PV1 wrote:For the discharge test, how about using one of these on the 12 volt battery while in READY? It'd provide a steady load and wouldn't be as wasteful as running a space heater for hours. I have one and use it with a couple of 12 volt panels as overcharge protection (you know, 'cause a real charge controller is too simple ).
http://www.ebay.com/itm/400-Watt-Power- ... 0444131961
I am starting this new thread to discuss a thought I had after reading PV1's post in the "Batteries & Battery Management" forum..
With my next electric billing cycle, I will be switching to my utility's Residential Real-Time Pricing program, which bills your electricity usage by the hour at that hour's market rate. I am making this switch from the fixed-rate program I am currently on through an aggregate retail electricity supplier. The purpose of my switch is to save money on my electric bill by shifting high-load operations in my home to the overnight hours where the hourly market rates are significantly cheaper. My current fixed rate is roughly $0.08/kWh. The current overnight market rate is around $0.02/kWh or less. In fact, frequently the overnight market rate can go negative due to excess power generation. So, theoretically, it is possible for me to get paid to charge my Meepster, run my dishwasher, or dry my bedding/towels overnight! Winning!
Anyway, PV1's post brought up an interesting thought. Something I want to do in the future when I have a little extra cash is to install solar PVs on my house specifically to charge my Meepster. I want to someday make my car truly zero-emissions by powering it via the Sun! Since I am only able to charge my car at night, I will not be able to directly charge my car via solar. So, this system will need to be tied into the grid. I will generate power during the daylight hours feeding it into the grid and receiving a credit for the power generated at the then-current hourly market rate (which is often near $0.14/kWh). I will then charge my Meepster at night from the grid at the then-current hourly market rate (which is often $0.02/kWh or even sometimes negative!).
This got me thinking. Would it hypothetically be possible to use a battery system to charge and store power at night and use a grid-tie inverter such as the one PV1 linked to in his post (or one with a larger output) to feed the stored power back into the system during the day? In doing so, I could charge the battery system at night at $0.02/kWh and then feed it back into the grid during the day and earn a $.14/kWh credit. Am I missing something here? Would this not be a way of earning money (or at least significantly reducing my monthly electric bill) simply by selling my stored power back to my utility at a higher rate?
In preparing for future solar power installation, suppose I was willing to pony up the funds for a 10kW Tesla Powerwall now (estimated cost $3,500 plus installation) or any other battery power storage system. Between now and whenever I finally getting around to installing the solar PV system, could I theoretically charge the Powerwall overnight via my electric utility and then pump it back into the system during the day? A 10kW system would cost roughly $0.20 to charge overnight (10kW x $0.02/kWh). I could then earn a credit of $1.40 by pumping the stored energy back into the system during the peak demand periods during the day (10kW x $0.14/kWh credit). I could earn a net of $1.20 per day. That would be a $36 credit on my electric bill every month or $438 per year.
Okay, help me out. What am I missing here? Other than the fact that it would take 8+ years for this fancy scheme to fully recover the cost of the Tesla Powerwall battery system? But then, isn't that roughly the same amount of time for the ROI on a solar PV system? With a big enough battery storage system, I'd be able to guarantee myself a fixed rate of $0.02/kWh all day every day. According to my TED system, my average daily electric use is about 28kW. I could get 3 of the 10kW Tesla Powerwalls and be able to store up enough energy overnight to accommodate my entire day's worth of electric usage. Once I finally install my solar PV system, then I can store the free energy generated from the Sun during the day. Granted, my solar system will NOT generate enough power during the day for my daily electric usage. However, I think I should be able to make up any difference by topping off the battery storage system overnight when the electric rates are at their lowest, right?