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Hi Elizabeth -- If you look at the last page of this article (http://www.luntmarymor.com/condensing_heaters.pdf), you'll see a nice description of how EF is measured. From this, and factoring in the additional consumption that Michael mentioned, you can probably come up with a reasonable approximation of the EF for your scenario. The toughest part may be to figure out the standby losses, if the DHW is a tank-type system.
Note that in REM the EF is entered as a decimal, not a percentage -- and for a heat pump, the decimal will be larger than 1.0.