# Forums

## Solar Panel dilemma

I look forward to my eventual purchase of a Model S. Meanwhile I have been using my calculator. The home quick charger (double charger) supplies perhaps 80 amps, at 240 volts, correct? This means that the wattage supplied is 80*240 = 19200 watts. Each solar panel supplies 200 watts or better (maybe 250). SO how many panels are needed? This should be 19200/200 or roughly 96 panels. I hear people talking about a "break even" point of around 8 panels to fully charge a model S. How does that work? How does the "Roof" at a Supercharger site supply maybe 10 lanes? Is my math wrong?

Bob the V

shs | 2 agosto 2013

Break even can mean several things. You can target breaking even in kWh (energy) or breaking even in dollars spent on energy. First, you are most likely not going to charge you Model S directly from the solar panels or even at the time of day when the solar panels are producing electricity. Most people with solar are on Time of Use (TOU) rate plans where you get credit in \$ for electricity produced during the day and then charge the car at night when rates are much lower, typically by a factor of 4 to 8.

As to your math, you will use about 100 kWh of energy when you fully charge an 85 kWh Model S. If you have 36 panels, (7.2 kW) like I do, they typically produce 42-45 kWh a day. So, if you will be driving 250 miles every day, and need a full charge every day, not likely, you will not break even in kWh. On the other hand if you drive much less than that on average, you might well break even in kWh and most certainly will in dollars if you are on a TOU plan and charge at night.

While breaking even in kWh is a fine goal, breaking even in dollars is also a responsible goal in that you are charging the car when the overall generation capacity of the grid has power to spare, and you are putting juice into the grid when it is needed the most – on a summer afternoon. My goal is to break even in \$ for our whole house which also uses electricity for winter heat making the best electrical generation/use strategy and choice of rate plans even more complex.

BobtheV | 2 agosto 2013

BobtheV

JPPTM | 3 agosto 2013

Maybe consult with a solar PV company like SolarCity to figure out your needs and options. You do not have to buy the system outright. You can lease or even just do a power purchase agreement. Lots of ways to achieve your goal. One key concept is to NOT feed the grid with more power than you will need, since (at least in my case) you are being paid at low rates for power you generate at higher cost to you. The SC mantra is to generate about 70% of the power you need to cover about 90% of your bill.

CalDreamin | 3 agosto 2013

I estimated that I need 1 240W PV panel for each 1000 miles per year I drive the Model S, to offset the annual kWh required to charge it.

negarholger | 3 agosto 2013

@CalDreamin - that is actually an excellent rule of thumb.

0.24 kW * 4 hrs * 365 days = 350 kWh per year
1000 miles per year * 0.34 kWh / mile = 340 kWh per year