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  • #31
    Originally posted by lkruper View Post
    My tiers are 0.15, 0.19, 0.26 and 0.31. When we get +100 temps for a month or more in a row, I make it into tier 4.
    So what. Battery power will cost you 60-cents and higher per Kwh in the USA.

    Taking the case to Hawaii where electricity cost more does not wash either. Yep electricity cost twice as much as the mainland in Hawaii, so do the batteries, panels, controllers, racking, wiring and everything else. Same piss poor scale you cannot win.
    MSEE, PE

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    • #32
      Originally posted by Sunking View Post
      So what. Battery power will cost you 60-cents and higher per Kwh in the USA.

      Taking the case to Hawaii where electricity cost more does not wash either. Yep electricity cost twice as much as the mainland in Hawaii, so do the batteries, panels, controllers, racking, wiring and everything else. Same piss poor scale you cannot win.
      I am still checking my calculations, but:

      Trojan T-105 get 1200 cycles @ 50% DOD
      Cost of 2 batteries = $280
      At 50% DOD the bank gives 1.35 kWh / day
      1.35 kWh X 1200 cycles = 1620 kWh lifetime
      $280 / 1620 kWh = 17 cents / kWh

      Did I do that right?
      Last edited by lkruper; 09-25-2015, 12:56 AM. Reason: Fixed units ... kW to kWh

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      • #33
        Originally posted by lkruper View Post
        I am still checking my calculations, but:

        Trojan T-105 get 1200 cycles @ 50% DOD
        Cost of 2 batteries = $280
        At 50% DOD the bank gives 1.35 kW / day
        1.35 kW X 1200 cycles = 1620 kW lifetime
        $280 / 1620 kW = 17 cents / kW

        Did I do that right?
        Are you assuming Peukert losses. You loads can never exceed 130 watts or 11 amps of battery current at any time. That same 225 AH battery is 140 AH at C/6.
        MSEE, PE

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        • #34
          Originally posted by Sunking View Post
          Are you assuming Peukert losses. You loads can never exceed 130 watts or 11 amps of battery current at any time. That same 225 AH battery is 140 AH at C/6.
          The calculation was done to simply show the cost per kWh of 17 cents. I can charge at night for 1 cent per kWh and discharge during peak hours when it would cost me 46 cents per kWh.
          Last edited by lkruper; 09-25-2015, 12:19 PM. Reason: Mistake on cycles for L16RE-2V removed

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          • #35
            Originally posted by lkruper View Post
            It's NOT something I am terribly interested in doing, but I calculate that 2 Trojan 105 batteries cost $280 and if they give 1200 cycles will produce 1620 kW in their lifetime. That comes out to 0.17 per kW. I am still checking my calculations, but at my baseline I can get a kW at 0.01 and use it during the day when it would cost me 0.46 or at 0.30. Still checking my calcs....
            You are going down a useful road, but you are being careless with your units, which will make the results less useful to others.
            You are trying to calculate the cost per kWh not per kW. If you make that change pretty much globally in all your posts on the subject it will help.

            Also, you need to make sure that those two Trojan batteries can deliver the power you need (in kW) and the energy you need daily (in kWH) before the comparison is complete.
            What depth of discharge are you using per cycle? And are those 6V or 12V batteries?
            Ah, I see you did add that information.
            You used 50% DOD. I have a hard time believing that you will still get 1200 cycles at that DOD in real world use, but Trojan should be pretty trustworthy and there has been time for them to run those actual tests rather than extrapolating the results.

            50% DOD for loads over, say, a 5 hour period, is going to discharge the battery at an average rate consistent with C[SUB]10[/SUB] rather than C[SUB]20[/SUB], so Sunking's point is well taken.

            And you have neglected to allow for the fact that you must put back more energy into the batteries than you take out (and are paying for, even if at the night rates) and are limited in available solar hours, so recharging from 50% DOD in one day is not going to happen either with solar, but could happen with storage.

            If your super off peak rate is .11 per kWh and you have to overbuy by 120%, and your battery storage cost is .17 per kWh then the sell back rate would have to be .11 x 1.2 + .17 = 30.2 (or higher) to make it worthwhile.
            Last edited by inetdog; 09-25-2015, 12:37 AM.
            SunnyBoy 3000 US, 18 BP Solar 175B panels.

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            • #36
              Originally posted by inetdog View Post
              You are going down a useful road, but you are being careless with your units, which will make the results less useful to others.
              You are trying to calculate the cost per kWh not per kW. If you make that change pretty much globally in all your posts on the subject it will help.

              Also, you need to make sure that those two Trojan batteries can deliver the power you need (in kW) and the energy you need daily (in kWH) before the comparison is complete.
              What depth of discharge are you using per cycle? And are those 6V or 12V batteries?
              Ah, I see you did add that information.
              You used 50% DOD. I have a hard time believing that you will still get 1200 cycles at that DOD in real world use, but Trojan should be pretty trustworthy and there has been time for them to run those actual tests rather than extrapolating the results.

              50% DOD for loads over, say, a 5 hour period, is going to discharge the battery at an average rate consistent with C[SUB]10[/SUB] rather than C[SUB]20[/SUB], so Sunking's point is well taken.

              And you have neglected to allow for the fact that you must put back more energy into the batteries than you take out (and are paying for, even if at the night rates) and are limited in available solar hours, so recharging from 50% DOD in one day is not going to happen either with solar, but could happen with storage.

              If your super off peak rate is .11 per kWh and you have to overbuy by 120%, and your battery storage cost is .17 per kWh then the sell back rate would have to be .11 x 1.2 + .17 = 30.2 (or higher) to make it worthwhile.
              Thanks, that is not the first time I have shortened kWh to kW. I have fixed my posts. I got the 1200 from the link Sunking gave me at -> http://www.trojanbattery.com/markets...ble-energy-re/ I agree that there is risk in batteries, however in this case, since the batteries will be charged with grid power at night, the recharging is not dependent upon the weather and should be quite reproducible.

              My point also was not that the numbers are dependent upon 2 T105s but to make the calculation easier to follow. It should be scalable to any size battery bank.

              I agree that I have not fine tuned the analysis to account for charging and inverter inefficiencies, but I am rather taken aback at how much SCE would be charging me for TOU during peak times as well as how little they charge at below baseline at night, effectively 1 cent per kWh.

              Initially my goal was to show that if one cherry picks the highest rates with a small system, that the batteries could be more than paid for by the difference. Now I am not sure if that is the extent of it. It may be that more of a system could at least break even depending on the load with the desired benefit of also ending up with a UPS of sorts for a good portion of a residence, paid for by the difference in TOU rates. After all, the delta is '46 cents - battery cost' up to the baseline.

              A simple system might be:
              [LEFT]
              Timer -> Charger -> Battery Bank -> Inverter -> transfer switch - > electrical panel

              where the timer only runs the Charger at the time period of the lowest rate. [/LEFT]

              I will probably look at more specifics in either my home or cabin to see if it really makes sense for me to consider this at all.

              ###
              Found Trojan T-105-RE on sale in San Diego for $125. That calculates to 11.57 cents / kWh. It is of interest that even Industrial does not have as good a price ratio (eg IND17-6V @ 17 cents / kWh)
              ###

              $$$
              Added Cost for Inverter and Charger (prorated them for 10 years - $744) so that now the cost per kWh is 12.44 cents. This is based on a sale price if T-106-RE at $125
              $$$

              ###
              My baseline is 13.8 kWh which for TOU would allow charging for 10 hours at 1 cent / kWh. This brings the cost of battery-produced power to 13.44 cents / kWh
              ###

              My tier 1 is 14.496 kWh. This gives me a surplus of about 1 cent / kWh.

              My conclusion is that the risk of premature failure of batteries or electronics outweighs any slight cost benefit and does not compensate for the labor of battery maintenance.

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