Grid-tie system sizing

JoshK

The members here seem have a lot of experience with system sizing and are eager to help, so could I get a recommendation please? I am interested in a grid-tie configuration. Last July we used 845kwh. And last January we used 508kwh. I am after a good ROI in the long run, not just a feel-good system. Thanks for the spec. help. Please don't get too caught up in my electric costs as it may change, but FYI it is $0.13 per KWH now.

solar_dave

The first step is to learn how your utility handles the power pushed back to them. Is it a kWh credit? a Dollar credit? ....
Then are there multiple billing plans to choose from? TOU etc.

JoshK

I called them Friday and the Customer Service person said she knew nothing about the programs available in "that state". Then she gave me another number to call, but nobody answered. I will try again Monday.

JoshK

Ug. Ok, so there is no "Net Metering" in South Dakota. So if I produce 20KW (@4 cents) per day and use 20KW (@13 cents) per day it doesn't even come close to cancelling. So if my bill would normally be $84 they subtract $24 and still bill me $60.

solar_dave

JoshK wrote: »

Ug. Ok, so there is no "Net Metering" in South Dakota. So if I produce 20KW (@4 cents) per day and use 20KW (@13 cents) per day it doesn't even come close to cancelling. So if my bill would normally be $84 they subtract $24 and still bill me $60.

That make your pay back really tough. The real problem then is if you want to clip them total by going off grid the cost go up a bunch. Batteries and the associated equipment will about double your outlay. If you can move your consumption to mostly daytime usage then the grid tie might still be OK but it is considered opportunity usage, as the power is generated you consume it all inside the meter.

BB.

Is it one meter or two meters?

If one meter, you are first using your GT Generated power, any extra you buy at $0.13; any extra power you sell at $0.04 per kWH.

If you have two meters (one on GT inverter, second to your main panel)... Then you buy "everything" at $0.13 per kWH and sell "everything" at $0.04 per kWH.

Different utilities, different plans. At least with one meter, your GT inverter output goes to power your local loads first.

-Bill

solar_dave

Yep Bill has that right you need to know how they measure it.

JoshK

He said it's a special meter with dual registers. Meaning the same as what Bill calls two meters.

JoshK

My only chance seems to be move my lighting and refrigerator and router to off-grid.

JoshK

I'm thinking if I ever build a house I would have two service panels, one for all 220v loads and one for 110v. An off-grid system should be feasible then. The grid would still handle the large 220v (intermittent) loads like the A/C and clothes dryer. The off-grid would service the 110v panel.

BB.

Actually, two registers is probably what I would call a single meter--But finer grain.

One true meter--That is a net meter for monthly.

One meter with two registers--Basically keeping track "exactly". If your net power is drawing power, they charge you the $0.13 per kWH. If you generate power at any instant in time, they pay you $0.04 per kWH.

The one meter/net billing is nice... Because you can generate excess power during sunny periods and draw from the credit during night/bad weather. You "true up" at the end of the month (or in my case, at the end of the yearly billing cycle).

With two registers, you want to use the power when you generate (i.e., run the A/C, electric heating/cooking/hot water when the sun is shining--So that you use most of the power right then, and keep your +/- power from/to grid near zero).

There are many different plans (some utilities offer different billing options to their customers). You have to see what works best for your household. Monthly/Yearly net metering is generally much better for the customer. Two Registers is generally better for the utility... Yearly net metering is usually the best--I generate excess credits in summer, and use them night/non-summer time. For those with high A/C bills, the non-summer credits help pay for summer time use.

However, Two Registers does not tell the whole story either. For me, I have two "net reading" registers because I have "time of use"--Peak noon-6pm for one week days. And "off peak" for other times (actually, it is more confusing than that in real life).

-Bill

JoshK

Yea I guess you're right.

I'm SOL, aren't I?

BB.

You need to find out the details to know for sure.
We are just guessing right now.

-Bill

solarix

Estimating your total yearly usage at 8000kWhr (which is below the 10,000 around here in out hot A/C climate), look up from NREL's PVWATTS the performance factor for your area, divide it into the 8000kWHr to get your ideal array size. ( in my area we can get upwards of 1660kWHr/yr per kW of array) This however assumes net metering which apparently you don't - so a good guess is that you use about half your power during times when the solar system is producing, so cut the system size in half to minimize the amount of power wasted by exporting to the utility. Not a great bargain, but better than going off-grid and never reaching payback.

JoshK

Bill, the guy they had me call flat out admitted he didn't know how the dual-register meter worked. He has no idea if the registers are both stopped, or both spinning, if your consumption equals your production. He also said there is nobody that can answer that and he's sorry. He did say he has a friend he might be able to ask.

Solarix, I assume you mean this number?

DC to AC Derate Factor:
0.770


8000 / 0.77 = 10389
Right now I am thinking I should build my proposed GC-2 backup system to correct size for taking my refrigerator and Internet modem/router to an off-grid solution. They are my persistent loads. This will give me a daily ROI on my backup system even when the grid is working.

JoshK

Alright, the guy called back and said the registers both stand still if your consumption and production are equal.

BB.

Yea--That is not a great billing plan...

-Bill