Hello,
I am seeking clarification on how BEopt's modified payback period is calculated. The help in v2.8 and v3.0 include a general description, but there are some ambiguities, and unlike the other financial metrics there is no equation to parse. Below is some pseudo-code indicating my understanding of the description (for a thirty year analysis period), but we are not able to faithfully recreate BEopt's values.
IF SUM(PACKAGE UTILITIES[1-30]) <= SUM(REFERENCE UTILITIES[1-30])
EXPENSE=PACKAGE INITIAL COST - REFERENCE INITIAL COST
SAVED=0
FOR YR IN 1 TO 30
EXPENSE+=PACKAGE REPLACEMENT[YR] - REFERENCE PACKAGE REPLACEMENT[YR]
SAVED+=PACKAGE UTILITY[YR] - REFERENCE UTILITY[YR]
IF SAVED>=EXPENSE
PAYBACK=YR-1 + (SAVED-EXPENSE)/PACKAGE UTILITY[YR]
BREAK
END
END
ELSE
PAYBACK = NaN
END
P.S. We need to calculate our own modified payback periods, because we want to produce payback periods for heat pump packages that use a furnace home as the point of comparison, since they are still the most prevalent equipment in our market.
