Why Net Zero Homes Lead to Electric Vehicles

By 2020, all new California homes are required to be net-zero.  There are many definitions of net-zero, but the one that counts for the California energy code is TDV net-zero.

Think of TDV as a weighted average.  The modeling software used to prove compliance with the energy code weights every hour's electric usage by the cost to produce that power in your climate zone.  So peak periods count FAR MORE than non-peak periods.  This means that west-facing windows (which take on heat in summer during peak periods for the utilities) get penalized more heavily than north- or east-facing windows, and in most cases south-facing windows.  To give you an idea of the TDV factors, the 2016 standards have TDV factors that average 20 kbtu/kwh.  Some days in August and September have TDV factors as high as 500 or 600.  This is how the utilities bring the reality they deal with every day of peak production loads into the equation, and it makes sense.  The energy code actively discourages designs that worsen the need for peak production, which typically is the most expensive and dirty electricity production.

Now that you know what TDV is, you can understand that TDV net zero is a less stringent goal than the purest definition of net-zero, which is site net-zero.  In site net-zero, a boundary is drawn around the property.  The sum of what goes in is less than or equal than the sum of what goes out.  If you produce as many btu's of electricity with your PV panels or wind turbines or hamsters in cages as you pull off the grid during the year, you're net-zero site.  By using TDV factors and applying them to the grid energy used, the calculation weighs the on-site production more heavily than the grid energy used.  Why?  Because grid energy has emissions associated with it and other societal costs, whereas on-site renewable energy production does not.  Also, PV panels produce a lot of their energy during peak periods, and grid energy is often drawn off-peak in a net-zero home.  

Bear with me; I'm getting there!  Our net-zero TDV house is about 65% net-zero site.  That is, we produce about 65% of the energy we use by a site definition.  That will be enough to qualify as a net-zero house by the 2020 California Energy Code.  If you use all electricity in your home, the calculation is easy.  But what if you're a "dual fuel" home, with gas and electricity?  In that case, we convert the natural gas usage to equivalent btu, do the same with electricity, and do our math in btu's.  The TDV energy produced by the PV panels has to more than equal the TDV energy used by the gas systems and electric systems in the house.

Hope you're still with me, because I'm just now getting to the point.  Most net-zero houses are more economical to build "dual fuel", because gas is the most established and efficient way to heat a home and dry clothes and heat water in most areas of the state.  In our case, 60% of our annual energy use is from natural gas; we would have needed an additional 4-6 solar panels to reach net-zero all-electric than the way we did it -- dual fuel.  What's this mean?  Most net-zero homes will produce more electricity than they pull off the grid.  Our house had a surplus of over 1,000 kwh electricity in its first year of operation.  This is partially compensating for all the gas usage in the house.  And our electric utility pays us poorly for these excess kwh -- about 4 cents each last year.

This is what makes an electric vehicle look attractive to a net-zero dual-fuel home.  They will find they have excess electric production, purchased by their local utility at very low rates, and will look for options what to do with it.  They could do wasteful things with it, like adding a big freezer in the garage or running their A/C more in summer.  Or they could use it to cheaply power an electric car.  Whatever they have in extra production will cost them $0.01 per mile or thereabouts, as compared to about $0.10 per mile for a typical gas car.  

​Net-zero + Dual-fuel = EV