Using Solar to Charge Electric Car

Hi folks,
I'm in a discussion group of people that are charging their EV cars with their rooftop solar.
I am intrigued by the notion of using your electric car as battery storage for your RV. They
can hold a ton of kwh then using power from the battery pack to supply the rv. We are batting
the idea around and I will let people know what the practicality of it is. Solarman do you have
any experience with this?

I actually do have some experience that is relevant to the idea.

I wired my golf cart to be able to turn a few switches and turn a 36 volt golf cart into a large 12 volt battery.

Then using connectors similar to the winch cable connectors I was able to connect the 4 golf cart batteries in my DP, the 4 golf cart batteries in my Stacker trailer, and the 6 golf cart batteries in my golf cart into one large connected 14 battery 12 volt bank with a capacity of over 1500 amp hours.

I fed it with a combined solar panel capacity of 10 panels of 160 watts each.  There is a more detailed write up on here and I will put the link on here in a minute.

http://www.rvforum.net/SMF_forum/index.php/topic,110702.msg997884.html#msg997884

butchiiii said:

Hi folks,
I'm in a discussion group of people that are charging their EV cars with their rooftop solar.
I am intrigued by the notion of using your electric car as battery storage for your RV. They
can hold a ton of kwh then using power from the battery pack to supply the rv. We are batting
the idea around and I will let people know what the practicality of it is. Solarman do you have
any experience with this?

Click to expand...

none actually using one for an RV, but the principles used are very simple..

1. voltage conversion between high voltage EV battery and 12 V for the RV
2. charging the EV

for item 1, there are commercial kits available for EV's that provide high current outputs at 12 V and
120 V AC with an inbuilt inverter.

for item 2, the simplest way is to use your existing 120 V charger and power it from the RV's battery/solar setup.

here are some thoughts to consider:

3. what powers the RV when you are driving around ?
4. EV's are power hungry, how will you recharge it ? where do these watts come from ?
5. if you intend to charge from solar, you will not be able to use the EV when charging. so how will you get around ?
6. if solar only then you will still need generator for backup in any case..

A lot depends on how you use the electric vehicle, lets use a Tesla model S as an example, depending on exact year, and version and if you are counting city / highway mileage the Model S uses between 3,300 - 4,000 watts of electricity to travel 10 miles. If looking at a typical 35 ft motorhome, it is difficult to fit more than about 1,000 watts of solar panels in the available roof space, and assuming they are fixed mount, you will be looking at getting maybe 4,500 watt hours per day out of them on a sunny day.  This means you can maybe get 10-12 miles of charge for the car per day out of the solar panels on the RV, if NONE of the power from the panels are going to charge the RV's batteries, or power electronic devices.  If we assume running only a propane refrigerator, fans, LED lights, flat panel TV, and maybe a furnace blower at night (no air conditioning)  A typical RV family can probably live comfortably with 500 watt of solar panels for RV use.  Say half of that 1,000 watt output going to RV life operations.  This leaves 500 watts worth of panel output to charge the electric car.  So maybe it will work if you only use the car for that 15 mile drive into town to buy groceries once per week.

Solarman I guess the 1st part of the equation is how many typical kilowatts can be produced by 2000 watts of
residential solar panels. The ones the group were using as a model were 72 cell panels that put out 36v and
8.5 amp at Pmax. They also come equipped with 3 bypass diodes to help with shading. What would you estimate
a typical days kilowatt output would be here in So Cal/ So Nev?
We are all just kicking around ideas. Talked about having some onboard batteries but using the EV for backup
storage if the panels are putting out enough.

How big are those panels and how many can you get on the roof or otherwise deployed?

I am guessing that they are 280 to 300 watts each.  Volts X Amps = Watts

If the EV car was going to be flat towed it might give you more usable capacity when boondocking, but you still have to be able to drive it out if not towing it.

The electrical capacity of the golf cart that I used to boost the battery bank size could be used up with no consequences.

Interested in what you find out from a practical standpoint.

I saw a commercial that seemed to indicate that some EV cars have a gas fuel backup.  That might work better.

One question that I still have is are you wanting to use the Solar to keep or add to the charge of the car or are you wanting to increase your overall battery capacity when boondocking.  Maybe both but does one have priority?

Edit:  If this does work out for you I can see one other possible advantage.  If you park your RV at home and in the sun you could have a quick disconnect method to always charge or at least top off your car battery at home when there.  Might be slow but if you have invested in the Solar anyway then it is close to free power.

There was an article in QST a couple of months ago about using the hybrid battery in Ford Focus to power amateur radio equipment in the field.

Of course the guys doing it were Ford engineers, so they weren't at all concerned about tapping into the high voltage bus. Interesting concept, but I won't be splicing wires on my hybrid anytime soon.

butchiiii said:

Solarman I guess the 1st part of the equation is how many typical kilowatts can be produced by 2000 watts of
residential solar panels. The ones the group were using as a model were 72 cell panels that put out 36v and
8.5 amp at Pmax. They also come equipped with 3 bypass diodes to help with shading. What would you estimate
a typical days kilowatt output would be here in So Cal/ So Nev?
We are all just kicking around ideas. Talked about having some onboard batteries but using the EV for backup
storage if the panels are putting out enough.

Click to expand...

insolation for say Las Vegas:

winter: 3 hours
Summer: 7 hours

with 2000 W of panels you get a total of 6000 Whr in winter and 14000 Whr in summer.


here is a dose of reality for off grid solar.. your not going to like this..


I did a costing for a guy here in south Texas for an off grid cabin and that included a
Nissan Leaf with a 24kWhr battery that he said would be down 50% each day typically.
the rest of the power budget was for the cabin.

his total daily Wattage came to 18000 W hrs or 18kW hrs ( 12000 car and 6000 cabin )


now using the calcs I laid out in my boobdocking post we get
a worst case in winter of approx 3 sun hrs.


Panel watts = 18000 * 1.5 / 3 = 9000 Watts

Battery capacity Amp hr = 18000 Watts * 5 days / 48 Volts = 1875 Ah

after juggling the panel size, cost and number I arrived at 30 x 320 W panels = 9600 W total.

Charge controller amps = 9600 / 48 = 200 Amps

using three strings of 10 panels in series we can use 600 Volt 80 Amp charge controllers
we need 3 of them

for batteries we have 24 Trojan SIND 02 2450 ( 2V 1849 Ah cells )
giving a total capacity of 48 V @ 1849 Amp hr ( 44.376 kW hr useable capacity to 50% DOD )

next is a 4400 W inverter/charger to power the car with 240 Volts.
we need a charger as well for backup, so a combo unit is a good choice.

and a smaller 120 Volt 3000 W inverter for the cabin.

we also need a level 2 EVSE for the car ( 240 V 16 Amp, 3840 W )

so that's the electrical side not including generator, wiring and construction materials..

add this up and we have:


Panels:                  $6000
Battery:                $15000
Charge controllers: $3600
Inverter/Charger:  $2300
120V inverter:        $1000
Lvl 2 EVSE:            $400

Total:              $28,300

that's a lot of green just to be off grid with a car that can drive 60 miles a day.. !



so you still think you can use your car as a backup ? yes you can, but how are going to recharge it when
you most likely don't have enough solar capacity for the RV's use ... ?

HueyPilotVN said:

Might be slow but if you have invested in the Solar anyway then it is close to free power.

Click to expand...

so you think solar is free power ?


it's actually the most expensive initially unless you are given panels for free..


consider this:

you buy 1000 Watts of panels for $1 per Watt.

you have an average of 5 sun hours per day so you
produce 5 kW/hrs of energy per day every day over a year.

you need some way to store the power or send it to the grid so you need a controller

to break even you will need to generate $1000 of power plus the controller cost.
so lets say you spend $500 on a controller.

at my POCO cost of 10 cents per kW/hr it will take $1500 / 0.1 = 15,000 hours

so 15,000 hours at 5 hrs per day = 3000 days
that's 8.4 years to break even.

this does not include battery storage, as soon as you buy batteries, you NEVER break even..
batteries are consumables and the replacement cost eats up any gain you might make.

Solarman those are the kind of calculations I was looking for. I didn't really think it was practical
but was trying to think outside the box on getting around the battery storage problem. It's info
that I will take back to the group and give them the rundown. The battery storage capacity always
seem to be a sticking point. Trying to keep weight and cost down with different ideas.
Thanks for the input. I will come back here with more wacky ideas I'm sure. By any chance is there
a site you recommend for more than just a basic under standing of solar systems? TIA

solarman said:

so you think solar is free power ?


it's actually the most expensive initially unless you are given panels for free..

Click to expand...

After spend thousands on Solar I agree and point taken.

My comment was to the fact that if you had purchased the Solar equipment for camping and boondocking that an added benefit of being able to use it for charging your EV while at home would not cost any more than not charging the EV at home.

Sometimes things do have double meanings.

As long as I had my Stacker trailer parked at home I always had my golf cart fully charged and did not use the A/C charger at all.  I can still do the same using the Coach Solar now.

Just saying probably not a big factor but it might help.

HappyWanderer said:

There was an article in QST a couple of months ago about using the hybrid battery in Ford Focus to power amateur radio equipment in the field.

Click to expand...

You would not happen to have the MONTH handy would you?????

June QST, page 66. I just read the article again, and it wasn't as I remembered. They simply tapped the 12 volt supply and used the on-board DC/DC converter to replenish from the HV battery.

HappyWanderer said:

They simply tapped the 12 volt supply and used the on-board DC/DC converter to replenish from the HV battery.

Click to expand...

That's cheating ....  ;D

butchiiii said:

Solarman those are the kind of calculations I was looking for. I didn't really think it was practical
but was trying to think outside the box on getting around the battery storage problem. It's info
that I will take back to the group and give them the rundown. The battery storage capacity always
seem to be a sticking point. Trying to keep weight and cost down with different ideas.
Thanks for the input. I will come back here with more wacky ideas I'm sure. By any chance is there
a site you recommend for more than just a basic under standing of solar systems? TIA

Click to expand...

it won't be practical until solar efficiency exceeds 50% and battery density is 10 times what we have now.
your going to have a long wait for that to happen..

sorry, no recommendations for solar sites.

solarman said:

it won't be practical until solar efficiency exceeds 50% and battery density is 10 times what we have now.
your going to have a long wait for that to happen..

sorry, no recommendations for solar sites.

Click to expand...

I've learned a lot here on this forum. Also quite a bit from you. Thanks everybody. Still learning.

HappyWanderer said:

June QST, page 66. I just read the article again, and it wasn't as I remembered. They simply tapped the 12 volt supply and used the on-board DC/DC converter to replenish from the HV battery.

Click to expand...

Thanks I'm going to take a look at that.

HueyPilotVN said:

How big are those panels and how many can you get on the roof or otherwise deployed?

I am guessing that they are 280 to 300 watts each.  Volts X Amps = Watts

If the EV car was going to be flat towed it might give you more usable capacity when boondocking, but you still have to be able to drive it out if not towing it.

The electrical capacity of the golf cart that I used to boost the battery bank size could be used up with no consequences.



Interested in what you find out from a practical standpoint.

I saw a commercial that seemed to indicate that some EV cars have a gas fuel backup.  That might work better.

One question that I still have is are you wanting to use the Solar to keep or add to the charge of the car or are you wanting to increase your overall battery capacity when boondocking.  Maybe both but does one have priority?

Edit:  If this does work out for you I can see one other possible advantage.  If you park your RV at home and in the sun you could have a quick disconnect method to always charge or at least top off your car battery at home when there.  Might be slow but if you have invested in the Solar anyway then it is close to free power.

Click to expand...

Mr Hueypilot. The cells are rated at 350 watts.  Size of the panels is 77 x 39. My question was just the feasibililty of using an EV for backup storage for generated power. The max I could practically put on the new 5er would be 8 but that would be a tight squeeze. Base on some of the calculations that solarman put out there I see that there wouldn't be a lot of excess power to save. I hate the idea of lead acid batteries, agm batteries because you've only got about a 25-30% allowable discharge every day. The darn Li batteries are so damn expensive. So we are back to looking at the re-purposed battery packs out of the Tesla vehicles. ON TO MORE RESEARCH 

solarman said:

insolation for say Las Vegas:

winter: 3 hours
Summer: 7 hours

with 2000 W of panels you get a total of 6000 Whr in winter and 14000 Whr in summer.


here is a dose of reality for off grid solar.. your not going to like this..

Click to expand...

Thank you for that analysis. Very practical.

Solar is valuable if you have few other options. For boondocking, I find it to be great. But that assumes that I value quiet, which I do. No, it is probably not economical, but when I am camping, economical is less of an issue.


There still ain't no such thing as a free lunch!  ;D


Frank