Help with system sizing Round 2

stereoman405
stereoman405 Solar Expert Posts: 56 ✭✭✭✭
I can be the "Ready, Fire, Aim" type, so for sake of clarity I'm going to start this from scratch. I have better numbers to work with and a better understanding of what i want to accomplish. (I took my kill-a-watt up here to see real world draw.) My original thread got pretty scattered as I was getting this all figured out. Seemed like a fresh start would make this more palatable. If you want to see where this started you can refer here: Original Thread

Here's the scoop. Wanting to put a backup PV system at our grid tied cabin. (Wind and snow can make power sketchy.) Already planning on putting in wood heat, and I have a propane grill with stove top if I had to use it.

Predicted minimum load:
- 120v fridge - 1.3kwh / day
- (6) 120v 60w lights - 3 hrs a day use (Could switch to 11w CFL's)
- 12v 50w HAM - (Minimal use, maybe 100w / hrs max.)
- Misc. - add 200w / hrs.

The gear I currently have:
- (4) REC 210w panels
Voc: 36.1v
Vmp: 28.2v
Isc: 8.1A
Imp: 7.5A

- Xantrex 12v modified sine inverters (XM 1800 Pro) Plus a couple 12v 1000 watt'ers.
- Xantrex Trucharge 10A AC to 12v DC charger.
- Honda EU2000i (Propane converted.)

Other useful info:
- Cabin is at 48.5 degrees latitude.
- Cabin is grid tied.
- Roof is southern exposed with minimal if any shade during the summer, heavy shade in winter.
- Cold winters with a lot of snow.
- Batteries would be exposed to freezing temps, but wouldn't get past 75-80 in summer.
- Low winter use from us. (Minimized power requirements.)
- PV should be within 75' of battery location.
- I plan on having inverters within a couple feet of bank. (Minimize 12v wire loss.)
- I have a couple 12v native appliances, but I plan on inverting to 120v for fridge, lights, etc...
- I don't have anything that should draw past 1000w at any given time.

Questions:
- What is the "ideal" array input and battery bank capacity I should have with these needs? (Do I have too much panel?)

- IF I were to put a large AGM battery bank in place (12v 1500AH) could I keep the batts healthy with this setup? I would get a larger AC to DC float charger that would be grid tied 98% of the time if that were the case. (I can get a 1500AH used setup for the same it would cost for a new 500Ah bank. Trying to justify if its worth it.)

Comments

  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    Re: Help with system sizing Round 2

    From the info you have given, I see your WH = 2700/day. Half of this is the fridge. Taking a popular "back of the envelope" method from this forum and working it backwards I take the expected usage, (2700WH), multiply it by 2 (to account for all losses, which becomes 5400) and then divide that by the hours of good sun that one can expect per day (for North climes in summer maybe 4.5) which yields 1200W. That is your solar array power requirement. One more panel and you've got it.

    Sizing the battery bank is more complicated. 2700WH = 225AH. A common battery capacity. But to prevent discharge below 20% we need to multiply the value by 5! Which is 1125AH. Now if you go 3 days without sun.... Would that become 3375AH? I think so.

    It is likely that many replies by some with more knowledge/experience have been posted in the time it took me to figure this out, but like Ginger Rogers, I did it backwards.

    Not sure that's the analogy I was looking for:blush:
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help with system sizing Round 2
    Predicted minimum load:
    - 120v fridge - 1.3kwh / day = 1,300 WH
    - (6) 120v 60w lights - 3 hrs a day use (Could switch to 11w CFL's) = 198 WH
    • note that I used 6 lights * 3 hours * 11 watt CFL's for power calc.
    - 12v 50w HAM - (Minimal use, maybe 100w / hrs max.) = 100 WH
    - Misc. - add 200w / hrs. = 200 WH
    So, your basic power need would be:
    • 1,300+198+100+200=1,798 WH per day
    Using PV Watts website, using Kalispell Montana, 1kW of solar panels, 0.52 derating:
    "Station Identification"
    "City:","Kalispell"
    "State:","Montana"
    "Lat (deg N):", 48.30
    "Long (deg W):", 114.27
    "Elev (m): ", 904
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 48.3"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 7.9 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 2.14, 34, 2.69
    2, 2.96, 42, 3.32
    3, 4.02, 63, 4.98
    4, 4.70, 69, 5.45
    5, 5.42, 81, 6.40
    6, 5.77, 80, 6.32
    7, 6.24, 87, 6.87
    8, 6.19, 88, 6.95
    9, 5.23, 72, 5.69
    10, 4.08, 61, 4.82
    11, 2.16, 31, 2.45
    12, 1.46, 21, 1.66
    "Year", 4.21, 730, 57.67
    Assuming November through February is deep shade, the minimum month is November at 61 kWhrs per month or:
    • 61 kWhrs ;per month / 30 days per month = 2 kW = 2,000 WH per day
    To size your panels, assuming 1,798 WH daily load:
    • 1,798 WH * 1/2,000 WH per 1,000 Watts of panels = 899 Watts of Panels
    So--that is real close to your 840 watts of REC panels. You will probably have a little generator run-time in March and October with this setup.

    Optimum layout for panels, assuming standard 60 amp MPPT controller (like Xantrex) would be 2 panels in series and the two strings in parallel... Will run from -40F to +95F and be able to charge a 12 or 24 VDC battery bank (not high enough for a 48 volt bank).

    For your AGM bank you are looking at:
    • 1,500 AH * 12 volts * 0,.85 inverter eff * 1/1,798 WH = 8.5 days of no-sun
    To recharge this bank--because it is an AGM, you can get away with as little as a 1% charge rate--but not much useful loads...
    • 1,500 AH * 0.01 = 15 amps (maintenance charging)
    • 1,500 AH * 0.05 = 75 amps (minimum recommended)
    • 1,500 AH * 0.13 = 195 amps (typical maximum recommended)
    Your 840 watts of panels on a MPPT charge controller:
    • 840 watts * 0.77 derating * 1/14.5 volts = 44.6 amps
    • 44.6 amps / 1,500 AH = 0.03 = 3% charge rate (pretty small)
    Now, for charging your battery bank, you Honda eu2000i can drive a non-power factor corrected 40 amp charger:
    • 14.5 volts * 40 amps * 1/0.80 charger eff = 725 watts
    To recharge your battery bank, assuming it starts at 50% state of charge and ends at 85% state of charge:
    • 1,500 AH * (0.5-85-0.50) * 1/40 amps = 13 hours run-time minimum
    Fuel consumed:
    • 1.1 gallons of fuel * )1/400 watts * 15 hours) * 13 hours * 725 watts = 1.73 gallons of fuel
    • 8.5 days * (0.85-0.50) = 3 days between recharging.
    You probably would want to get at least 1x 40 amp charger and possibly 2x 40 amp charger. You could run 2x 40 amp charger when your grid comes back up to quickly recharge the bank.

    For your 75' distance from panels to charge controller and using copper wire, you will want between 3% to 1% maximum voltage drop at rated current. 7.5 amps * 2 strings @ 2x 28.4v -- or -- 15 amps 56.8 volts or 1.74 to 0.57 drop (most folks aim for 1-2% drop. Using this site for voltage drop calculations, we get:
    • 8 awg and 1.7 volt drop
    • 6 awg and 1.1 volt drop
    • 4 awg and 0.7 volt drop
    • 2 awg and 0.4 volt drop
    I think I have run out of things to type for the moment.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with system sizing Round 2
    BB. wrote: »
    • 1,500 AH * 12 volts * 0,.85 inverter eff * 1/1,798 WH = 8.5 days of no-sun

    Does this assume only depleting the battery 50% or would I have to change this to 4.25 days of run time?

    My other battery alternative would be (2) 255AH 12v AGM's in parallel for 510AH. Depending on the above formula it would look like this unless I have to cut it in half right?
    • 510 AH * 12 volts * 0.85 inverter eff * 1/1,798 WH = 2.89 days of no-sun

    I'm just trying to figure out if the 1,500AH is overkill here.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help with system sizing Round 2

    That is 8.5 days to 0% state of charge (dead). Or 4.25 days of no-sun.

    We normally recommend 3 days of no sun + 50% maximum discharge for: 6x daily load.

    The bank is a bit large--but certainly not hugely over sized.

    In engineering terms, usually things within a factor of 2x (or x1/2) or less are about the same. Things that are 10x (or x1/10) or larger--the smaller one can pretty much be ignored.

    For the loads you asked to use (assuming CFL's for lighting)--Your 1,500 AH bank is not an issue. (in my humble opinion).

    For off-grid use--it would not hurt to almost double your solar array--it is pretty small for that size battery bank. You are current ~3% rate of charge from solar. We like to recommend for 5% to 13% (your AGM could even go above 13%--but there is no reason to do that--unless you need very quick charging for some reason).

    What will be limiting (again, in my opinion) is the 12 volt battery bank... Your maximum inverter would be around 1,200 watts:
    • 1,200 watts * 1/10.5 volts min * 1/85 inv eff * 1.25 NEC safety factor = 168 minimum branch circuit and fuse/breaker (round up to next standard).
    That is not a small amount of current...

    But frankly if you needed to use more power (saws, sump pumps, etc.) you can just use the grid or purchase a cheap 5kW genset too...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with system sizing Round 2

    Thanks Bill. What would be the 40A non power corrected AC charger that you would recommend?

    Something like the 45A Iota?

    Wouldn't that only be pulling 500w - 600w from my genny though? Wouldn't I be better with the 55A Iota to maximize my generator a little better?

    All the Iota's say "regulated" is that the same as "power corrected"? Why should I get a non power corrected model?

    Also, if I did want to add panels down the road shouldn't I be fine with another brand as long as I stat within 10% of my REC's output?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help with system sizing Round 2
    Thanks Bill. What would be the 40A non power corrected AC charger that you would recommend?
    Others probably can do better at recommending various power supplies.

    Iota has a good reputation and has a simple interface (jumper in, bulk/absorb charging; Jumper out, float).

    After years of working with various commercial power supplies for computer systems--I really like Power Factor Corrected power supplies. Basically, an average power supply may be rated at PF=0.65 or so... That means, for a 15 amp rated circuit, you can only pull 65% of the rated power of the circuit.

    Or, basically, you are pulling 15 amps from the circuit, but only 65% of the current is doing "work"... The other 35% is heating wires, transformers, generator windings, but not doing any work for you...

    You do have to look at the ratings for your power source... Many will have Watts=VA (remembering that Watts=V*A*PF), others may actually have Watts<VA rating... It all depends. Unless you know better, there is a maximum supported current and you should plan on worst case Watts=VA.

    So, for the Honda eu2000i:
    Rated voltage=120V
    Rated frequency=60Hz
    Rated current=13.3A
    Rated output=1.6kVA
    Maximum output=2.0kVA
    So, we assume 13.3 amps maximum current and 120 VAC (=1.6 kW)
    Something like the 45A Iota?
    The specs. for the 12 volt 45 amp supply is here:
    DC Output Voltage (No Load) approx.      13.6V (DC)
    Output Voltage Tolerance (No Load)     + or - .7%
    Output Amperage, Max Continuous     45 Amps
    Output Voltage (Full Load) approx.     >13.4V (DC)
    Maximum Power Output, Continuous     600 Watts
    Ripple and Noise     <50 mV rms
    Input Voltage Range     108-132V (AC)
    Input Voltage Frequency     47-63
    [B]Maximum AC Current @ 108VAC     11 Amps[/B]
    
    So, the VA rating of the power supply appears to be (assuming 80% efficient):
    • 108 VAC * 11 amps = 1,188 VA (not watts)
    • 600 Watts * 1/0.80 * 1/1,188 VA = 0.63 Power Factor
    These devices are "constant power" type units... If the voltage is higher, then the input current is lower:
    • 1,188 VA / 120 VAC = 9.9 amps (estimated)
    The DLS-55 is rated as above, but the differences are:
    13 amps at 108 VAC
    Maximum continuous output is 750 Watts
    • 13 amps * 108 VAC / 120 VAC = 11.7 amps
    • 11.7 Amps * 120 VAC = 1,404 VA
    The Honda eu2000i is rated for 1,600 Watts (VA) continuous and 2,000 Watts (VA) for short periods of time... A DLS-55 is right on the edge.

    It should work (it is close)--but I could not guarantee it would--somebody else (like NAWS our host) may be able to tell you more.

    To show you what a Power Factor Corrected supply could output (maximum) using PF=0.95 and assuming 80% efficient:
    • 120 volts * 13.3 amps * 0.95 * 0.80 = 1,200 watts into the battery bank
    So, you can get significantly more power (or run other loads on the genset at the same time) with power factor corrected devices.

    On the other hand, a 1,200 watt PF corrected power supply does more work than a 750 watt non-PF corrected supply--so the larger supply will consume more fuel per hour.

    The old Xantrex TC (True Charger) line was a very nice and had power factor corrected power supplies. The new TC2 line has been difficult to get (NAWS has not listed any units for sale yet)--but if you could find a TC2-60 (PDF Sales handout)... That would be a good fit. They also have an optional remote battery temperature sensor.
    Wouldn't that only be pulling 500w - 600w from my genny though? Wouldn't I be better with the 55A Iota to maximize my generator a little better?
    Yes--but I would confirm with somebody who has knowledge that a eu2000i has successfully operated the DLS-55 on that genset...

    You have Grid to the cabin--if the 55a does not work on the eu2000i, you could always relegate it to Grid charging and use the 45a for eu and parallel for faster grid charging with to AC Mains powered chargers (your bank can probably take in excess of 195 amps.
    All the Iota's say "regulated" is that the same as "power corrected"? Why should I get a non power corrected model?
    Regulated is talking about the 12 volt regulated voltage output.

    Power Factor Corrected (PFC) describes the AC "front end" of the power supply. From an electrical point of view PF=1.0 makes the power supply look like a resistive load.

    PF=0.65 sort of makes the power supply look like an inductive/motor load (voltage and current out of phase--or for many electronic power supplies, the current is not a sine wave--but zero current with brief, large spikes of current, near the peaks of the voltage sine wave).
    Also, if I did want to add panels down the road shouldn't I be fine with another brand as long as I stat within 10% of my REC's output?
    Yes--for parallel connections should match Vmp within 10%

    Series connections, Imp should match within 10%

    However, "non-standard" voltage / high current panels can be difficult to match (no real standard once you get a way from the "12 volt panels" used to charge batteries). So--in the worst case, you would need a second charge controller to connect to panels whose Vmp/Imp cannot be match with the existing panels (it does happen).

    In your case, you have 840 watts of solar panels on a 60 amp MPPT controller connected to a 12 volt battery bank. So--you would need an additional solar charge controller to add more panels anyway... Therefore, "panel matching" is not really an issue for your current planned configuration.

    Outback does have the FM-80 controller which you may put one more pair on--but it is very close to exceeding the useful array size (oversize array would be limited by FM-80's 80amp battery output spec.).

    I hope everything makes sense--it is late and my batting average has not been good when I am tired. :roll:

    -Bill

    PS: I see NAWS has listed the TC2-20 amp charger... Perhaps the larger 40 and 60 amp units are not far behind--calling your supplier would be the next step.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset