independence-house-2011editors-blog-entry3A favorite topic we’ve returned to again and again from different angles and perspectives here at SolarChargedDriving.Com is the question of whether driving a solar-charged EV saves you money.

Because there are so many factors, from the cost of the gas car you use in your calculations to whether you’re using a new or a used gas car as your point of comparison to the cost of gas where you live to the gas mileage of the gas vehicle you’re talking about to the cost of solar where you live to the amount of sunshine you receive where you live — the list goes on, believe me — the answer to the question of whether solar-charging an EV will save you money is a big fat, ‘It depends.’

Many factors in play
I’m not going to go over all of the different answers we’ve arrived at depending on the different factors involved in the gas car vs. solar-charged EV cost comparison here. Instead, I’ll point you to the list of entries at the bottom of this one in which we’ve previously addressed this topic.

Right now, I’m going to meld three different streams of analysis I’ve focused on in the past and compare the cost of us driving one of our gas clunkers – our 20-year-old 1992 Acura Integra — vs. the cost of buying a brand new EV and solar-charging it vs. the cost of buying a brand new gasoline econo-car and tanking it with gas.

The goal: To see approximately how long it would take for the new EV + home solar equation to beat out the gasoline + clunker car that runs forever scenario.

leaf-volt-solar1Desperate to add EV to PV
I’m doing this because I thought it might be interesting/useful for others and because I’m desperately trying to find a way to justify plugging ourselves into solar-charged driving ASAP, before we likely head off to Germany for a full year about 17 months from now.

In fact, looking back, I came pretty close to doing the same calculation I’m going do here in a March 2, 2010 entry called ‘High short-term costs barrier to long-term savings’. However, I didn’t accurately calculate true maintenance costs for our ’92 Acura Integra. While the Integra has been a great car, over the past five years, it’s cost us $1,500 per year in maintenance costs.

Before I tackle the calculations, some important background: We’ve already amassed 7,000 extra kWh hours with the 5.59 kW solar system REC Solar installed on our rooftop in June of 2010, that’s 24,500 miles of EV driving (3.5 miles/kWh).  

Simplified EV + PV calculation
Okay, here are the rough calculations:

$24,000 = Our out-of-pocket cost for new Nissan LEAF after Federal & Colorado State Tax Credits
$8,000 = Our out of pocket cost for 5.59 KW solar system (installed in June 2010)
$0 = “New” car cost of our 1992 Acura Integra
$1,500 = Annual average maintenance costs for ’92 Integra
10,000 miles = Total number of miles we drive annually in our Integra
25 mpg = Mileage for our ’92 Acura Integra
400 gallons of gas = Yearly gasoline consumption for ’92 Acura Integra
$4 per gallon x 400 = $1,600, total annual fueling costs for ’92 Integra

Basically, to see how long it would take before buying a new LEAF and powering it with the 7,000 extra solar kWh hours we’ve banked with our utility pays off, we need to add the annual maintenance and gasoline costs for our ’92 Integra and then divide these into $32,000 – which is the approximate cost of our 5.59 kW solar system plus the cost of a new Nissan LEAF.

Here goes –>

$32,000 (Cost of new Nissan LEAF + 5.59 kW solar system) ÷ $3,100 (yearly maintenance /fueling costs for our ’92 Acura Integra) = 10.3 years to payoff.

In this simplified comparison between keeping our ’92 Integra and buying a new Nissan LEAF + home solar system (which we already have), solar-charged driving takes forever to pay off.

More nuanced EV + PV calculation
But let’s move to a more complex analysis, one that takes into account the following:

  1. The fact that our home solar system won’t just power 100% of our miles in a new EV, but would continue to power 100% our home electricity use as well;
  2. The interest costs on a six-year, 7.5%, $20,000 loan for a new Nissan LEAF
  3. The fact that we could probably sell our ’92 Integra for about $3,000 (it’s got just 155,000 miles on it!)
  4. Rising gasoline and electricity costs over time, let’s say 5% per year;

utility-meter-roundWe use about 4,500 kWh of electricity per year. Right now, electricity costs us about 11 cents per kWh, which means we’re using about $500 of electricity per year at current costs.

$41,000 (cost of new Nissan LEAF + 5.59 kW solar system, including $9,000 in interest costs on $20,000 loan for the LEAF)
– $3,000 (money for selling our ’92 Acura Integra)
= $38,000
– $38,000 (8 years of gasoline costs with 5% increase + 8 years of electricity with 5% increase + 8 years of maintenance costs for ’92 Integra)
= 0

Eight years to pay-off?
As you can see, according to a finer-grained comparative analysis that takes into account rising gasoline and electricity prices as well as interest paid on a new Nissan LEAF (the interest on the loan is the big killer here), it looks like it would take us about eight years to reach pay-off in our ‘keeping our ’92 Integra forever’ vs. buy a new Nissan LEAF and power it with home solar comparison.

That’s not very encouraging, to say the least.

We might be able to get a better interest rate on a loan for a LEAF than 7.5%. Additionally, I’m guessing that annual maintenance costs for our ’92 Integra, which have averaged $1,500 per year for the past five years, will almost certainly rise if we hold onto it for another eight years. In fact, one might even say it’s questionable whether our Integra will last another eight years.

acura-integra-front2Gas clunker wins again
The moral of this comparison: A reasonably efficient gas clunker that you’ve already paid off is likely to be cheaper over the next five to 10 years than buying a brand new EV and plugging it into home solar, even in a scenario such as our own, where we’ve already banked nearly 25,000 miles worth of EV driving.

However, as I note in the ‘High short-term costs barrier to long-term savings’ entry I wrote two years ago, across a seven-year time span a new EV + home PV wins out over a new gasoline car in our case.

Of course, in that scenario, I compared a Honda Accord to a Nissan LEAF (because we’d buy an Accord if we bought a new gas car, and a LEAF if we bought a new EV), a comparison many might question. If you put a new Nissan Versa up against a new Nissan LEAF + home PV, the LEAF + PV vs. new Versa + gasoline doesn’t look as favorable.

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It’s worth noting that across a very long period of time – five decades rather than just one decade – it sure seems as if EV + PV pays off, big time, over gasoline driving, to the tune of about $200,000 in savings, as I note in ‘Checking the math on solar-charged driving’, a follow-up to veteran solar-charged driver Peder Norby’s interesting long-term comparative look at EV + PV vs. gasoline + stinker car.

However, five decades – about the driving lifetime of most humans, give or take 10 or so years – is an awfully long time horizon, too long, I think, to be appealing to all but the most hard-core advocates, and practitioners, of solar-charged driving.

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