# grid tie size vs production

Solar Expert Posts: 192 ✭✭
(First can a moderator point me to a discussion on micro vs central inverters? I searched and wasn't successful. thanks)

Now to my topic. I am new to the grid tie side of solar. I am planning on putting in (or having installed) a grid tie system on the house. I've been reading and I can't get a grip on how you figure out from the size of the system (say 5kw) how much you'll get (or should produce) in a day? I get it on the off-grid side of things, with batteries and all but maybe someone could help me here. I saw a previous poster just put a 6kw system in and they were talking about getting 30 kwh a day. How do I figure out what I should be getting from a 5kw system per day? For that matter how do I figure how big of a system I need/should get? Thanks for the help.

Tim

Re: grid tie size vs production

The above is just this search string (with the site:wind-sun.com tag to limit the search to this forum):
• enphase vs central inverter site:wind-sun.com
Regarding how much a fixed array may generate. Using PV Watts for Albuquerque NM, 5kW array, 0.77 default derating, fixed array tilted to latitude:
"Station Identification"
"City:","Albuquerque"
"State:","New_Mexico"
"Lat (deg N):", 35.05
"Long (deg W):", 106.62
"Elev (m): ", 1619
"PV System Specifications"
"DC Rating:"," 5.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 35.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (\$)"
1, 5.33, 631, 54.90
2, 6.06, 632, 54.98
3, 6.44, 742, 64.55
4, 7.16, 764, 66.47
5, 7.40, 792, 68.90
6, 7.10, 716, 62.29
7, 7.13, 740, 64.38
8, 7.02, 736, 64.03
9, 6.71, 686, 59.68
10, 6.55, 723, 62.90
11, 5.73, 635, 55.24
12, 5.14, 610, 53.07
"Year", 6.48, 8407, 731.41

You will get around 600-700+ kWHrs per month, or:
• 610 kWH per month / 30 days per month = 20.3 kWH per day minimum December
• 700 kWH per month / 30 days = 23 kWH per day typical ~7 months a year
If you know how many kWH per month you will need and particular month, you can take the above numbers (like 610 kWH per month per 5 kW array) and figure out how many kW array you would need:
• 1,200 kWH per month December * 5kW array / 610 kWH per month = 9.84 kW array to meet December needs.
If you have Time of Use metering and Summer / Winter pricing differences, it gets more complex (for example, from noon to 6pm summer weekdays, I get pay 3x as much for the power I produce and then pay the reduced price for off-peak usage (cooking, washing clothes, lighting, etc.).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 192 ✭✭
Re: grid tie size vs production
BB. wrote: »

The above is just this search string (with the site:wind-sun.com tag to limit the search to this forum):
• enphase vs central inverter site:wind-sun.com
Regarding how much a fixed array may generate. Using PV Watts for Albuquerque NM, 5kW array, 0.77 default derating, fixed array tilted to latitude:

You will get around 600-700+ kWHrs per month, or:
• 610 kWH per month / 30 days per month = 20.3 kWH per day minimum December
• 700 kWH per month / 30 days = 23 kWH per day typical ~7 months a year
If you know how many kWH per month you will need and particular month, you can take the above numbers (like 610 kWH per month per 5 kW array) and figure out how many kW array you would need:
• 1,200 kWH per month December * 5kW array / 610 kWH per month = 9.84 kW array to meet December needs.
If you have Time of Use metering and Summer / Winter pricing differences, it gets more complex (for example, from noon to 6pm summer weekdays, I get pay 3x as much for the power I produce and then pay the reduced price for off-peak usage (cooking, washing clothes, lighting, etc.).

-Bill

Thanks as usual Bill. I didn't think about using the PVwatts.

So in essence if I'm using 1200kwh in a month, then a 5kw system will get me half way there (pay for half my usage) on average. If I use 600kwh in a month then a 5kw system will help me break even. That is what I wanted to know. I hope I interpreted your figures correctly.
Re: grid tie size vs production

Yep... Note that the figures are for December... Other months are a bit better.

Also, really depends on how your power company does their billing for GT connected systems. In California, we have to take Time Of Use billing with Summer/Winter Tiered rates).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,973 ✭✭✭
Re: grid tie size vs production
bobdog wrote: »
Thanks as usual Bill. I didn't think about using the PVwatts.

So in essence if I'm using 1200kwh in a month, then a 5kw system will get me half way there (pay for half my usage) on average. If I use 600kwh in a month then a 5kw system will help me break even. That is what I wanted to know. I hope I interpreted your figures correctly.

Neither your usage nor your production from PV is the same month to month. You have to consider how your utility deals with excess production/net metering when sizing a GT system. If they let you carry a kWh balance forward every month, you can cost effectively build a bigger system than you can if they zero/pay it out every month.
• Solar Expert Posts: 192 ✭✭
Re: grid tie size vs production
ggunn wrote: »
Neither your usage nor your production from PV is the same month to month. You have to consider how your utility deals with excess production/net metering when sizing a GT system. If they let you carry a kWh balance forward every month, you can cost effectively build a bigger system than you can if they zero/pay it out every month.

I'm attached to El Paso Electric in NM. There are 2 programs as best I can figure. One is net metering and the other is called REC (renewable energy credit). The net metering pays squat as best i can tell and the rec pays for 12 years 0.12 KWH gnerated after my use. So REC seems to be the way to go. I'm not sure why I would go with net metering frankly.
Re: grid tie size vs production

1 year net metering can actually be a great deal... The utility pays you "retail" for energy you export to grid, and you buy it back at "retail" when you need it later (free AC Battery Bank with 1 year storage capacity).

Ideal for GT systems that generate excess power in some seasons, and not enough in others (for me, generate during summer, use during winter). With Time of Use rates, you can sell at ~\$0.30 per kWH and buy off peak at \$0.09 per kWH--Makes my array look 3x larger during summer afternoons.

You really need to understand the different plans to make a good decision about what is best for you.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 2,389 ✭✭✭✭
Re: grid tie size vs production
bobdog wrote: »
I'm attached to El Paso Electric in NM. There are 2 programs as best I can figure. One is net metering and the other is called REC (renewable energy credit). The net metering pays squat as best i can tell and the rec pays for 12 years 0.12 KWH gnerated after my use. So REC seems to be the way to go. I'm not sure why I would go with net metering frankly.

Well it depends on how the net metering works and how they carry your kWh forward and your rates per kWh.

Example: your rates are \$0.15 per kWh, you have a system that excess generates Jan - May & Oct - Dec that builds a bank for use in Summer for AC usage. It saves you any bill from the utility year round. Requirements are how the utility carries your excess, whether they cash out your excess at some point.
• Solar Expert Posts: 192 ✭✭
Re: grid tie size vs production

Maybe I have this wrong, but is TOU and net metering the same?
• Solar Expert Posts: 414 ✭✭✭
Re: grid tie size vs production

In PG& E Northern CA they allowed me to stay keep my Tiered Summer/Winter but full bi-directional digital (NET) meter. Was not forced to go on TOU metering. I could of gone TOU but then I couldn't come back to just teired rates. Wife likes the A\/C in summer and hard to control for drying clolth during the day. Like scooking and eating before 07:00 PM at night?? Online in 08/10/2010 just completed first year doing true up this month. Purchased total of 1,633 kWh from them. My 4600W with 3.8GT Zantrex inverter produced 7,104 kWh for the 12 month period. CEC rating was 6368 kWh predicted by PV Watts 77% factor. My system is 218° True and 18° Tilt fixed mount. Planned next phase of system will be rotated back 90° to 128° 20° Tilt same size depending on panel pallet (20ea) prices at the time. Will go to 240W panels if reasonable price to gain output in winter and summer. System will limit in spring & fall with better angles and cooler temps.

Check with your utility to see which metering scheme they will allow as mention to help determine planning your system size.
GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
Re: grid tie size vs production

No:
• Time of Use: 2-3 periods of day with different charges. Usually seasonal differences in billing too
• Tiered Pricing: use 0-300 kWH per month, \$0.12 per kWH. Use >900 kWH per month, \$0.52 per kWH for any usage >900 kWH per month
• Net Metering... There is 1 Month N.M. and 2 Year N.M. (that I have seen). Meter is "bi-directional"... Goes forwards or backwards depending on power generated vs power used. At end of month you pay (or bank) the difference (banking for 1 Year N.M. plans).
You can mix/match any of the above together... Billing gets really complex and even the Utility/PUC will disagree on how to do the tiered/TOU/Net Metered billing. Tiered pricing seems to be more of a East/West cost sort of thing (forced conservation and transfer of payments from "rich" to "poor").

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,973 ✭✭✭
Re: grid tie size vs production
bobdog wrote: »
Maybe I have this wrong, but is TOU and net metering the same?
They are independent of each other. If you have TOU (time of use) billing, that means that they charge you a different amount per kWh at different times of day, usually more (can be a lot more) in the afternoon/evening than at other times. Net metering has to do with generating systems that feed power back into the grid and how the utility deals with that power.
• Solar Expert Posts: 9,572 ✭✭✭✭✭
Re: grid tie size vs production
bobdog wrote: »
Maybe I have this wrong, but is TOU and net metering the same?

No ! Sometimes ToU is tied to net metering, usually to the determient of solar, peak period runs 1pm - 10 pm. and you suffer buying power at high rates after sunset !

Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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• Solar Expert Posts: 2,389 ✭✭✭✭
Re: grid tie size vs production

My TOU is 9am to 9pm Mon - Fri @ about \$0.18 a kWh and about \$0.06 the rest of the time, with a seasonal variation. And my net metering is based on those 2 TOU time frames, I am not so lucky to get to buy back at low rates for power generated @ high rates. I bank mostly high \$\$\$ kWh for summer AC usage, and end up buying some off peak power anyway.

There is another TOU plan that run noon to 7PM Mon - Fri @ about \$0.26 a kWh peak rate and have been considering changing. Down side is I can't return to the old plan if it doesn't work as well. The challenge will be to train the "other" users of power here to stay away from afternoon usage.