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Solar panels to charge MS

I realize this is a little off topic, but I get the feeling there are a few experts here on solar panels. How big do the panels on your house need to be in order to be effective? What cost are we looking at? Is it worth it financially if electricity is .09/KWH where I live?


I think that gas prices are more important to compare with than electricity -- it gives you better numbers :-)
The other very key things to think about are:
1) What region you live in. Some places get more sun exposure than others. Look at a solar insolation map to see how many kWh/m2/day you can expect for your area.
2) Your house. Do you have a roof facing south? Do you have trees that shade your house?
Get several professional solar installer to come out and give you a quote. They have tools that can calculate roof shading and facing to figure out whether it make sense for you.
We're in a high solar insolation region with an exposed, south facing roof. It's awesome. We've already bought our fuel for the next 25 years or so.

I suggest you call Solar City and/or another Solar Systems Installer, and have them do a free energy analysis and site plan for you. Then you can evaluate with very specific information at hand. Get a couple of evaluation/plans to compare. Competition is a good thing, and if you like the plan of one, but the bid of the other is cheaper, that makes for good negotiation on final price, if you decide to go for it.

There are others on these forums who have had great success installing solar panels. (search through Definitely worth getting the free evaluations and quotes.

I have a 20kwh system on our roof and our electricity rates are pretty low (about $.10/kwh). I also live in Arizona and my power usage soars in the summer time when our AC units are running 24 hours a day when it's above 100F outside:-)

My system will pay for itself rather quickly (in fact my first solar installation will have paid for itself within 4 years - I got it when the incentives very very high).

You really need to figure this out for yourself - too many details are missing in your post to give you any meaningful answer (where do you live - what is your energy usage - is it a flat rate that you pay or does it depend on time of day etc. etc.)

What every one has share up to now with you is right on. Another factor is 30% Federal Tax Credit.That helps, but does your state offer a local/state income tax credit too? For instance, Hawaii offers 35% up to cap credit of $5,000 Combine the state and Federal and you have a 65% tax credit. That means spend $ 1 and you get back 65 cents in tax credit. That makes it more affordable.

Wow. Great replies! Thanks guys. I live in Salt Lake City, UT. I'm high on a hill with great South and West exposure. Lots of roof line and nothing to block it. Winters are sunny, and I might be able to generate some useful energy during the winter months if I can keep the snow off the panels. Sounds like I need to call Solar City and get an estimate. If they don't pay for themselves in 4-6 years, it isn't worth it to me. I will have to ask my wife what our electric bills were last summer in our old house. We moved into our new house a couple months ago. This house is bigger, with huge windows, and will probably be very expensive to keep cool May-September. I also need to make sure that the tax credit from the panels and the MS don't cancel each other out in some way.

I'm a big fan of solar (I have panels on my roof as well). However, it's not the cheapest way to lower your rates. In fact I would call it one of the last things you should do. First make sure you are using the lowest amount of power possible. Swap out all incandescent bulbs with CFL and LED bulbs. Hook up timers for exterior lights. Find out what's causing vampire draw and eliminate/reduce it (getting a Kill-a-watt meter is great for this). Take a closer look at your daily habits and see if they can be changed in anyway to reduce your consumption (turning lights off when you leave, adjusting the AC differently). Once you have your usage down as much as possible, then solar would be the next best step and be the most cost effective.

One other thing to be aware of, your roof needs to be in good condition. You don't want to install a solar system only to have to pull it down to replace the roof in the next few years. Pulling the panels and reinstalling them adds a lot of cost and can increase your ROI to a much later date.

BTW, Solar panels don't have to be mounted on a roof. You may or may not have enough sq ft of roof facing in the right direction, either S or SW, to provide the necessary power. If you have the land to have your panels with a ground mount, you can face them in the right direction, easily clean and adjust their altitude, and more importantly, they will likely produce more juice on hot summer afternoons as roof mounted panels get too hot and their output goes down. Typically much better airflow on the backs of ground mounted panels and so they remain cooler.

the $0.09 per KWH is very relevant and extremely cheap. If its a night rate that you use for charging the Tesla it may still make sense to do solar for other needs. If that is your rate regardless of TOD I cant see how solar can compete.

the $0.09 per KWH is very relevant and extremely cheap. If its a night rate that you use for charging the Tesla it may still make sense to do solar for other needs. If that is your rate regardless of TOD I cant see how solar can compete.

I have solar on my house in Orlando and have done quite a bit of research on the topic. I agree with the above comment about starting with energy conservation first. My bill before solar is about half that of others in similar houses. The local utility company will often do a survey for free.

To recharge about 50 miles you need about 14.2kWh of electricity (per Tesla's calculator).
In Florid you can count on 4 to 6 hours of sun per day on average. You have to derate the system from the "sticker value" on the panels to account for losses that occur. A typical factor is 77%. There is a great calculator and other resources here: (

So if you figure an annual average of 5 hours of Sun, with 77% derate, you would need about 3.7kW system to offset driving your Tesla 50 miles per day.

Also, you will want to "grid tie" your system rather than use a battery back up. This is basically using the utility company as your battery. Batteries only make sense in remote locations or with unreliable utilities. During the day your will spin your utility meter backwards and an night you will use the electricity from the grid.

The cost should be in the ballpark of $4 to $5/Watt installed. So, the system will run about $15k out of pocket. Keep in mind you will get 30% back as a tax credit--be sure to ask for it when you do your taxes. So you net cost will be about $10k.

Also check the website for any local/state rebates.

If you are going to install solar and can afford the upfront costs, I recommend installing 5kW or more to offset up to 80% of your house electric. The return in savings on your electric bill is better than any savings account at a bank. (BTW, I pay about $.12/kWh from the grid)

Be sure to get a least 3 quotes. You will be surprised to find some companies charging over $6/Watt and getting away with it.

Good luck.

Great post Solar Scout! Thank you for the numbers. Very helpful!

Yes be sure to get (3) quotes!
The quotes varied by $10K for our 12KW system

First: Solar Scout great post.

If you live in the North East you may also be eligible for SRECs (Solar renewable energy credits). SRECs can be sold for cash. See for details.

My electric bill separates the cost of generation ($0.06) from the cost of delivery ($0.065). Both are charged per KWh. So the real cost of my power is $0.125 per KWh.

When measured at the power meter my MS uses about .450 KWh per mile (Verses the .3 KWh per mile uses to move). The .450 KWh includes all battery standing loss, charging inefficiency, computer drain, etc.

FWIW, SolarCity is doing a 15kW solar install for me soon under a power purchase agreement. SolarCity was founded by a couple of direct relatives of Elon Musk and he is a principal in the company. SolarCity really understands the Tesla and knows the infrastructure you need to charge/feed your Model S. On the Tesla Motors web site under Charging, there is a direct link to SolarCity to help you in this regard. FWIW, they are heavily discounting the installation of my 2 NEMA 14-50 outlets as part of my solar package. They ask up front how many miles you plan to drive your Model S a day. Their planning calculators factor in your kW usage, and they spec a solar system to meet your needs. One stop shopping--very professional. Maybe not the cheapest, but I want a company who is likely to be around for a while and can support and service my system.


What state are you in? Here in New Mexico we get production credits so the cost of my electricity after ~5 years is $0. On average dropping ~$15000 on solar panels will power your MS ~100 miles/day. I spent $12k after the Tax Credit and get 50-60 kWh/day.

Place a call to SolarCity. They can evaluate based on your location, rates, etc and provide a solution. If you sign up thy will also do an energy audit on your home.

Since there seems to be a bias (that I'm ok with), I'll add a couple of counter philosophies to solar PV. Solar City offers a truly good deal leasing you your PV. They will also sell or allow you to buy your PV array. Other companies offer similar plans.
The other approach is to just go out and buy your array. Some states and power companies offer some very tempting rebates and deductions for doing so. One can finance them with home equity if desired. This way, you essentially fix the price of your electricity for the life of your PV array.
When purchasing PV, there are 2 basic philosophies:
1) - buy cheap, low efficiency panels
2) - buy expensive, high efficiency panels
The tradeoff trades labor costs (Installing more panels take more labor) with more expensive panels. I've seen the tradeoff go both ways so I'm not promoting either, just suggesting you don't categorically eliminate options.
Other considerations:
If you happen to have limited roof space, of course, you'll want to bias toward higher efficiency to get more energy. I'll put a shameless plug in for Sunpower as they have the highest efficiency. They got their start making the panels for the solar powered airplane that I worked on. They can recommend installers wherever you are.
Some Lower-efficiency (poly-crystal Si) is better at collecting energy that is not 100% incident to (directly hitting) the panels. This may make them cheaper if you don't have a good south (Northern Hemisphere) exposure.

Well, I looked at my electric bill for the new house, and it is not good. We have not turned the air conditioners on yet, we try not to leave lights on, and we are still using 90kwh/day. We do have a pool pump that is running 24hrs/day, and a hot tub pump, 4 radon fans, 4 furnaces that have been running a lot until recently, etc. When the two giant a/c units start running, I wouldn't be surprised if our usage doubles. With our low Utah rates, our cost was $240 this month. I think $3,000/year or more is in the cards. Spending $20K on solar panels after rebates will be worth it if they can generate enough power. I will get some estimates and report back....

@rdalcantro - so the MS doesn't even make a dent in your electrical usage. Holy cow with that usage I would pay north of $1000 every month in CA. Good news is the MS makes aware of the electrical usage.

Just out of curiosity - why do you have your pool pump running 24/7? Pool pumps consume an awful lot of energy. We have a 5,000sqf home with pool and hot tub and we are using about 50kwhs/day - IF the AC is not running. My pool pump runs for 5 hours/day. Once the 4 AC units kick in our power consumption goes up to 150kwh/day - that's when it's 110F in AZ:-)

All our electricity needs are covered 100% by our PV system - that includes charging our P85.

I am new to pools, but I'm pretty sure I had checked the pump specs, and to turn over the 17,000 gallons, it needs to run all day. However, I will check again. The hot tub only runs a few hours at a time, several times a day. Our house is really big, so there are a lot of lights and other appliances. But, the roof is big too. I just got an email from Solar City that they don't service my area. I will have to ask a few people we know who installed solar panels recently for their homes who they used and get estimates from them.


You can reduce the number of hours you run the pool pump during cooler months, because there's less chance of algae growing in cold water. I run 3-4 hours/day in the winter and 6-7 hours/day in the summer (mostly to get the most out of solar heating), and I have a 20,000 gallon pool with a high-efficiency pump and 2" plumbing pushing the water up 2 stories to the roof.

Get some advice from an expert -- you'll be able to save a *lot* of money if you can reduce the hours that pump is running. My pump is by far the biggest energy draw, even though I have a big house, too.

Pump manufacturers and pool installers always exaggerate the hours a pump has to run. You really can't go by what they tell you. Do you have a pool guy looking after the pool? He/she would be best to consult. If you monitor the pool yourself just check the chems on a regular basis and keep reducing the hours the pool runs until you notice a significant imbalance in the chems.


The pool is indoors, so it is 87 degrees year round. I will try to cut back on the pump hours and see what happens.... Thanks guys!

Rdalcanto- here in Australia it is hot too and we have heaps of pools. If your pump is correctly sized for your pool, 6 hours in summer and 4 in winter should be plenty. As we run on time of use, and our CHEAPEST rate is 12 c ! In winter my 4 hours runs just after midnight on. It is off during the entire day. Regarding sanitizing the pool, mine is not on chlorine at all. I use an ozonator, and a silver copper electrode system. Never have a problem with my indoor pool.( 13 years now.) water is great too. Silver kills bacteria and copper kills algae. Ozone kills everything except humans!! The silver copper system simply plugs into the lid of the skimmer box.
Our expensive rate is 34c. Amongst the dearest electricity in the world!!

@deborn perfect scenario for achieving best value for solar system.

Rates have gone to a 2-tier system here in British Columbia, Canada. Over the base (6.9) we get slammed with 10.4¢/kWh. Shocking! Slightly ..

I called an installer who did a neighbor a couple years ago. Like people have said, it is about $4.50/W. Therefore a 10kW system would be $45,000. But, he would drop the cost to $40,000, or about $27,000 after tax credits. In Utah, I would average 60kW a day, or 1800kw/month (more in the summer, less in winter). At 10cents per kW, that would save $180/month, and would take 12 years to pay for itself. Doesn't seem worth it, but he is coming out tomorrow night to look at the roof.

Interesting. I learned that Rocky Mountain Power in Utah charges differently in the summer. From May-September, after 1,000kW, the rate goes up to 13cent/kW. I need to find out how much the panels make during the summer months, instead of just having an average for the year. Since I will probably be at 3,000kW of usage or more during the summer, the higher summer rates could make it more financially rewarding than I previously thought if the panels put out more juice.

Yep, there's more sunlight in summer. It's a rule.

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