We’ve produced 2,300 kWh of electricity more than we’ve used since we went online with our 5.59 kW home solar system in late June 2010 here in sun-soaked Aurora, Colo.
Thanks to our utility’s ‘indefinite rollover option,’ this means we’ve been able to bank these extra kWh hours for future use.
With winter approaching, thankfully, rather gradually so far – we have not turned on the heat once yet and it’s the end of October – a decision about whether to use some of these banked kWh to heat our house is fast approaching.
I’m waffling. This is mostly because there are so many different variables it's enough to make one's head spin.
Heating with solar PV
If you’re thinking of adding a solar system to your home to solar-charge a future electric car, you, too, may face the question of how best to use the kWh your system produces, especially if it overproduces – which you might well want it to do, particularly if you go solar before you get the electric car, and your utility allows you to bank any extra kWh you produce.
Beyond that, you can heat your home with solar PV if you use electric heaters. I’m not talking about those watt-sucking, inefficient, cheap portable heaters you get at Home Depot. We’ve got four high-efficiency, highly effective Econo-Heat wall heaters (see sidebar), which consume between 400 and 450 watts per hour when turned up to high, and which use roughly one-third the electricity of the typical cheesy 1500-watt space heater.
Before moving on to the calculations, a few sentences about why we have these Econo-Heat heaters in the first place.
We had two of our four econo-heaters prior to this past year in our basement. We added two more last year in a quest to increase our annual kWh use so that we could install a solar system big enough to cover 100 percent of our home electric use and 100-percent of about 12,000 miles a year in a future electric car. We needed to increase our electric use last winter because our utility, Xcel Energy, does not provide rebates for residential solar systems which produce more than 120-percent of a home’s electric use.
Finessing the rebate rules
Basically, we shifted a portion of our heat from natural gas to electric last winter in order to finesse the Xcel Energy rebate rules. We succeeded and had a 5.59 kW system, complete with a $3.50 per watt Solar Rewards rebate, installed this past June.
So, now that we have all of these e-heaters but we’re still waiting on an electric car, what to do?
Our 2,300 banked kWh = 9,200 miles worth of fuel for an electric car. At 20 m.p.g. and $3 per gallon, that’s $1,380 worth of gas. Using our econ-heaters is definitely going to eat into those 2,300 kWh, although it’s pretty hard to figure out exactly how much.
Our REC Solar bid sheet says we can expect our system to produce 2,200 kWh from Dec. 1 to April 1. If we never turned on any of our econo-heaters, we’re probably going to use 1,400 kWh during that time, meaning we’d bank 800 kWh.
1,200 extra kWh
In fact, we’ll probably bank about 1,100 more kWh during this upcoming four-month span based on the fact that our system has consistently produced 100 to 150 kWh more per month than the REC Solar estimate.
If we’re relatively frugal with our econo-heaters, say, on average, we have two of the four heaters on 12 hours per day for 109 heating days, that’s 1,200 kWh, or a total roughly equal to the number of kWh we’d overproduce if we didn’t use the heaters.
Done deal, you say. Run the two econo-heaters for 12 hours in the kids’ and your bedrooms at night, turn down the natural gas furnace as low as possible (last year, we set our home thermostat to 52 degrees at night and the temperature in our kids’ bedroom stayed at about 62 to 65 while it hovered between 58 and 61 in our much larger bedroom with econo-heaters in both rooms) , and you’ll still have the 2,300 kWh you've banked so far at the beginning of April 2010. At that point, you’ll start banking kWh again and keep on banking them until an EV arrives in your garage -- which could be as late as a year from now.
Not so fast. Those 1,200 kWh hours which would go to our econo-heaters = $720 worth of “gasoline” for an EV at 20 m.p.g. and $3 per gallon. It’s still $576 worth of fuel at 25 m.p.g. and $3 per gallon.
So, how much would it cost us to keep our gas furnace set to come on at, say, 60 all night instead of at 52, which is what we’d have to do if we didn’t run the econ-heaters in our bedrooms?
Offset natural gas -- or gasoline?
The answer: I don’t know. That's because it's very hard to calculate this out to any degree of precision. All I’ve got to go on is a monthly natural gas bill from Xcel, which tells me how many “therms” we’ve consumed. And this total isn’t just for our gas furnace, but includes our hot water heater as well.
What’s better for the environment, using our extra solar-produced kWh hours to reduce consumption of natural gas via our home furnace, or saving the equivalent kWh to offset the consumption of gasoline?
I can say that last year, when we had at least two, and sometimes three, econo-heaters on probably 20 hours a day for approximately 120 heating days, our gas bill was about $400. Of course, our December kWh usage hit 1,200. We did have a nanny at home with the kids all day last year. That’s not going to be the case this winter.
Of that $400 in natural gas costs, I’m guessing about $100 went to hot water. That brings us down to an extremely rough $300 for natural gas heating costs last year with our econo-heaters on much of the time. That’s a lot less than $720 or $576, which, again, is how much 1,200 kWh are worth in terms of "gasoline."
Better to use solar as 'gasoline'
Overall, after ‘eyeballing' things in admittedly pretty rough fashion, it seems like, economically speaking, we’d be better off not turning on our econo-heaters and saving the extra solar-produced kWh for our future EV.
Of course, we did spend $200 to buy the extra econo-heaters last October, and it’s painful to think this was a “waste.” It would also be kind of cool to partially heat with solar this winter, while keeping our natural gas bills down to a level our neighbors would – if they heard about how low they were, and they will! – drop their jaws at.
There’s also the question of whether we’ll actually be able to catch up and use all the extra kWh hours we’ve banked so far once we get an electric car. If we continue at the rate we are, and the Nissan LEAF doesn't arrive in Colorado until next September, we may have 20,000 miles of EV fuel banked! We do hope to eventually own two EVs, one pure EV and one plug-in hybrid electric (PHEV), so maybe we will eventually catch up to our banked kWh.
What's better for the environment?
Finally, there’s the whole environmental part of the equation: What’s better for the environment, using our extra solar-produced kWh hours to reduce consumption of natural gas via our home furnace, or saving the equivalent kWh to offset the consumption of gasoline?
I’m not going to crunch the numbers here – in part, because I don’t actually know how to do it – but, I’m guessing that solar-produced kWh have a considerably bigger and more positive offset impact in terms of gasoline than in terms of natural gas. In fact, I may need to figure out how to calculate these different environmental impact numbers in order to determine whether to heat with our solar kWh or to drive on them.
For now, though, I can't quite decide what to do. I'm leaning toward using our e-heaters sparingly --enough to continue to bank a few extra kWh over the winter while also hopefully signficantly reducing our natural gas bill.
What would you do?
- Solar-charged driving 101
- Savings add up fast with EV+PV combo
- Solar can replace oil -- and eliminate your gasoline costs
- Solar before the EV the way to go for many
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Web blogs by current solar-charged drivers
-- Peder Norby's Electric BMW ActiveE Blog
-- Darell Dickey's EV Nut Web Site
-- Doug Korthof's Live Oil Free Pages
-- The Solar-Charged Electric Car Page
-- Solar Power and Electric Cars
-- Sun Powered EVs
-- Ecogeeco Web Site