My name is Sam Pritchett; my wife & I are designing a new residence near the coast in San Diego (Climate Zone 7). We wrote an e-mail to the CEC’s general Title-24 “contact us” e-mail address earlier this week & I spoke with Javier Perez a couple of days ago, Javier was quite helpful and he resolved one of our two questions I had asked. For the second question, Javier proposed one resolution but I proposed an alternate approach... so Javier referred me to you guys. At the time, my wife & I were still blissfully unaware of our state of confusion, but as we investigated this more (before contacting you), our shared confusion became more & more apparent.
First & foremost, my wife & I want to translate something like 55,000,000 TDV/year into a MWh/year estimate. We realize there is not a clean one-to-one correspondence between kTDV /ft^2-yr & MWhr/ft^2-yr... since TDV represents something like the average net present value of the (hourly) time value of energy averaged over a year that also takes into account the escalation of energy cost/environmental impacts over time.
This “approximate conversion” (from kTDV/year to MWh/year) addresses our principle desire to estimate our annual energy costs before adding solar PV. In addition, this approximate conversion should enable us to discuss the monetary impact of some (continuous insulation) construction implementation alternatives with each other as well as with a builder. Although my wife & I are both engineers, neither of us are competent operators of the CEC’s Title-24 energy estimation software; therefore, we do not wish to run full (Title-24) simulations for each scenario.
But after some study, we found the language & units that revolve around TDV quite confusing... TDV is first a (Time Dependent Valuation) of energy, which we recklessly labeled (in our minds) as a “generic” energy cost metric ($/kWh, $/kBtu or $/Therm) for three unique energy sources (electric, natural gas & propane). Then in some situations it appears TDV is divided by its natural gas form (i.e. the natural gas heat energy cost metric) and after division, TDV becomes an energy/energy (i.e. dimensionless time dependent valuation) energy value metric or in some situations an (dimensionless... i.e. like miles/meters is dimensionless) energy savings metric, yet after transforming (via division) from a cost/energy metric to a dimensionless (energy) metric it retains the identical TDV variable name. Although moving in the opposite direction, let us use fluid dynamics’ Reynolds number (i.e. Re... a dimensionless variable that describes a fluid’s ratio of inertial & dissipative forces) to demonstrate our point... if we multiply the Reynolds number (Re) by the kinematic viscosity (in m^2/s), is it common practice/does it promote clarity to still call the result the Reynolds number and employ the exact same variable (Re) to represent both quantities?
In our experience as engineers & technology researchers/developers, we believe we risk a given variable’s/notation’s entire utility when we do not consistently preserve its dimensions & sight recognizability. As a result of TDV’s apparent (at least to us) assault on dimensional analysis, when we see kTDV/ft^2-yr in the CF1R under the title of annual energy usage, our initial reactions are that it is not annual energy usage but is instead annual energy usage density, which we must first multiply by an area before we truly have annual energy usage. For example, our planned residence has ~5200ft^2 of conditioned floor area with a 2013 Title-24 Compliant (Climate Zone-7) Energy Use (density) Budget of 10.65 kTDV/ft^2-yr, which upon multiplication yields a 55,380,000 TDV/yr compliant energy use budget for our planned residence. Up to this point, we thought all we needed was an (approximate) way to convert from TDV to kWh.
But then we came across Table 2 of the CEC’s Staff Report, “Solar Offset Program Pre-rulemaking Draft Regulations,” dated September 2010 (CEC-300-2010-009-SF), wherein a 2 kW “reference solar energy system’s” Climate Zone-7 annual kWh & annual TDV are 3,837 & 61,446 respectively (in terms of CEC’s 2008 energy standard). But in the CEC’s PV calculator (Version 5) the identical 2 kW reference solar energy system’s Climate Zone-7 annual kWh remains 3,837 (when normalized to 2 kW AC), but the annual time dependent valuation of the same 2kW Reference Solar Energy System is now 95,119 TDV kBtu (in terms of CEC’s 2013 energy standard). So upon comparing these two reference points, as the CEC migrated from the 2008 Title-24 energy standard to the 2013 Title-24 energy standard, not only has the time dependent valuation scale inflated 50% in five years (for the identical 2kW reference solar energy system) but the units/dimensions have also migrated from TDV to TDV kBtu. That is tough to wrap our heads around!
Yet for Climate Zone-7 in Tables 6 & 7 of the CEC’s Consultant Report, “Impact Analysis California’s 2013 Building Energy Efficiency Standards” dated July 2013 (CEC‐400‐2013‐008), the first year TDV savings (density) for the CEC’s two single family reference homes is consistently ~9.15 kTDV/ft^2. But if we apply this TDV savings density to a 5,200ft^2 single family (two-story) residence, we get 47,580,000 (first year) TDV savings; which is 86.5% of our planned 5200ft^2 residence’s entire 2013 Title-24 Compliant Energy Use budget. Sure scaling the TDV savings by 2x will not be exactly linear, but our planned (5200ft^2) two-story residence’s roof area is not much more than the CEC’s 2700ft^2 reference one-story single family home... so the savings between the 2008 & 2014 energy standards reaching 86.5% of our planned 5200ft^2 residence’s entire 2013 Title-24 Compliant Energy Use budget appears quite extreme.
Please know that the foregoing is not intended to be a critique, but is instead intended to demonstrate our shear confusion. We realize the CEC’s task is daunting and that CEC staff have cleared numerous huge hurtles to evolve this science to where we are today. But in spite of all that accomplishment, at least two of your fellow citizens & fellow taxpayers (i.e. my wife & I) are quite confused.
So we ask that you help us understand how to recognize the appropriate TDV units/dimensions incarnation to apply to a given situation and how to translate our planned residence’s annual energy usage into an approximate first year (SI unit) annual energy estimate.
