors (although its an interesting question of how they handle the heating function where it replaces an existing furnace, which should not be additionally taxed, and I don't know the answer to how the assessors actually handle heat pumps that provide both heat & cooling when they replace just heating)

Spreadsheet extract below:

Scenario:	“Least Cost” residential space heating 2019 EMP Technical Report
Building Electrification Percentage in Target Year: 2030	21.0%
MMT saved (without adjusting for electricity consumed)	3.0
MMT newly generated from “cold climate” heat pump electricity	0.7
Net MMT saved by “cold climate” heat pump	2.3
% Residential Heating GHG Reduced	15.9%