Grid Parallel Inverter

2 Posts
Aug 15, 2008 03:47 pm
Grid Parallel Inverter

I am wanting to install a 2 - 3kW system in my home. I live in Mississippi, and currently the power company does not allow for net metering. Actually when I spoke to them they had no idea what I was asking for. I did find information on it from their website which states to tie to the grid I must purchase another meter for $18/month, and hook the PV system to this. That's not exactly what I had in mind.

There is no such thing as a local certified PV installer in our area. So, I will be relying on my own research, and the help of an electrician. I was curious about the following set up.  Lets assume I purchased a GVFX3648, an MX60, mate, Batteries, and all the various assorted components/panels that go with it,and hook up precisely like the diagram Outback Provides for a grid connected + battery backup system. I asume i would have to turn off sell mode in the inverter. When the grid power is up and running ( and it is very reliable )does the power from the PV array actually get used on my household loads, or does it simply maintain the charge on the batteries?

Obviously since i can't put power back on the grid, I would like to use everything the PV Array makes.

Thanks,
cd
 
351 Posts
Aug 18, 2008 01:58 pm
Re: Grid Parallel Inverter

Power from the PV array would be used to charge the batteries (if low) and to supply power to the “backed up” loads. (Loads that have been transferred to a new distribution panel off of the inverter) If your power usage on those circuits is low during peak sun hours, you will end up with very little power production from your PV.

I would suggest you rethink your position and type of inverter. Buying a fully integrated grid tie inverter with battery backup, so that you do NOT sell power to a “Reliable grid”, doesn’t make much sense to me.

You can build a solar system and transfer certain loads to it. Operate it as a stand alone system with grid backup.  That would allow you to use all of the solar that you generate.

At the other end of the spectrum, is a grid inter tie without backup. If you do not buy a large battery bank, you could invest that money into more PV.  With a larger array, you could sell more power and possibly justify the additional meter and the monthly fee.  As long as you are generating the solar and making income from it, does it really matter to you, who is consuming it ?

I looked at 
http://www.solarpowerrocks.com/mississippi/
They indicate that you may have a five cent spread between your purchase price and the solar selling price.
 
2 Posts
Aug 18, 2008 03:31 pm
Re: Grid Parallel Inverter

Thanks for the reply , Ken. Building a Stand alone system with grid back up is what I had in mind.

That 5 cent spread is only from TVA, and I am on Mississippi Power. The spread on that is not so good. We pay 11 to 12 cents, and they will buy back power at between 4 and 8 cents. http://www.mississippipower.com/pricing/pdf/CSPP-3.pdf

What Inverter would you recommend for a stand alone application with grid powered back up? I would prefer one that is grid-tie capable just in case MS Power decides that it would be OK to allow net metering sometime in the near future.

Again, thanks for the info.

-cd
 
351 Posts
Aug 19, 2008 02:34 pm
Re: Grid Parallel Inverter

I do not make "early" recommendations of equipment. My experience is that people who start with or concentrate on inverters and associated equipment, end up spending way too much money on bells and whistles. They then end up skimping on batteries and PV panels. 

If you want to build a good reliable system that really does the job, start with a load inventory. It will drive your design. It will show you where you can conserve energy. Conservation is always cheaper than generation.  It will also help identify which loads must be put on the system for the few outages that you have.  It will help make the decisions about which loads can be easily transferred to the system, and how much re-wiring will be required.

The loads (device and monthly/weekly/daily draw) that you decide to put on the system, plus the reserve factor (e.g. you want 4 days) will drive the sizing of your battery bank and PV array.

Often, this is the point where the budget realities start kicking in. You may re-evaluate how many loads you are going to put on it. So it may take a couple of iterations to arrive at a good sizing.

Once you have a good sizing on the battery bank and PV array, you start looking more closely at the equipment. This is what you need to do the job for this size system. Finally, you consider future expansion and make reasonable allowances for it.

Ken
 
Aug 23, 2008 05:37 am
Re: Grid Parallel Inverter

Assuming 4 hours of equivalent full rated output of a 3,000 watt PV array, per sunny day, during winter months,
12,000 watt hrs.
Assuming a 48 vdc nominal battery bank sized to average a 20% depth of discharge, based on PV array output not an assumed load,
1,250 amp hours at 48 volts nominal.
You might have 250 amp hours at 48 volts nominal available each day/night. It all depends on how badly you want to stay in that top 20% of battery capcity and of course the weather. But then, there is always the grid right there as a safety net.
250 amp hours at 48 vdc nominal would translate, roughly, into 5 - 100 watt bulbs burning for 24 hours.

At $0.15 per kWh., 12 kWh. would cost about $1.80, times 365 days would be $657.00 a year, thats 1 PV module not installed. 25 years and the PV modules alone will have payed for themselves.

One can think of electricity as a necessity for life, like the air we breath, or one can feel that electricity is a luxury for indulgence, like one of them southern fried tenderloins with egg on a cheese bisquit and a cold Miller beer.
 

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