Net metering in Texas

[ngc1068]

I live east of Houston in an area where Centerpoint is operating the grid. Im investigating installing a small grid tie system, around 2 to 5kW. My average consumption is close to 3000kWhr/month…so this is obviously not about trying to be free of energy bills.

I’ve been researching net metering, it seems net metering is optional in Texas, so if you want it, you must find a retail electrcity provider (REP) who supports solar and sign a contract. It seems several different arrangements are offered, buyback, credits at end of month, etc. However the rate is very high, $0.15 to $0.16/kWhr. It seems like there no way it makes sense to go solar when the rate is 50% higher than the $.085/kWhr I pay with my current REP contract.

Now my questions…

This will probably seem like a stupid question, as its probably answered at a very basic beginner level, but I only find articles that gloss over this question. Its about how the “net” difference is calculated.

A) Is the metering so detailed that each time slice (minutes, seconds) the kW power value feeding into the house is positive they integrate (area under the power curve = energy) and those kWhrs are charged at $0.15, and on the flipside, any negative instantaneous power readings are integrated and this negative kWhr total for the month is bought back at their “buy” back rate (much lower than $0.015)..and the difference is my bill?

B) Or is it that they just go off the meter kWhr reading for the month, and as long as you used more than you generated for the whole month (even if there were times the meter counted in reverse) I will be billed only for the “net” consumption (difference between consumed and generated for that month?

For the small system Im looking at, Im sure my monthly consumption will always exceed production. My thought was, with the small system, I would rather get permitted with Centerpoint for the system, and once inspected and the system is emergized, I guess I’d like to keep my regular REP, so that I could pay the $0.085/kwhr consumed, and not have any buy backs or credits. I imagine the REP or Centerpoint dont like this option, and force customers to switch to a net metering contract or cut you off, disconnect you or make u disconnect your solar setup, and therefore u have to go with A) …Am I unfortunately right?

Anyway, i know that was a long post, hope my question makes sense, and I appologize if it has been answered eleewhere, I’ve looked and searched for a few days and havent found a detailed explanation.

Thanks in advance for your guidance!

 

[svetz]

Welcome to the forums!

... Houston ... investigating ... grid tie system, around 2 to 5kW

Assuming south facing with a tilt angle of 60° for optimum year-round performance. Your insolation varies from 3.1 in December to 5.8 in Jul.
Let's say the insolation averages out to 4.7 for the year, so a 5 kW array would generate on average 4.7 x 5 = 23.5 kWh/d

. My average consumption is close to 3000kWhr/month…so this is obviously not about trying to be free of energy bills.

3000 / 31 = 96 kWh/d.

So, in a day you'll never make more than you can use.

Does a net-metering agreement make a difference? Let's say you use 30% of the power at night and 70% during the day, so on average during the day 96 x .7 / 12 = 5.6 kW (note the "hour" is missing, we've switched to power rather than energy).

5.6 kW consumed > 5 kW maximum produced from a 5 kW array at noon. So, if the power consumption were continuous a net-metering agreement would not bring any benefit as the consumption would always be greater than the production.

But most things are constantly turning on/off, if your using 8 kW for 30 min and 2kW continuous, then for 30 min you might be throwing away 5 kW - 2 kW = 3 kW of power. But, you might also be able to arrange things so you're using the most power when the most solar is available (e.g., run the pool pump from 10 to 3). If you can do that, you have no need for a net-metering agreement, you could just set the system for zero export.

However the rate is very high, $0.15 to $0.16/kWhr. It seems like there no way it makes sense to go solar when the rate is 50% higher than the $.085/kWhr I pay with my current REP contract.

1 kW of Solar at an average insolation of 4.7 and a 20-year life span at 90% efficiency provides 1x4.7x365x20 = 30,000 kWh, at $0.085/kWh that's a value of $2624 ($2.62/W). Forum members report DIY at under $1/W installed, so that's ~8 year payback at just 8.5¢/kWh. But, it can be hard in the U.S. to find a "professionally" installed system at under $2.50/W.

This will probably seem like a stupid question, as its probably answered at a very basic beginner level, but I only find articles that gloss over this question. Its about how the “net” difference is calculated.

It can be different everywhere. You'd have to ask your REP. In Florida, the rate is generally the same so it makes no difference. In California they have TOU tariffs so horrendous even batteries might make sense to sell power back to the utility from 4 to 7.

I guess I’d like to keep my regular REP, so that I could pay the $0.085/kwhr consumed, and not have any buy backs or credits.

Probably depends on local laws. But you're probably fine with a system that supports a Zero-Export feature (different vendors may call it something different, but basically it's programming the system to not export power). You can do this with some string inverters (e.g., Sol-Ark) or some microinverters (e.g., Enphase with a zero-export profile). What might worry the local building inspector the most is the anti-islanding (tech to ensure no back-feeding), but all grid-tied systems should have that built-in.

If you don't get a net-metering agreement, you might want to optimize panel tilt for when you use the most energy. In my case, I'm not optimized due to hurricane-force winds (mine are flat to reduce wind forces). It's a bit odd for me really, you'd think being flat in summer would be better as that is when I use the most energy, but it's also the cloudiest months. You can find out more about weather impacts and the best tilt for your system location with a tool like NREL's SAM.

Hope that helps some!

 

[ngc1068]

Hi, Thanks so much for all the information. Its all extremely helpful.

Wow, $1/kW installed. Would you advise where to look for purchasing hardware to support a small system at those prices? A friend with a 10kW system informed here, the utility is calculating on a 15min period. With that in mind, even for a small kW system, I’m pretty sure if I dont set the system for zero-export, there will be periods (mild weather, when nobody is home) where the system is producing.

If I can source the hardware, I think I can handle the install myself. I think microinverter setups are attractive from installation simplicity standpoint.

 

[svetz]

Hi, Thanks so much for all the information. Its all extremely helpful.

Wow, $1/kW installed. Would you advise where to look for purchasing hardware to support a small system at those prices?

Nowhere. Sorry to get your hopes up like that. From the earlier post it's a ~$1 per watt for U.S. DIY, not kilowatt. ; -)
Everybody's mileage varies a lot on that.

@upnorthandpersonal talks about a system on this thread, a 10kW solar system with 28kWh LiFePO4 battery, inverter, BMS and charge controllers for under $12k U.S. dollars.

Example:
$3000 - 16x Q Cells 310 Watt Mono Duo Cell (4960 watts, 20 amps@240V)
$1400 - 5 kW string inverter
====
$4400 (not including taxes, shipping, wiring, permits, mounting, etc.)

A friend with a 10kW system informed here, the utility is calculating on a 15min period. With that in mind, even for a small kW system, I’m pretty sure if I dont set the system for zero-export, there will be periods (mild weather, when nobody is home) where the system is producing.

The image to the right shows how quickly solar power falls off. A 5 kW array will only produce a maximum of 5 kW around solar noon for a few weeks out of the year. If you can you reset some timers to turn on your devices around solar noon (e.g., hot water tank timers, pool pumps) you might be able to finagle it where you're only throwing a little bit away. But you're right being grid-tied with a good net-metering agreement maximizes the return.

I think microinverter setups are attractive from installation simplicity standpoint.

Definitely! Microinverters are the cat's meow for this, typically with long warranties, high reliability, great efficiency, no single point of failure, reduced BOS costs, per panel diagnostics. I have them and love them. The main gripe you see about them is the added cost (e.g., 16 IQ7s for the example above increase the costs about $800 on just the inverter (although to be fair, they typically also reduce the BOS costs)) and adding a battery later can be expensive.

 

[Texican]

I have a 10kw gridtie in Texas in oncor area. I give back for free probably 15kwh per day except during summer (5 months long)
because as you correctly said, any REPs who allow net metering charge way more than regular REPs. actually saves money to give it away. at 9-10 cents kwh batteries still dont pencil out

 

[Herb]

Well, net metering have changed a lot since I got my agreement 6 years ago. I live in TX but not in Houston. My monthly bill will never be more than $32.00 for 25 years around 20 years left on it ATM. My monthly bill ranges from -$10.00 to around $22.00 during hot summers when the 3 tons Central AC is on 24/7. My last bill was for -$6.00, my consumption is very low, the way I accomplished this, converted the house to a "smart" house, all is on the network and controlled by smart devices. Since there isn't much you can do about the AC consumption, except getting rid of it and going with split units, that is basically what I did. I installed one upstairs and one downstairs, both run 24/7 during summer time and keeps the central unit from working all the time. Average daily consumption averages from 6kW-12kW! For a 2 story house with central air that is very low! Even though I'm grid tied (Enphase) I run a 3k Growatt with 12 kW LiPo4 battery bank which support the 2 mini split units w/o any issues. The bank provides up to 4 days of 24/7 energy to the units, at night it cools down so they just stand by, but the Growatt is not big enough for the central unit to run, for everything else such as lights, small appliances and such I use an older 5kW LiOn battery bank and that provides energy for around 2 days, all this w/o using solar to charge the batteries, since I have a smaller PV system just for the emergency battery banks I just flip flop from one bank to the other while one charges up the other one is working, I normally deplete the batteries to about 20% before charging them to 80%, and try to avoid small charging cycles, rinse and repeat. The issue with the Grid tied systems is that you will loose it if the power goes out, yes you can purchase a battery backup system for the Enphase system but their prices are out of my reach and with what I got is enough to sustain us for a few days/weeks/months as far as energy is concern in case of an emergency.

 

[Azeem]

@Herb. That's very efficient, I am surprised that even with the load of 2 ACs you are consuming 12kw per day! Do you have heat pump installed supporting mini splits or inverter ACs?

 

[Herb]

@Herb. That's very efficient, I am surprised that even with the load of 2 ACs you are consuming 12kw per day! Do you have heat pump installed supporting mini splits or inverter ACs?

The AC 12K BTU units are split units, inverters I think they call them. They are not heat pumps, they can de-humidify there is a setting for that, but I don't think they are heat pumps. I only use them to supplement the central Air so that it don't run a lot and they do a good job specially during the hot summers, we had a few days w/o electric power here a few months ago and the temperature outside was 105f, thank God we had this backup system. The Enphase PV system is grid tied and generates most of the power for the day, but during summer time the panels output decreases due to heat, that and the high temperatures can mess up your day if the power goes out you will need emergency power to get going. The stove and the water heater are natural gas, the only thing that runs on electricity besides the central air and (heat is natural gas also but motor runs on electricity) is the drier, which is a Samsung about 9-10 years old and that one is an energy hog. I checked on a gas drier but they would have to do some modifications to install it and is not cheap. But we don't use it a lot. All outdoor lighting is solar powered, all interior lighting is LED on a Zigbee network, it took me a while to get everything together but slowly but surely got it done, don't regret it at all and I can sleep better at night.

 

[Azeem]

That's very efficient, with two AC's non inverter I consume 40 units; which is my PV generation for a day (20 from PV and 20 at night from grid) on 24 hour basis.