Note: From time to time, I solicit guest posts from various individuals on different topics. While I like to think I know a little something about a lot of things, I’m fortunate to be acquainted with a number of people who know a whole lot about certain topics, and who are willing to share some of that knowledge here. In this particular case, I’m welcoming back someone who has written on this particular topic before.
We’ve now had a solar system on our house, and an electric car charging from the house, for just over a year. Also of note, in my post last May, I suggested that what we need is a retail electric plan that sells to you at a competitive rate (versus the inflated prices at Green Mountain) and buys from you at the wholesale price (which can climb impressively high on hot summer afternoons, when your solar system is cranking out the juice). Well, plans like this are starting to appear on the market. MP2 Energy has such a plan and others are looking into it.
Today, I want to discuss a few related questions:
- How much electricity did our solar system generate, and our electric car consume, in the past year?
- Based on our year of data, would we do any better to stick with Green Mountain or go to one of the newer plans?
Of course, I’m only writing about our own usage, with our house and our car. Your house and your car and your, umm, mileage will vary, but you might be able to extrapolate from our numbers to reach your own conclusions about whether you want to go solar.
How much electricity did our solar system generate?
Below is a graph of the energy-per-day produced by our solar array. You can see the system generating more energy in the summer months, with the correspondingly longer days. You can also see the occasional days with bad weather. No sun = no power.
In total, over this twelve month period, our solar array (36 panels, 250W peak production per panel) produced 10.3 MWh of electricity. At the $0.12/kWh buyback rate we’re getting with Green Mountain Energy, that means that our solar array saved us roughly $1240 this year. (Our panels are facing east and west, as a result of the way our roof is built. If your house has a large southern-facing roof, you could get this much power from fewer panels.)
How much juice did our Tesla consume?
According to the Tesla’s dashboard measurements, after a year of owning it, we’ve driven a total of 7033 miles, and used 2476 kWh to do it. That’s 352 Wh/mile. Assuming you were paying $0.10/kWh for your electricity, then we’re talking about 3.5 cents/mile. Contrast that with a comparably large sedan with comparable performance (e.g., an Audi A7), doing the same sort of city driving and thus getting something crappy like 15 miles/gallon, then with current $2/gallon prices, you’re looking at 13.3 cents/mile. You’d have to have some kind of amazing 57mpg hybrid to achieve the same cost per mile. (A Prius is almost there. Big luxury cars, not so much.)
Another way to think about it: the “long tailpipe” problem. Some critics of electric cars note that they still burn fossil fuels, just somewhere far away from home. Our solar array produced enough energy to run our Tesla for nearly 30,000 miles. So if you want to have a “solar powered electric car”, you can do it with even a modest-sized solar system.
What if you drive a longer commute? The prior owner of our Tesla lived up in the Woodlands and commuted back and forth to Houston. He was averaging an even more amazing 300 Wh/mile, driving twice as many miles per year in the same exact car. He upgraded to a Tesla P85D (the four-wheel-drive version that goes insanely faster) and his mileage stayed roughly the same. Supercar performance, tiny hybrid efficiency.
All that said, I don’t have a really good handle on the overhead of the Tesla. Sure, it consumed 2476 kWh in the past year, but that’s going from the car’s battery to the tires. There’s some fractional overhead beyond that, going from the wall outlet to the car’s battery. Charging a battery creates heat, which represents wasted electricity, and also requires additional energy to remove. The Tesla will thus use extra power to run the A/C compressor while it’s charging. For now, let’s just say that measuring the charging overhead is future work. (Hey OffTheKuff readers: if you’ve got measurement infrastructure that I could borrow for this, let me know!)
Lastly, I’ll note that we did several road trips in the Tesla, using their Supercharger infrastructure. I’d estimate that somewhere around 500 kWh of that energy was “free” from the Supercharger network (i.e., included in the cost of buying the car).
Should we stick with Green Mountain or switch elsewhere?
Green Mountain has the best net metering plan on the market, but there are only two competitors. In a nutshell, Green Mountain buys and sells power from you at the exact same price: $0.12/kWh, inclusive of all fees and taxes. But there are plenty of standard retail plans that will sell you electricity at $0.08 or $0.09/kWh. Can we do better than Green Mountain’s net metering plan? The real issue, once you strip away all the dumb politics, is that the underlying pricing model isn’t at all a flat rate for electricity.
Roughly speaking, there’s a wholesale price for the electricity coming from a commercial generator and then there’s a distribution price to get it to you. Wholesale prices vary all day long, with overnight lows below a penny and mid-afternoon highs as much as 3 cents/kWh, with occasional peaks that are much, much higher. CenterPoint charges 3.8 cents/kWh for delivery of that power, no matter what, alongside a flat monthly charge of $5.47 per residential customer. All those charges are often rolled into the pricing plans you see from other retail electricity providers, who are essentially gambling that they can buy power at variable wholesale rates and sell it to you at a flat retail price while still somehow making a profit.
When a retail electricity provider wants to get into the solar buyback game, their actual costs to get power downstream to your house (so far as I can tell) are the wholesale price plus the distribution price. Your excess solar power production is worth the same to them as the spot wholesale price when it flows back upstream. CenterPoint doesn’t give any sort of rebate for upstream electricity flows. CenterPoint’s argument: Somebody else is receiving the power you’re sending upstream, and they’re paying to get it delivered. CenterPoint charges for that delivery.
Can a retail electricity provider offer a competitive pricing plan that’s closer to the wholesale market structure while still buffering consumers from the sometimes insane spikes of the raw wholesale market? One such provider, who prefers not to be named yet in public, approached me privately and offered me the chance to test drive a new plan they’re working on. Their proposal is to pass through all the CenterPoint charges, as is, and then have a flat 3 cents/kWh for buying and selling power, downstream or upstream. I ran these numbers through my spreadsheet for the same 12 months of data I’ve already captured. Here’s what came out: Green Mountain’s $0.12/kWh net metering plan cost us $692.84 for the year. If we had this new plan instead, it would come out to $712.07 for the same usage in the same year.
Evaluating MP2’s spot-price “solar buyback” plan is a bit more complicated, because the upstream price they pay you varies all day long. Conveniently, MP2 did this analysis for me. I emailed them all our data and their conclusion was that our annual bill would be $904.32, so not especially competitive with Green Mountain’s net metering. MP2 also offers a net metering plan, similar to Green Mountain’s plan, but it’s presently offered as part of getting your solar system installed through SolarCity. Thus, not an option for us.
Call me modestly bullish on this. Even though MP2’s solar buyback plan isn’t a good deal for our house, other firms are looking to offer variants on the same business model that are competitive. As an added bonus, I’d now be incentivized to put a big battery on our house to capture the excess daily production and reuse it at night. With standard net-metering, there was no incentive, but now I’d save those distribution charges. I’ll still wait for the cost of battery packs to drop, but it’s fun to think about.
Some Thoughts About the Future
There are always going to be a few days in the summer where the demand on the grid peaks out. In those cases, all the market-rate adjustments in the world won’t cause a new industrial generator to be constructed and placed online. That means high prices and brownouts. (If anything, there’s a reasonable fear that generators might deliberately go off-line to force price spikes. That’s beyond the scope of today’s post.)
Solar has a big role to play in stabilizing our grid, because those hottest hours of the day are exactly when solar panels will be generating the most power. Solar also happens to do the job without pollution, and without incurring large infrastructure costs for long-distance power distribution. On top of that, solar’s one-time purchase and installation costs are rapidly shrinking.
Consequently, it’s sensible and desirable for the Federal government to continue its solar subsidy, and it would make a lot of sense for the Texas state government to get in on the game as well. The solar on our roof helps our neighbors, not just us. I’m not suggesting that we’ll stop burning fossil fuels, but rather that a diversified set of sources is a desirable way to meet the needs for a stable and scalable power grid.
The biggest objection to solar, so far as I can tell, comes from shills who misrepresent the financial structure of the electricity markets and claim that residential solar production leads to “mooching” off the grid. What I like about MP2 and some of the other buyback plans coming online is that they address this concern head-on. By passing through the monthly CenterPoint connection charge and pricing power consumption somewhere only marginally higher than wholesale rates, these new plans make it clear that solar systems aren’t mooching at all. They’re paying their fair share, and they’re improving the reliability of the grid while they’re at it.