electric output required to charge 85kWh battery pack

electric output required to charge 85kWh battery pack

Would one of you Electrical Engineers take this one?

I've been checking out the companies on the internet that have designs for solar carports for charging EVs. Each of the companies give the DC output of their panels but I don't know the amount of electricity required to recharge the 85kWh Tesla Battery Pack. I only am interested in the amount of DC 10kW output required to recharge the 85kWh Tesla Battery Pack using the single charger because that is what I'm planning on purchasing but feel free to figure the other battery packs and 20kW Twin charging as well.

Also, are there any current Roadster owners that are using solar collectors to recharge your Roadster?

stephen.kamichik | May 20, 2012

Check out the specs page on TM's website. I believe it is on the order of 400+ volts DC.

DallasTXModelS | May 20, 2012

Thanks, I will look there. Everything about charging on the website is talking about the Roadster.

DallasTXModelS | May 20, 2012

Actually I had already looked there. This is all that is shown on the SPECS page:
10 kW capable on-board charger with the following input compatibility: 85-265 V, 45-65 Hz, 1-40 A (Optional 20 kW capable Twin Chargers increases input compatibility to 80 A)
Peak charger efficiency of 92%
10 kW capable Universal Mobile Connector with 110 V, 240 V, and J1772 adapters

DallasTXModelS | May 20, 2012

I did find this under Model S/Facts

■If you’re interested in installing a home solar system to charge your Tesla, we recommend working with a local solar installer to develop and install a system that supports your total daily energy demand. Assume average energy usage per mile is approximately 300Wh/mile (188Wh/km). Multiply 300Wh/mile (188Wh/km) by your daily driving distance to estimate your daily vehicle energy consumption.

jbunn | May 20, 2012

It doesnt much matter if you have twin chargers, or the size of your pack unless your car is home everyday mid afternoon. As Dallas rightly points out, its about averages. Average driving, sunny days, panel efficiency, ect. But you dont draw directly from the dc panel. You draw from the grid, and the panels feed into the grid.

gjunky | May 20, 2012

The on board chargers (single or twin) take AC power. If you are talking about DC (level 3) charging then you would bypass the on board chargers but it will need to interface correctly with the car like a Tesla supercharger would.

All this is irrelevant because you can't charge directly from your solar panels.

Solar charging is really a bad term. It simply means you are producing electrify by solar and you are using electricity to charge your car. Just not at the same time and not connected directly to each other.

EcLectric | May 20, 2012

The idea of having a solar carport charging your DC battery directly sounds good at first, and would be efficient if your car was sitting in the carport a lot during the day. Practically, however, it's better to think of solar power as a separate entity that provides electricity during the day that offsets the electricity you use at night to charge your car.

I would recommend that you design your solar system completely independently of your Model S. it doesn't matter how much power / current / voltage your Model S requires for charging. The more electricity produced by your solar panels, the more you save on your electric bill and the more you help the planet. Think of the electrical grid as a giant battery: when your panels are providing juice, they provide much needed electricity to the grid that you sometimes get paid extra for (depending on your utility company and rate plan). At night when your car is charging, you get some (or all) of that 'stored' electricity back when demand for electricity and rates are low.

In this case you are saving money because you provide a load balancing utility to the electric company. During the day, demand for electricity is higher (air conditioning, etc.) and because of this high demand, the electric company has to resort to costlier and sometimes dirtier ways of generating electricity. When your solar panels provide power to offset some of this demand, they have less requirement for these undesirable generation methods. In addition, the fact that this power is provided closer to the load (your neighbors) means there may be less transmission losses in the system. At night, when you are charging, the overall demand for electricity is less, so the power company is using its best (cheapest, cleanest) generation methods to charge your car.

Instead of designing around your Model S specifically, I would consider such things as which available areas (roofs, etc.) face the sun the most often during the day, and have no shade. I know that many solar cells are wired in series to an extent. When they are wired this way, whichever solar cells are wired in series must all be entirely in sunlight in order to produce any electricity. If there is, for example, a tree in the way partially shading the solar array, it will severely reduce the output of the array. If you are in a warm climate and use air conditioning a significant part of the year, you might save more electricity by putting solar panels over your living space rather than over a carport. The panels absorb sunlight that might otherwise heat your house, and would reduce your air conditioning load.

My point is that there are many other things with solar that should be considered before you worry about the specifics of the charging demand of your Model S.

Rod and Barbara | May 20, 2012

Dallas, we have a 3.5 year old Roadster and power our house and Roadster electrical needs with two solar panels. There is no easy way to charge the car directly from the output of the solar panels. Most solar panels feed into the electrical grid through an inverter so that your electric meter runs backwards during the day. In the evening, of course, you draw electricity from the grid and your meter runs forward again. EcLectric describes many of the considerations regarding solar panels very well.

In an attempt to answer your original question, let me offer the following. The size of the solar panels, usually expressed in kW (DC) is an important factor in how much electric power your panels will generate. Other factors include the orientation of the panels (both vertically (roof pitch) and directionally (south is usually best)), the number of sunny hours per day available, the latitude, and any shading issues. For example our, here in Southern California, our 2.48 kW system faces southwest and generates 3721 kWh annually (1500 annual kWh/kW of solar panel size) while our 4.68 kW system faces southeast and generates 6033 kWh annually (1289 annual kWh/kW of solar panel size).

To estimate the electrical power needed to charge your car requires an estimate of the number of miles you will drive and the Wh/mile that you will experience. The Tesla suggestion of 300 Wh/mile seems a little optimistic to me. We keep detailed records on our Roadster and our average after 3.5 years is 338 Wh/mile (includes charging losses, sitting losses and power to propel the car). From the Wh/mile graphs Tesla has published the Model S should take about 10% more power than the Roadster, which would work out to 372 Wh/mile. So if you expect to drive 12,000 miles a year, you will need a solar system that will generate 4,464 kWh annually. For our location in Southern California, that would be on the order of a 3.3 kW (DC) solar system.

DallasTXModelS | May 21, 2012

Thanks guys, that's great information. The designs I've been seeing online have all been self contained systems with one or two support pillars, a roof covered. In solar cells and not connected to the electric grid. The formula supplied by Tesla Model S facts. Page had me puzzled because multiplying 300miles or even 60miles (my daily commute plus 8 miles of wiggle room) gave me huge numbers. I realize the result would be in Wh/m not kW but still was not making sense to me.

EdG | May 21, 2012

If, for philosophical reasons or bragging rights, you want to be completely off the grid, fine.

Otherwise, just do as EcLectric said: forget the fact that you're going to charge your car when you're figuring out your PV system.

You won't have pay for equipment (batteries) to store your power for when your car comes home. Instead, you get top dollar (top price of the day) for your power when you generate it, and are charged bottom dollar (if your rates go down at night) when you ask for it back while charging your car.

You minimize your costs, both installation and maintenance (no batteries to replace), and you don't waste energy you're generating but not using.

Beaker | May 21, 2012

We just talked to our solar rep. To cover ~10,000 miles/year of driving he recommended adding 1.880kW (2,346 kWh annually) to our system. For us that means 8 more Kyocera 235 panels, to match the existing ones. (we're using enphase micro inverters so it's easy to add more panels.)

Like EdG pointed out we feed the grid during the day, and feed off it at night.

Timo | May 22, 2012

10000/300 * ~90kWh (with charging losses) ~= 3000kWh. 2346kWh isn't enough, you need a bit more.

Brian H | May 22, 2012

It's all approximations anyway. If you drive in the city, you'll use less, etc. Just get into the right ballpark and you'll not only cover your battery charging, but collect the rate difference, which is the financial equivalent of extra generating capacity.

Mike_ModelS_P457 | May 22, 2012

Man do I wish I had room for more panels right about now!

I have 42 Sanyo 210's across two inverters (two distinct arrays).

Alas, I am out of roof.

Regardless, I only spent about $500 on power in 2011 in total.

I've been actively swapping out higher energy usage appliances and light bulbs to LED in the hopes of reducing my usage prior to the EV charging needs.

Regardless my electric costs will go up, but far less than my gas costs go down.

Beaker | May 22, 2012

@Timo, You are correct sir.