Off grid garage system for car charging

[jahi120]

In Denver Colorado I have a two car detached garage, which can fit 9 or 10 ~400 kw panels on the south facing roof. My original plan was to have an installer do a grid tied system, but estimates are 17-19k pre incentives and replaces 30% of my use. In addition, with changes in TOU coming from my utility, the credits don't make as much sense. I was thinking of a DIY off grid system with a goal of filling a battery and then charging my EV from that power overnight. I estimate I could generate 3-4 kwh per day, which would cover most of my daily commute. I have a sub panel in the garage and a 240v outlet. My car charger can run from a 120v or 240v outlet. I am a total newbie, so simpler is better. Am i thinking about this correctly? What is the most time and cost efficient way to do this?
thanks

 

[BentleyJ]

Since there is already a subpanel in the garage it would be fairly easy to add a hybrid inverter, batteries and solar panels on the garage roof. That said, we generally don't recommend charging an EV entirely with storage batteries simply due to the cost of all the batteries and potentially long pay back period. Although if your commute is not too long and 15 to 20 kWh of battery storage is sufficient to keep the car charged then its more practical.

So 10 x 400W panels is a 4kW solar system, why is the daily kWh hour solar production value so low at 3-4kWh? Did you use a solar calculator to estimate solar output based on your address and roof orientation.

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

 

[jahi120]

Thanks for the reply- I guess I was just multiplying the panel w by the number of panels...oops. PV wats website estimated 5859 kwh/yr and the installers had estimated between 4241 and 4539 kwh/year from the roof

 

[BentleyJ]

PV wats website estimated 5859 kwh/yr and the installers had estimated between 4241 and 4539 kwh/year from the roof

I've found PV Watts to be quite accurate as far as actual production compared to the estimates for both our systems. Its interesting that the installers came up with such low numbers. I suppose its because they don't want to set expectations too high since they usually have to give the homeowner some kind of output guarantee.

 

[SunDave]

Hi. Welcome to the forum. Looks like a doable project if you do your research. It won't be cheap in the short-term, but could work out in the long run. There is a lot to figure out, like if the system you install could handle your car charger. A small setup would likely handle a small charger. A faster charge requires much more gear. Figuring out if your area requires roof setbacks. Making sure what you install is relatively safe. There is a lot to it.

Factor in losses. You lose some of the energy to heat. If using batteries, charge and discharge loses some energy. 85% of production turned into usable would be somewhat realistic. Smaller systems tend to lose more to standby losses (just the small power draw the solar related equipment uses 24/7.

It won't always make a lot of power. There are usually cloudy seasons. December was bad here. Consider snow as well. They could spend some time covered, which also severely limits output.

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Multiple ways to tackle it. There are all in one units that will use battery until depleted, then switch over to grid. That could be useful to pair with a small sub-panel for car charging. Watch the differences in units. Some are made for grid tie, and if you don't grid tie, you don't really want those. They can back-feed a bit to the grid even when settings say they shouldn't.

You can also build it with separate components for the charge controller (uses solar to charge batteries), and inverter (to convert the battery DC to AC).

Hopefully a forum member with a similarly sized system for car charging can chime in and let you know what has worked for them.