Solar capacity needed for my Leaf?

(Apologies in advance if I'm not using quite the technical right terms. I'm an accountant, not an engineer.)

I have an existing 5.9KW solar system for my home, which covers our home plus about 1k KWh to spare over the course of a year when we have a mild summer (Southern California had a very mild summer this year). I suspect that a normal summer would wipe out that surplus. Since I just got my Leaf, I'd like to add more panels so I can drive on sunlight.

How much capacity do I need?

I expect to drive my Leaf about 12-14k miles per year. Over the first month, I'm averaging 4.0 miles per KWh per the Leaf's dashboard. That would 'suggest' that I would consume 3,000 - 3,500 KWh per year.

Now I'm assuming that is the amount drawn from the charged battery, not the amount drawn from the wall outlet as there would be overhead and inefficiencies in the charging process. I will be installing a 30amp level 2 charger (Scheider EVPlus). But I have no clue how to estimate the amount of solar production I will need to get that 3,000 - 3,500 KWhs into my battery. Can anyone guide me on this point?

And then, how do I convert the KWh production need into a KW specification when requesting bids from my solar installer?

Thanks!

Back-of-the-envelope number I use is about 5 MWh per year for 15,000 miles at your efficiency which would equate to about 3000Wp PV panels (grid-tied). (I included charger efficiency.) At $2/watt, that comes to about $6000. If you are using TOU rates, that would cut the amount of PV by about half.

Again, just a rough guess.

I'd say Reg nailed it. Figure 3 miles/kWh. Then it just depends how many kWh per installed watt you'll get from the installation. In SoCal his guess of 1.5 kWh per installed watt seems right. But can you really get $2/ installed watt? Seems very cheap. Also where I am TOU only increases production/decreases cost by about 25%.

SanDust said:

But can you really get $2/ installed watt?

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Don't know. Just a guess of the cost after incentives. He'll have to plug his actual costs and incentives in there.

RegGuheert said:

would equate to about 3000Wp PV panels (grid-tied).

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So 3,000 W is the spec I need to tell the installer that I want to add to my system? Or am I mis-translating? What is a "Wp"?

Thanks!!!

What utility? Are you at home running the air at 74 from 10a to 6p weekdays?

Otherwise when you go on TOU-EV rates and charge at night you may have even more excess credit than you do now. Don't buy new panels until you see how the TOU shakes out.

In summer with SCE I sell electricity at about 40 cents during the day and buy at 10 cents overnight. The shoulder hours are about 30 cents but it all works favorably. I also run the air pretty hard at night so it does not need to kick on until 6p after peak rates.

Utility: SCE
Metering: NEM (Net Energy)

I have family at home most weekdays, so the AC does run quite a bit on hot days. I really only bank power in the spring and fall when the panels still produce well, but the AC isn't needed. During this past mild year, I've had a net surplus since we didn't need much AC this summer. But that won't exist in normal years.

My focus is more from a perspective of wanting to be completely solar, not so much on trying to maximize the disparities in rate at which power is sold to SCE during the day versus purchased in the evenings.

I'll look into the TOU metering option though. It may well be a good deal that I shouldn't pass up. But I'd really like to expand to fully cover my power footprint, but don't want to significantly over produce.

Assume about 15% of your wall power will be lost to charging inefficiency.

I've got a 5.16 kW DC system in So Cal, south facing 20 degree roof unshaded with a central inverter installed in 2007. I've been running a LEAF for 2.5 years on SCE TOU-D-TEV single meter plan. I run about 4 mi/kWh average per the car's dash and about 3.5 mi/kWh per the wall meter.

My system generates about 8,800 kWh solar per year and we routinely use about 2,000 more total kWh than that. But with the TOU rates and charging over night during Super Off Peak, I usually cruise in at the end of my NMY with a bill of zero. We just installed central AC this spring, so we'll see how that impacts us after a while. This summer so far, it hasn't hurt much, but as you say, this hasn't been a typical summer.

CAVEAT: A new law, AB 327 (Perea) is working its way through the legislature and could have major impact on rates for new residential solar, but the details will depend on CPUC decisions later on. Anyone considering a new NEM solar setup should pay attention.

For my usage and billing details, check out the numbers section of my blog (link below in my sig) for detail.

Cheers and good luck!

DarthPuppy said:

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So 3,000 W is the spec I need to tell the installer that I want to add to my system? Or am I mis-translating? What is a "Wp"?

Thanks!!!

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You will need to produce about 3000kWh per year beyond what you're currently producing, maybe a bit more given your anticipation of reduced production in 'normal' years, though our production has been pretty consistent for 3 years. In SoCal, depending on your roof orientation, you should be able to get at least 5 (hours per day) times your AC rating, as an annual production average. You didn't say whether the 5.9(?) kW rating on the system you have is the DC or AC rating (DC is higher,there are losses when converting to AC and then feeding back through your meter to the grid).

As an example we have a 3.44kW DC system, which rates down to about 2.98 AC. We produce about 5800kWh per year. That covers our house, about 8K miles per year on the Leaf, and a bit extra which we get poorly compensated for, but we're at our objective which is the same as yours, to actually net to zero usage, not just zero dollars.

The 3000kWh estimate is based on 13,000 miles / 3.2 miles per kWh (that's what we get from the wall) = 4000kWh minus the 1000 excess you have right now.

To make 3000kWh a year means about 8.2 per day, divided by 5 "nominal full sun hours" is 1.6kW AC, so you need about 1.85kW more in panel rating.

As to how you might add that sort of capacity to your existing system - various factors there. If you went with microinverters it should be pretty straightforward (you've probably got a 6K inverter right now, you can't just add panels to it, and a dedicated second inverter for just 1.85kW in panels may not be cost effective. )

You could go for a 2kW AC (2.3 or so DC - eg. string of 10 230 watt panels on a dedicated inverter or microinverters) and you'd probably be looking good.

We might end up in a similar position if we were to someday get a second EV.

Thank you wsbca! That was an excellent response.

And yes, the 5.9 is the DC measurement, which is converted through an inverter that is maxed out. So the expansion of the system would be AC units with the micro-inverters. I was hoping to cover the load with 4-6 extra panels. Our past year's excess production is read from the SCE meter, not the inverter, so that is already after the conversion to AC.

My expectation of not having the current surplus in normal years isn't due to a loss of production, but an increase in the use from running our air conditioner more.

From the discussion I've had with the installers thus far, it sounds like the 250 watt DC panels are now pretty standard. My existing panels are 327 watt DC by SunPower. Not the most affordable, but it was the way to get the production I wanted for the section of roof it was going on. I have another less convenient section where I can place an additional 6 - 12 panels. which is why I'm looking into adding more. So for this install, I can go with the lower cost units. Especially if 10 of them will do the job.

Thanks!!!

At what point does it make economic sense to skip the micro-inverters and do a second inverter? I was originally thinking 4-6 panels, but am now thinking 10 panels. Would I be better off with 10 panels and 1 inverter or going with 10 of the AC units with the micro-inverters?

Also, I'm guessing that 1 inverter has more risk as it could be a single point of failure that knocks 10 panels out. While if a micro-inverter dies, I still have 90% of the panels producing. So that might shift the point at which it makes sense to switch from micro-inverters to a single inverter.

Of course, I'm an accountant, not an engineer, so I could really use some insight on that. :?

Thanks!

Two advantages of micro-inverters over string inverters.

- Micro-inverters allow each panel to generate their max power when one or more panels are being shaded. In a string-config with large inverter, if you have a string of ten 240 watt panels with nine generating 200 watts and one being shaded by a chimney and only generating 100 watts, the power output of the string would be 1,000 watts. For micro-inverters, you get would be generating 1,900 watts from that array.

- Micro-inverters allow per-panel diagnostics. Since you are collecting per-panel production data, you can easily spot any panels/inverters that are underperforming and investigate.

Thanks swaltner! Those are good points to consider.

swaltner said:

Two advantages of micro-inverters over string inverters.

- Micro-inverters allow each panel to generate their max power when one or more panels are being shaded. In a string-config with large inverter, if you have a string of ten 240 watt panels with nine generating 200 watts and one being shaded by a chimney and only generating 100 watts, the power output of the string would be 1,000 watts. For micro-inverters, you get would be generating 1,900 watts from that array.

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This is not correct, if one panel is shaded, a diode across the panel shunts the current and you will get 1,800 watt minus the loss in the diode.

pchilds said:

This is not correct, if one panel is shaded, a diode across the panel shunts the current and you will get 1,800 watt minus the loss in the diode.

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It really depends on the type of shade, but typically with the sort of soft shade you get the diodes don't have that sort of effect.

There's a good reason why the string-inverter guys are selling "optimizers" to compete with micro-inverters.

If a couple simple bypass diodes would take care of the issue, there'd be no reason to sell an "optimizer".

Unless I missed it, what is the total output of your existing system annually? Because there are various things that affect output (e.g. number of sunny days per year, air quality, temperatures, etc.) taking the total output would let you know on average what each existing panel is generating. From that you have already done the math on how much extra power you think you need. Just working with rated panel output ignores the real-world factors.

Good point. When I'm back at home, I will take a look to see what total production was for the past year.

Darth,



What you'll find here are primarily on-grid folks. Also you'll find folks that primarily are retrofitting a PV system to an existing roof . That requires adjusting for less than optimal panel siting. And that's where the conversations normally start to look like spaghetti.

A properly sited system has no shading and therefore doesn't need work-arounds to minimize the damage shading causes.

Another argument you'll find from the micro-inverter crowd is that micros allow each panel to work independently and provide max energy harvest. Except that most of the microinverters are installed on oversized panels - so the max collection days are limited by the smaller inverter, not the capability of the panel.

One cool thing about microinverters is the per-panel monitoring that gives the instant-gratification smart phone crowd pretty pictures to look at. Yes, there are per-panel MPPT devices like the Tigo maximizers that provide panel-level data and panel-specific energy harvesting should one be forced to work around shading. But that's market driven, not because of some failure of central or string inverters. Put another way, a PV system is a marathon, not a sprint. Annual inspection and preventive maintenance is normally sufficient.

Nothing made by man is perfect, and hot electronics are not happy electronics. At some point inverters will fail. My personal choice is a central inverter that's mounted at chest-height. It can be gutted and completely rebuilt in less than one hour if necessary. Because the devices are normally placed in better environments than rooftops, they normally don't require rebuilding very often.


http://www.mynissanleaf.com/viewtopic.php?f=45&t=10736
http://www.mynissanleaf.com/viewtopic.php?f=45&t=11807
http://www.mynissanleaf.com/viewtopic.php?f=45&t=12282
http://www.mynissanleaf.com/viewtopic.php?f=45&t=11966

You can use microinverters, a central inverter, or a hybrid system with individual MPPT units and a central inverter. They'll likely give similar service in both harvest and lifespan. It'll most likely come down to what your local installers are used to working with and what they charge for labor. Installers generally like micros because they can do the job faster yet get the same income from the installation - they generally push them because it's better for them, not necessarily better for you.

I have NFI, just passing it on - you might want to check out this post - all parts for a microinverter based PV system of about the size you need, for what seems to me a very good price per watt, and maybe very near you.


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Ok, I checked the SCE meter and it appears that over the past year, my 5.8kW (DC) system produced 7400 kWh AC, which exceeded what was used by the house by a little over 1,400 kWh.

Using the same factors would suggest an additional 2.5kW (DC) system (10 panels) would produce just under 3200 kWh AC over the course of a year. That should pretty nearly cover my Leaf even if the next summer is hotter and I don't have the 1,400 kWh of over-production from my existing system due to using the air condition more.

Thank you all for your wonderful counsel. I'm now collecting proposals for an appropriate size system.