Grid-tie for low battery transfer vs. sell-back mode

MountainpwrMountainpwr Registered Users Posts: 3
I have a power system with battery bank installed at my home in Colorado. For years, I was simply using the power grid to supplement the house power when the batteries got too low. The invertors (lbx mode) would cut over to grid power for a few hours each day based upon the battery voltage and timers.

Recently, I enabled sell-back on the invertors. Now the PV brings the batteries up to a full charge for 30 minutes each morning, then the sell-back starts. I was surprised to see a huge difference with grid-sell-back enabled vs. using the grid input like you would a backup generator to supplement power.

Advantages as I see them:
1. not cycling battery bank each night down to 60% as we were before. Should see the bank last longer.
2. Looks to be much more efficient selling back vs. charging the batteries and discharging the batteries as we have done for the last 19 years.
3. less battery maintenance.
4. Grid power bill has dropped significantly and building up credits each month.
5. have a fully charged battery bank ready for our typical power outage periods.

Wish I had enabled sell-back earlier and recommend those of you that have grid access consider using the grid sell-back option vs. cutting over to the grid on low battery voltage only. The power companies do not make it easy with all their roadblocks and requirements to sell-back power (currently fighting with SRP on my Arizona system), but I think it is worth it in the end.

system
24 Kyocera 240w panels
2 Outback gvfx 3624 invertors
2000 amp hour battery bank

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grid-tie for low battery transfer vs. sell-back mode

    Welcome to the forum.

    Your experience is not surprising. We always say "if you have grid, use it" because it's nearly always cheaper than producing off-grid power however you do that. In this case you have definitely increased system efficiency by making use of the power the panels would put out once the batteries are full if they had someplace to send it. Now they do. :D
  • BB.BB. Super Moderators, Administrators Posts: 29,208 admin
    Re: Grid-tie for low battery transfer vs. sell-back mode

    Do you have an agreement to sell power back to the utility?

    Some utilities (Eastern Europe, Southern California, etc.) are starting to limit connection of "hybrid" AC inverter systems to the grid.

    They "fear" that people will charge the system at night (cheap power) and sell during the day. In theory, that is not solar/RE power (for which people are currently being subsidized) but power shifting of fossil fuels with RE subsidies.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ButchDealButchDeal Solar Expert Posts: 35
    Re: Grid-tie for low battery transfer vs. sell-back mode
    BB. wrote: »
    Do you have an agreement to sell power back to the utility?

    Some utilities (Eastern Europe, Southern California, etc.) are starting to limit connection of "hybrid" AC inverter systems to the grid.

    They "fear" that people will charge the system at night (cheap power) and sell during the day. In theory, that is not solar/RE power (for which people are currently being subsidized) but power shifting of fossil fuels with RE subsidies.

    Solar city received a stay from the judge on the case and has started to install them again. It is kind of a ridiculous claim by the power company as it would be a net lose to any homeowner doing this. The batteries would wear out very quickly with this kind of cycling, there would be inherent energy loss in the conversion of the energy as well.

    The batteries will last much longer in grid tie mode and you will generally have backup power available when the grid does go down.
  • BB.BB. Super Moderators, Administrators Posts: 29,208 admin
    Re: Grid-tie for low battery transfer vs. sell-back mode

    I agree, people will almost never save money by time-shifting power with a hybrid system.--But when has common sense been the answer to many of the regulations we have today...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MountainpwrMountainpwr Registered Users Posts: 3
    Re: Grid-tie for low battery transfer vs. sell-back mode

    I think I just get a credit in Colorado and the credit expires each year to keep us from banking too much. I have heard of people building up millions of watt hours in credit, but I don't think that is very smart. I am not an expert, but my goal is to create enough to meet my needs, no more and no less.

    The great thing about the grid tie/sell-back is your local loads are supported first, then anything left over is sold back. I have outbuildings that are not tied to the invertors (air compressors, welders, lighting,etc.) due to their current draw. With the grid sell-back enabled, the sell-back supports (partially or entirely) the outbuilding loads prior to sending excess power to utility meter 3/4 miles away. Without sell-back enabled for 19 years, I would simply go to float about 1 PM each day and the outbuildings were grid powered full-time and spinning the utility meter.

    Since someone brought up Solar City, I wish Northern Arizona Wind and Sun would create some knowledge base articles and explain the drawbacks to all these leasing programs. I read 85% of the residential PV installed in AZ is leased these days. The numbers to own vs. lease are very different and the leasing companies are misleading the public.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grid-tie for low battery transfer vs. sell-back mode
    Since someone brought up Solar City, I wish Northern Arizona Wind and Sun would create some knowledge base articles and explain the drawbacks to all these leasing programs. I read 85% of the residential PV installed in AZ is leased these days. The numbers to own vs. lease are very different and the leasing companies are misleading the public.

    Although helpful, it would be rather outside of the venue of the forum. And the facts written down today will not necessarily be applicable tomorrow. They just keep changing the rules - and still expect you to know how to play the game. :p
  • ButchDealButchDeal Solar Expert Posts: 35
    Re: Grid-tie for low battery transfer vs. sell-back mode
    I think I just get a credit in Colorado and the credit expires each year to keep us from banking too much. I have heard of people building up millions of watt hours in credit, but I don't think that is very smart. I am not an expert, but my goal is to create enough to meet my needs, no more and no less.

    In CO it is cashed out annually. Here is the net meter page : http://www.dsireusa.org/solar/incentives/incentive.cfm?Incentive_Code=CO26R&re=0&ee=0

    But I do see how you have the option to carry it forward:
    Net metering customers of an IOU may make a one time election in writing on or before the end of the calendar year to have their NEG carried forward from month to month indefinitely. If the customer chooses this option, they will surrender all their kWh credits if and when they terminate service with their utility.

    I would agree not terribly smart. I guess if you expect the cost of power to go up in CO faster than inflation and planned to live in a house longer than the lifetime of the solar installation... a lot of IF's though.
  • bill von novakbill von novak Solar Expert Posts: 843 ✭✭✭✭
    Re: Grid-tie for low battery transfer vs. sell-back mode
    Recently, I enabled sell-back on the invertors. Now the PV brings the batteries up to a full charge for 30 minutes each morning, then the sell-back starts. I was surprised to see a huge difference with grid-sell-back enabled vs. using the grid input like you would a backup generator to supplement power.

    Thanks for the post. One of the more common questions here is "I have grid power but I don't want to go through all the hassle of grid tie; can't I just charge some batteries and use the grid as a backup?" (i.e. LBX mode) Your post is a good example of just one reason it's a bad idea.
  • bill von novakbill von novak Solar Expert Posts: 843 ✭✭✭✭
    Re: Grid-tie for low battery transfer vs. sell-back mode
    BB. wrote: »
    Do you have an agreement to sell power back to the utility?

    Some utilities (Eastern Europe, Southern California, etc.) are starting to limit connection of "hybrid" AC inverter systems to the grid.

    SDG+E did, but fortunately they recently started approving them again. The issue was how to integrate it into net metering, since stored power returned to the grid is (by law) considered differently than solar grid tie power. So they had to ask a judge for a ruling, which took a while.
  • ButchDealButchDeal Solar Expert Posts: 35
    Re: Grid-tie for low battery transfer vs. sell-back mode
    SDG+E did, but fortunately they recently started approving them again. The issue was how to integrate it into net metering, since stored power returned to the grid is (by law) considered differently than solar grid tie power. So they had to ask a judge for a ruling, which took a while.

    My power company though had no problems approving my system (FlexPower One).
    I ran into problems getting registered for SRECs (out of state, through PA SREC market) as they thought I would do this. I argued that :
    One) I was going onto their "estimated production" plan (only available for <= 5kW systems)
    Two) the OutBack Flexpower system is incapable of using battery power to send power to the GRID, it can only use battery power for the emergency panel.

    Surprisingly they did not accept reply One but did accept Two after reviewing the documentation.
  • bill von novakbill von novak Solar Expert Posts: 843 ✭✭✭✭
    Re: Grid-tie for low battery transfer vs. sell-back mode
    ButchDeal wrote: »
    Two) the OutBack Flexpower system is incapable of using battery power to send power to the GRID, it can only use battery power for the emergency panel.

    The Flexpower systems use the GTFX/GVFX inverters, and they are quite capable of "selling" from the battery - you just move the sell voltage setpoint down, and the battery is effectively drained into the grid. (Note that the system does not do this intentionally; it merely pulls power into the grid until the voltage in the battery decreases to the sell point.)
  • ButchDealButchDeal Solar Expert Posts: 35
    Re: Grid-tie for low battery transfer vs. sell-back mode
    The Flexpower systems use the GTFX/GVFX inverters, and they are quite capable of "selling" from the battery - you just move the sell voltage setpoint down, and the battery is effectively drained into the grid. (Note that the system does not do this intentionally; it merely pulls power into the grid until the voltage in the battery decreases to the sell point.)

    Um, sssh, the PA SREC guys might hear you. I sent them the manual etc. AFTER the first claim which they rejected.
    My system is on Estimated production so it really would have NO benefit to me to put battery power on, Further WV does NOT have time of use so power at night costs the same as power at any other time of day. Bureaucracy

    The really difficult part was that they didn't like my one day meter reading. For estimated production they just use PVWatts style estimations and they want an initial reading off the transformer to verify. I filled out all the forms and mentioned that we were having record setting cold weather and snowfall and that the panels were covered with snow. They wrote back that the PVWatts took into account typical weather which should account for this. I am not sure they understand the words "Typical" and "record setting". But after making some minor changes with my TED5000 meter (key was to put both the IN and OUT wires from the transformer through the same CT clamp), I was able to provide them accurate production values.
    http://www.pvoutput.org/list.jsp?id=31117
Sign In or Register to comment.