How many solar panels does it take to charge a Leaf?

A four panel 1 kw system, with Enphase M215 inverters, in California with PGE rates, will get you 21,000 miles per year. Here are the calculations-


1,000 watt system
x1.75 multiplier used to predict production (determined from data from real installs)
1,750 kwh’s produced per year (south facing panels)

1,750 kwh’s
x 3 multiplier for solar produced power with PGE time-of-use billing
5,250 kwh’s available from PGE for charging at night for 1,750 kwh’s made during day

5,250 kwh’s
x 4 miles per kwh of charging
21,000 miles driven per year


How much is that worth to you?

21,000 miles / 25 mpg = 840 gallons @ $3.35 per gallon = $2,814 saved per year


If you drive 21,000 miles per year, a 4.25 kw solar system will pay itself off in approximately 25 months (by combining the electric savings, the PGE rebate, the federal tax credit, and your gas savings). Is that cool or what?

Mothernaturesolar said:

A four panel 1 kw system, with Enphase M215 inverters, in California with PGE rates, will get you 21,000 miles per year. Here are the calculations-


1,000 watt system
x1.75 multiplier used to predict production (determined from data from real installs)
1,750 kwh’s produced per year (south facing panels)

1,750 kwh’s
x 3 multiplier for solar produced power with PGE time-of-use billing
5,250 kwh’s available from PGE for charging at night for 1,750 kwh’s made during day

5,250 kwh’s
x 4 miles per kwh of charging
21,000 miles driven per year


How much is that worth to you?

21,000 miles / 25 mpg = 840 gallons @ $3.35 per gallon = $2,814 saved per year


If you drive 21,000 miles per year, a 4.25 kw solar system will pay itself off in approximately 25 months (by combining the electric savings, the PGE rebate, the federal tax credit, and your gas savings). Is that cool or what?

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No it's not cool at ALL! There is a huge flaw in your calculations and this is VERY VERY misleading. I'm a supporter of you and obviously PV but please reconsider the logic behind your 25 month ROI.

You can not include the cost of gas in a ROI for installing PV. Installing PV does NOT save you a drop of gas. Installing PV saves you from buying electricity. Buying an EV saves from buying gas. The two ROIs are not directly ralated.*


IMO: The question you answered is not really relevant at all to the ROI of PVs or EVs but is more so just a fun trivia. The real question that needs to be asked is how much PV can you afford, how much PV will fit on your roof, what effect does increasing the size of the system have on the ROI efficiency (Factors such as rate tiers, rules regarding excess production, using less efficient roofs, but also decreased cost from spreading the fixed costs over more capacity, etc., etc.)


Fine print *: When it comes to the complicated rate structures, peak rates, tiered rates, etc. I can see situations where installing PV and EV could save more money combined than doing each individually but I don't believe this is the point you are making.

Also, I'm a bit envious of how much better of a deal solar is there than here. Here we use a rule of a thumb of 1 to 1 vs your 1 to 1.75 and then we have no time of use, very little tiers, and our electricity is dang cheap in comparison. Although we do have a 15 cent per KWH state production incentive for the next ~8 years.

"Installing PV saves you from buying electricity. Buying an EV saves from buying gas." Hmm, I see what you mean, QB. You have to have the Leaf first, and then you're already saving on the gas. What a concept! How did I miss that? They shoot Mythbusters near me, I'll see if I can get you a job there, haha.

Well, the four panels per 21k miles driven, in our PGE area, is certainly correct. A 4.25 kw system will pay itself off in less than 48 months, without my Mythbusted gas savings calculations

How are things in the Northwest? It just hailed here an hour ago, you would've felt right at home. When are you adding your new panels?

Mothernaturesolar said:

"Installing PV saves you from buying electricity. Buying an EV saves from buying gas." Hmm, I see what you mean, QB. You have to have the Leaf first, and then you're already saving on the gas. What a concept! How did I miss that? They shoot Mythbusters near me, I'll see if I can get you a job there, haha.

Well, the four panels per 21k miles driven, in our PGE area, is certainly correct. A 4.25 kw system will pay itself off in less than 48 months, without my Mythbusted gas savings calculations

How are things in the Northwest? It just hailed here an hour ago, you would've felt right at home. When are you adding your new panels?

Click to expand...

I'm glad your logical abilities are so great! I had this same argument with a local Washington solar non-profit promoter after they had a meeting where a vendor promoted a similar claim and could not get through to him at all.

I'm still completely amazed that all it takes is 4 panels to offset the cost of driving 21k miles. And it's just pure insane that a 4.25kw system has a 4 year ROI, installed by professional installer using high quality equipment. There's no reason everyone homeowner down there doesn't have a system.

Last weekend we got about half way through cutting down the large trees that blocked my east/west exposure, next weekend we'll finish the rest of the job. Then I'll be ready to get a quick solar path analysis just to verify my understanding of which roofs will be best utilized, and then I'll be submitting my permit applications.

I like this way of thinking! As an owner of two solar systems, one installed in 2004 and a second one installed by the author of this topic (which is working wonderfully -- Hi Mark!), I'm in complete support of the premise of PV for EVs. I think the raw numbers here are a bit optimistic, however. What makes it extremely complex is the TOU (time-of-use) rates combined with the tiered billing rates, which vary depending on where you live in California.

"1,750 kwh’s
x 3 multiplier for solar produced power with PGE time-of-use billing
5,250 kwh’s available from PGE for charging at night for 1,750 kwh’s made during day"

The 3x multiplier is probably a bit overly-optimistic. The higher rate only applies from 1pm-7pm weekdays for the currently-available TOU rate schedule, and only during the "summer" (which actually runs 6 months). So in the morning and on weekends, you're only getting a one-for-one rate on the produced kilowatt-hours. So maybe 60-65% of your solar generation would happen at the higher rates, even taking into account that summer mornings are more likely to be foggy than the afternoons . The other thing to look out for is if you use a lot of power during the high-rate TOU hours, that may erode the benefit of TOU, since you would have been paying a much lower rate with no TOU and no solar, provided you stayed within the first two price tiers for each month.

Oddly enough, the 3x may in fact fully apply during the winter, but not because of TOU, but rather because of the tiered rates. If you're in the third tier during the winter (whether you have an EV or not), much or all of your winter solar production may effectively offset somthing close to 30c/kWh. But then, the "winter" 6 months of production may only account for about 1/4th (or 1/3rd at most) of your annual production, because of poor sun angle and coudy/rainy skies.

The math is complicated enough that I don't know if I'll ever know exactly how much my solar panels offset my car usage, in dollars (kWh are a lot easier to calculate, and perhaps in some sense, more important!). My best current guess is that I needed the additional ~2kW system to offset somewhere between 15 and 20K miles of driving efficient EVs (Leaf and Fit EV). Part of this is the reasons above, part of it is that I do have some minor shading from tall neighbor trees at certain times of the day. Maybe I'll know for sure next August at my next true-up! In any case, none of this changes my firm belief that at least in CA with PGE's odd tiering structure, PV solar is definitely the way to go to power an EV, provided you have a decent place to install it.

-Steve

Hey Steve, PGE doesn't make figuring this out easy, do they? You are technically correct in your analysis of the summer production being around 60-65% of the total yearly production figures. The 3x multiplier is only a guideline, with the byzantine PGE billing system, you can never know for sure exactly what will happen.

You get the best multiplier if your solar production only takes care of tier 3 and higher charges. In your case, Steve, your solar production reduces your bill to almost zero (lucky guy!), so any credits that you receive for your solar production are all within the baseline rates, so for the reasons you stated, quite correctly, the 3x figure is overstated (are you related to QueenBee?)

But, in general, rather than being overstated, the 3x figure is often an understatement, since most systems that I install are designed to take care of only tier 3 and above electric charges. For example, if your summer solar production is in tier 4 during the summer, you would produce 1 kwh in the summer at $.49032, and then, if your winter usage is less than 130% of baseline, you could use over 4 kwh during the winter at off-peak tier 2 $.11947, a multiplier of 4.10x. It's wildly complicated, and PGE does that on purpose, so we don't know how badly they're robbing us.

The reason that PGE does a once a year true-up is that most people over-produce during the summer, and then use those summer credits during the winter. If your solar credits are in the baseline area, your credit multiplier will be approx 2.2x. If your solar credits are in tier 3 and above, your credit multiplier will be over 3.5x.

I hope that you and Heidi and Megan had a good Christmas, and I'm looking forward to a ride in your Leaf!