There is much to like about Electric Vehicles (EVs). There is much to like about Solar Photovoltaic power collection (PV) too. But when you put the two together, something extra happens… You can now think of your solar system as being paid off, not by offsetting the cost of the electricity that runs your house, but by offsetting the cost of the gas you’re not buying.
Below, I am going to list the real-world drive data from our two company cars, a Nissan Leaf EV and a Prius Prime PHEV (Plug-in Hybrid Electric Vehicle). Sorry hybrids, you don’t get to play this game. Then I’ll list our real-world solar array data and calculate the value of the electricity we have harvested from our PV array. And answers some important questions; How much PV do I need to offset driving an EV or PHEV?
3.24 kW PV array:
Number of panels: 12
Power per panel: 344 watts
Annual production: 4,128 kWh
Cost after rebates: $5,796
The calculations will vary, of course, from place to place and car to car and driver to driver. These cars live in Carbondale, Colorado. The climate is pretty good for EVs; not too hot, not too cold. Typically highway driving punctuated by small towns. The road is never level, up valley, down valley, up the hill, down the hill. We run Blizzak winter tires for about five months, and Michelin Energy-savers the rest of the year. I’m no engineer; there is much rounding and estimating used in these calculations. For projections I will use these fuel values:
Street value of a kWh: $.11 (summer 2019)
Street value of a gallon of gas: $2.60 (summer 2019)
Total annual value of electricity produced by PV based on utility cost of kWh:
2013 Nissan Leaf EV, 24.0 kWh battery
Miles driven annually: 13,000
Miles/kWh: (average) 4.4 miles/kWh
Total kWh consumed: 3,022
Percentage of public charging: 10%
kWh provided by PV: 2,720
Leaf consumes: 66% of our production
Leaf consumes: 8.0 panels
Miles per panel: 1,463
2018 Prius Prime PHEV, 8.8 kWh battery
Miles driven annually: 19,000
Hybrid ratio: 50% EV : 50% ICE (Internal Combustion Engine)
Gallons of gas: 118
Cost of gas: $307
Miles/kWh: 5.4 miles/kWh
Total kWh consumed: 1,713
Percentage of public charging: 10%
kWh provided by PV: 1,542
Prius consumes: 37% of our production
Prius consumes: 4.5 panels
Miles per panel: 2,111
Total annual value of electricity produced by PV based on gasoline offset:
If 22,500 miles were driven in a gas drive vehicle (e.g. 2005 Subaru Outback, 32 MPG)
Gas offset EV portion of Prius: $772
Gas offset Leaf: $1,056
Total value of gas offset: $1,828 (703 gallons)
Every solar PV panel you put on your roof will push your EV 1,500-2,000 miles.
So, the bottom line is that our PV system makes us $520 worth of electricity annually, not bad. It would pay itself off in about 11 years at that rate. But when we put that electricity into an EV, it saves us from buying $1,828 worth of gas over our old ICE car, which translates to a 3.2 year payback!
Confluence is excited to announce that Prince Creek Home (outside of Carbondale Colorado) is framing. This home is a modern reinterpretation on an existing ranch home foundation. It will be net zero! To achieve net zero the home uses SIPS, good foundation insulation, heat pump heating system, proper window location and shading and PV.
Recently more jurisdictions have adopted the 2015 IECC or the IgCC and we have been helping several architects & designers with energy and sustainability code compliance.
Go custom! You don’t have to follow the recipe. Make the energy code work for your project.
Farm out the energy work to Confluence. We will be responsible for any or all of these, bring value to the performance of the building, and take the load off of your hands:
- Energy code compliance
- Infiltration (blower door) testing
- Assembly UA trade-off (calculation software)
- Total UA trade-off (REScheck or HERS Rating)
- Performance path compliance (REScheck or HERS Rating)
- ERI path compliance (HERS Rating)
- Code compliance/optimization and Construction Documents
- Ventilation calculations
- Sealed crawlspace & ventilation details
- Continuous Insulation details
- Back-ventilating siding and attachment details
- Efficient framing details
- Fenestration flashing details
- Radon mitigation details
- Thermal and pressure envelope delineation
- Vapor retarder specifications
- Air-sealing details
- Local/municipal Green/Efficient Building Checklists
- Carbondale, Basalt, Town of Snowmass Village, Telluride, Mountain Village
- Above-code/Net Zero design and certification
- LEED, Passive House, HERS Rating, etc.
Confluence can perform very large residential and commercial infiltration tests, or blower door tests.
We set up eight fans in three doorways for this test at Habitat For Humanity’s new ReStore warehouse In Glenwood Springs, Colorado. Although, not the biggest test we have conducted. That honor goes to a 56,000 square foot house built for a Saudi Arabian Prince in the Starwood neighborhood near Aspen, Colorado.
I don’t understand MPGe. A better metric would get me closer to knowing what I really want to know; how much does it electricity does it take to charge my car’s batteries? How far will that get me? What does it cost? How much solar do I need to offset the power consumption of an EV?
We’ve been driving our 2013 Nissan Leaf for one and a half years now and I have some data… the metric that makes the most sense to me is miles/Kwh. We live in a climate that is less than perfect for electric cars; little too hot in the summer, little too cold in the winter and lots of mountains. But still we average 4.5 miles/Kwh annually. I can’t find much difference in efficiency between the different makes and models of EVs. It seems to have much more to do with your climate, geography, topography, and a driver’s tendency to show passengers how fast an electric car will take off from a start. In 2016 we drove 12,108 miles. Assuming 4.5 miles/Kwh, then 2,691 Kwh went into powering the car.
Our solar array is officially rated at 3,240 watts. It was predicted to make us 4,753 Kwh annually, but in 2016 it gave us only 4,000 Kwh (16% less than estimated). The solar guys say this is because their software doesn’t de-rate for “losses” like; snow on the panels, age, azimuth and orientation. Sounds like a weak excuse to me; regardless, 4,000 Kwh is what we get.
The solar panels made enough power to push the car 4,000 x 4.5 = 18,000 miles. Each one of our twelve panels made us 1,500 miles worth of driving electricity. We drove only 12,108 miles, so the rest went into powering the house. To zero-out our total electric consumption, we would need to make a total of about 8,000 Kwh of power, or have a 6,500 system. 2,700 Kwh for our 12,000 miles of driving (34%) and 5400 Kwh for the house (66%).
So, how much solar do you need to offset your drive. Impossible to calculate for sure, but here’s starting point… wattage of PV array required = (miles driven annually / 4.5 miles/Kwh) X .8 If you have a lead foot, get a couple more panels.
If we bought the electricity to drive the car 12,108 miles (2,691 Kwh x $.138) it would have cost $371. It would cost me about $1,000 for the gas to drive our 2005 Subaru Outback the same distance. Solar is good when offset your home electric uses, but when it keeps you from buying gas- it pays back three times faster! And don’t get me started on maintenance and repairs; oil changes, transmission oil, power-steering fluid, fan belts, timing belts, head gaskets, catalytic converters, mufflers, oil filters, air filters, fuel filters, hoses, plugs, tubes, valves, sensors, etc. EV’s still have/need; insurance, tires, shocks, air conditioners, windshield wipers, windshield washer fluid, brakes and brake fluid. But I really don’t miss the regular stops at the gas and oil change stations and repair shops. If you have the means and it fits your commuting- buy one! You’ll love it.
This is a re-posting of an article from Roaring Fork Lifestyles magazine.
Check this Tumbler scrapbook about the construction process, very interesting.
Confluence Architecture & Sustainability was the HERS raters for this home. The HERS is an outstanding -10! The negative means is actually beyond net-zero, it is net-positive. As in, the occupants of this home should never have to pay for heating, cooling, lighting or hot water. Attention to detail got this house crazy air tight. Even with salvaged windows and doors, Steven was able to get this down to .69 ACH50. I’m sure it would have bested Passive House requirements (.6 ACH50) if not for the less-than perfect windows and doors.
Congratulations Steven and Bailey- you have a beautiful, high-quality home. Here are a few teaser photos…
What is energy modeling and why would I want it?
Basically, energy modeling software creates a mathematical simulation of your building over time to estimate how much it will cost to operate. It figures out how much heat is lost through every square foot of the envelope and how much heat is gained by the sun shining through the windows every year. It uses historical weather and solar data to calculate how much heat you will need to put into or remove to keep the indoor environment comfortable. And it can put this data in terms of dollars spent on fuel and utilities.
What’s the point? Optimization. We can swap different windows, adjust overhangs, try differing amounts of insulation, etc. and see what the result to the loads are, so we can find the sweet spot for your particular building. Knowing how much it costs to operate your building will allow you to calculate a return on investment for monies spent on energy conservation upgrades. For example, solar will pay for itself in the long run, but what is the payback period 5 years? 10 years? 20 years? Is it better to add continuous exterior insulation or buy better windows? Is it more cost-effective to add insulation to the attic or buy a few more solar panels?
So what is a HERS Rating?
The RESNET HERS Rating protocol is a nationally standardized energy modeling system, just for houses, that lets us get to the answers relatively quickly and makes results consistent and comparable across the country.
The value of a HERS Rating over just energy modeling comes in four ways.
- Code compliance- HERS Ratings supersede the rigid rules of the energy code and gives you flexibility.
- Third-party inspections- we inspect at rough to look for problems with air barriers and insulation. Having an air-tight envelope not only saves energy, it helps prevents rodents, bugs and dust from getting into the house. We grade insulation on the quality of its installation. If we find less than perfect installation, we bring it to the attention of your GC and he can have it corrected before it is too late.
- Rebates- many rebates are only available to those that receive a HERS Rating.
- Resale- the score goes on the MLS report so buyers get a sense of how efficient the house can be (like a MPG sticker for a car). Embedding the value of energy features in the value of the home also makes it easier to invest in features that have a longer return on investment.
What energy modeling doesn’t do…
- The model doesn’t know how much anything costs, except gas and electric. So it will not do a cost benefit analysis without help from the rest of your team.
- It is not the same kind of zoned load calculations used by HVAC designers to size ductwork or radiant tubing layouts. You will still need a heat/cooling/ventilation distribution design. It will tell us how many kBtu the furnace or boiler should produce, but not how to get the right amount of heat to every corner of the house to maintain comfortable temperatures.
- It is just an estimate. Occupant factors, like where the thermostat is set, will skew the numbers accordingly.
Why choose Confluence Architecture and Sustainability?
The team at Confluence has been practicing architecture in the extreme climate of the mountains of Colorado since the turn of the century. We started offering sustainability services when the 2009 International Energy Conservation Code was adopted. We know how to play with Owners, Architects, Engineers, Builders and Code Officials. We bring our experience to the table on tangential matters like; air barriers, vapor retarders, ventilation rates, indoor air quality, mechanical systems and optional solutions and alternative methods for code compliance. And we can make recommendations to increase the quality, comfort and durability of your structure.
Hers are some other related blogs…
Here in western Colorado, local jurisdictions are busily adopting the 2015 building code. The 2015 IECC adoptions are the most contentious and most heavily modified. It is hard to keep track what needs to be done to comply with the energy requirements for residential construction. Below is a cheat sheet table of adoptions and modifications in Western Colorado. If you need assistance getting the most sustainable home or just figuring out what a jurisdiction wants – give Confluence Architecture a call. How is the 2015 IECC different than 2009? See this blog. We do HERS ratings, blower door tests, and code compliance documents for the IECC.
|Adopted Residential Energy Codes and Amendments|
|Jurisdiction||IRC||IECC||Amendments to Code||HERS Required||HERS usable for permit||Blower Door Required|
|Garfield County||2015||2009||No Amendments to IECC||no||no||no|
|City of Glenwood Springs||2009||2009||No Amendments to IECC||no||no||no|
|Town of New Castle||2003||2009||Requires third party checklist, blower door (7 ACH50), CAS test, and ENERGY STAR appliances||no||yes||yes|
|Town of Silt||2009||no||none|
|City of Rifle||2009||2009||No Amendments to IECC|
|Town of Carbondale||2009||2009||No Amendments to IECC||no||yes||maybe|
|Pitkin County||2015||2015||No Amendments to IECC||no||yes||yes|
|Town of Basalt||2006||2015||no||yes||yes|
|Town of Snowmass Village||2015||2015||No Amendments to IECC||yes||yes||yes|
|City of Aspen||2015||2015||IECC -many amendments||no||no||no|
|Eagle County||2015||2015||Fenestration U-factor .30 max.||no||yes||yes|
|Town of Vail||2015||2015||R21 on wood framed can be substituted for continuous insulation||no||yes||yes|
|Town of Avon||2015||2015||all gas fired boilers used for snow melt minimum 92% AFUE||no||yes||yes|
|Town of Eagle||2012||2012||No Amendments to IECC||no|
|Town of Gypsum||2003||no||none||no|
|San Miguel County||2009||2009||San Miguel County Prescriptive Code||if over 5000 sf||yes||if over 5000 sf|
|Town of Telluride||2003||2009||Eliminated whole code and replaced with their own that has higher insulation levels, required HERS, blower door, and duct blasting, min. HVAC efficiencies, low flow fixtures, and rough for solar||yes||yes||yes|
|Town of Mountain Village (Telluride)||2012||2012||HERS rating required, mechanical engineering required, permit fee reduced for good HERS or low exterior energy use||yes||yes||yes|
If you haven’t been through a blower door test yet, chances are you will soon. As Pitkin County, Aspen, Basalt and Carbondale gear up to adopt the latest round of building and energy efficiency codes. The 2015 IECC (International Energy Conservation Code) have houses going for a maximum of 3 ACH50 (Air Changes per Hour at -50 Pascal) and commercial buildings going for a maximum of .40 CFM/square foot of envelope area at -75 Pascal. Most builders I work with could get to 7 ACH50 without doing anything extra. Getting to 3 ACH50 will take some extra care. If you are unfamiliar with the techniques of air sealing, then read up or get an expert on the team. A great place to start reading is the ENERGY STAR Thermal Bypass Checklist . Awesome document- do this stuff and you will pass the blower door test the first time.
I’m often asked, “what do I need to have done before we test?” Completely done, done, done is ideal; but in reality… below is my checklist of this that should be done before testing so test results are not significantly degraded:
- doors and windows installed
- door and window hardware and weatherstripping installed
- door thresholds installed
- hatches to unconditioned attics and crawlspaces installed and gasketed
- dampers in place
- fireplace doors installed
- plumbing traps filled
- conduits leading outside sealed
- air handlers and ductwork complete
- light fixtures installed
- plate covers installed
- any other gap, crack or hole between inside and outside that you can find
Setting up the Building
When we test a building for air infiltration the building must be setup in a prescribed fashion. The IECC has it’s section (2009 IECC R402.4.2.1 or 2009 IRC N1126.96.36.199) that describes how to setup a house. RESNET has their official protocol as well, the document ANSI/RESNET/ICC 380-2016.
- Exterior windows and doors, fireplace and stove doors shall be closed, but not sealed with tape;
- Dampers shall be closed, but not sealed; including exhaust, intake, makeup air, back draft, and flue dampers;
- Interior doors shall be open;
- Exterior openings for continuous ventilation systems and heat recovery ventilators shall be closed and sealed;
- Heating and cooling system(s) shall be turned off;
- HVAC supply and return registers shall not be sealed.
Running the test
I usually takes me 30 minutes to set up the blower door equipment and check that the house is prepared. I need an exterior door that is not too small or too big to set up in, power nearby and a space to work in. If the house is more than 5000 square feet or so, I will set up double fan equipment. Then I will need to shut down the air handler and exhaust fans. At this point, anyone opening a door would void the test. But typically I only need the doors closed for five minutes to get an accurate reading. If it hasn’t been done yet, I will need the drawings to calculate the volume of air inside the house. Then do the math; flow (the results of the test) X 60 divided by the volume of the house = the number of air changes per hour. In the end, I create a certificate, that need to go to the building official.
If you want/need someone else’s eyes on the job, then give us a call. Confluence Architecture has a lot of experience with construction detailing, building testing, improving test results and also does HERS ratings, RESchecks, COMchecks, blower door tests, duct blast tests, IR camera inspections, etc.