Generator / charger sizing

Hi lads,

I am having a real struggle sizing an off-grid generator--I'm just not confident in my figures. Much of what I have read gets so far into the technical I'm lost and
cannot apply it to my situation.

Simply put, I DO know that I need a battery capacity of 868AH @ 48V (gives 3 days of autonomy). I envision 2 strings of 8 batteries, 6V at 450AH = 900AH.

I am using two Xantrex SW4048 inverter/chargers, stacked for 240V. The specs are:

AC Input 120V, 60A passthrough, 30A charging
AC Output(RMS) 120V. 33A
DC Input 48V, 100A
Contiuous Chg rate 60A DC
PF (allowed) ? -1 to 1 pf

Here's what I did to size the generator:

5566W/day power usage
6679W after 20% charge/discharge loss

63-117A -- Charge current range 7-15%
120A -- Charge current available (2x60?--is this correct?)
5760W -- Max sustained pwr from chgr (120A x 48V--is chg V actually higher?)
6,776W -- Adj for 85% charger inefficiency
28A -- 6776W/240V (use 240V, not 120V, correct?)
30A -- Adj for gen inefficiency 95%
31A -- Derate gen 4%/1000' for 1200' elevation
7,476W -- gen output needed for charging
2,524w -- passthrough for other loads while charging
75% load on gen while charging = good (80% Rule of thumb)

Generac Guardian 10Kw
120/245V
41.6A
Main Line Breaker 45A

21,600W to replenish battery bank @ 50% DoD.
3.75 hrs chg time (21.6Kw/Max sustained chg pwr)
2.00 hrs absorbtion time (mfg recommedation)
5.75 hrs total chg time (roughly)
11.1 gal LP for 10Kw generator @ 1.93 gph (1.25 half-load)

Any glaring errors?
Is my use of a 10Kw gen correct?
Suggestions?

Thank you, in advance. Love this forum!

Comments

  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Generator / charger sizing
    Any glaring errors?
    AC Input 120V, 60A passthrough, 30A charging
    2,524w -- passthrough for other loads while charging

    Yes. 2 inverters each accept 30A charging,
    60A total for 2 chargers running full out.
    21A for your pass thru loads

    81A to be supplied by genset.

    I'd guess a 100A @ 120V or 60A 240V genset would work. 15KW genset roughly.

    I'd lean toward a 240V, and split the phases to feed each SW it's own phase.
    I'd also have a spare, 60A 120V genset - just in case the big one is down for oil change or something, so you can limp along. Although having a clone to get spares from is a valid argument too.

    This is some big power, so diesel is your only option, and look at 1800 RPM gensets. Consider an option for a propane/natural gas kit, so you can lean out the diesel to reduce carbon loading at lighter loads.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • CallMeChazCallMeChaz Registered Users Posts: 3
    Re: Generator / charger sizing

    Hi Mike,

    Wow, now I'm really confused. I went through all the steps to calc this that I pieced together on numerous websites, but you pulled out 2 specs and made a recommendation. Bear with me as I try to become educated, as I have another system to design. (BTW--I already have the two SW4840's)

    1. Why do we assume the chargers are running full out (60A). I thought that's what all the calc's I did were for (I hope you are not LOL as you read them;-) Don't you have to do all that to conclude ya' need 7476 watts for charging (31A @ 240V)? In other words, what the heck are all those calcs for from various sources if you just need the gen/inverter/charger specs??

    2. I'm not sure where your 21A passthrough figure came from that boosted my need to 81A. I hope I didn't misspeak when I put in "2,524W -- passthrough..." I was assuming that's how many watts I would have left on a 10Kw generator after figuring the charge watts needed, that could be used if generator is running and charging. Did I err?

    Does it matter that the Xantrex units will proportion the charging and other loads as it sees fit. If I understand, it will use excess gen power for loads while charging, and borrow battery power if needed.

    In light of the above, do we still need a 15KW to be able to charge out battery bank?

    Thanks for your input.
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Generator / charger sizing

    To size a generator, you need to know what the load will be. I just took the portions of your data that spoke to load:
    AC Input 120V, 60A passthrough, 30A charging
    2,524w -- passthrough for other loads while charging
    If you called the loads or pass thru something else, I can't help that.

    Passthrough = Amount of load going thru the box. Like the gadgets that are turned on that drain your batteries, Fridge, radio TV, computer. Do you want those on while you are charging batteries - or will you charge in the dark with no lights?

    30A charging if there is a way to regulate this, let me know, I can do it on my XW, but I don't know if the SW has it, so I'm assuming when you have low batteries, and you turn the charger on, you pull 30 A. from the AC source. 2 chargers= 60A

    So when these condtions change, you can change the genset. I allowed a bit extra, and called 15KW, instead of 12.64KW. But you can't go too much over - diesels like to be loaded to about 50%. Altitude does have an effect too.

    I think you just got too involved and thunk too much about it.

    There is the end of charge taper, as the batteries come up, but the genset still has to support the beginning charge rate.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Generator / charger sizing
    Any glaring errors?

    it's a Xantrex SW4048 which are no longer made because they don't meet current UL grid tie spec.

    I don't know if it has special generator input, but it may fail to sync and regonize a genset if it's expecting clean grid power.

    Another thread covering s/w versions:
    http://www.solar-guppy.com/forum/post4181.html
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • RCinFLARCinFLA Solar Expert Posts: 1,397 ✭✭✭✭
    Re: Generator / charger sizing
    CallMeChaz wrote: »
    Hi lads,

    Your calculations are very good.

    The SW4048 will suck up 33 amps AC maximum for charging. (35 amp pushbutton breaker on top of unit)

    The amperage the batteries will get depends on their voltage at a given point in recharge profile. The SW's regulate current based on AC input current and have no measurement capability for DC amperage.

    Best way to figure battery current is power-in times efficiency loss of inverter, divided by battery voltage.

    Inverter max capability is 33 amps x 120vac = 3960 watts (there is a 35 amp pushbutton breaker on top of unit)

    For 240 vac input @ 33 amps for each leg (each SW4048 ) that is a 7900 watts from generator.

    Charging DC for each unit will be 3960 watts x 0.87 = 3445 watts to battery. At 57.2 vdc battery that will be 60 amps per inverter or 120 amps total. 13.3% of battery AH rating current. At lower state of charge with lower battery voltage, the DC current will be higher. At 49.6 vdc battery, the charge DC current will be 3445 watts / 49.6 vdc = 69.5 amps, or 139 amps for two inverters. 15% of battery AH in DC current.

    The SW's will automatically back off on charging current, as required, if there is ACout current needed to stay within the set AC2in AC amps max setting. The AC2in max amps must be equal or higher then the AC charge amps as the AC2in amps max sets the limit on generator feed current.

    If you have a 10kW gen you can set the AC2in max amps to match the gen max of 41 amps and set the Chg AC amps to 30 amps. This will give you some margin for ACout loads.

    10 kW will be fine, you are right on! A higher amperage generator will give you more margin for ACout loads. AC2in max can be up to 60 amps AC.

    Sometimes when gen load goes from low load to surge load the gen will bog down and drop rpm's. This can cause the SW's to release from generator due to gen freq change. If this happens you have to lower the AC2in max to avoid the gen surge bog down. The inverter will suppliment ACout surge to supply the heavy output load. It will automatically revert from pushing current into battery to pulling current from battery to suppliment the ACout heavy load.


    I have two SW5548plus series connected and two SW4048's in series connection. I have a 15kW Generac for generator.

    If your generator is relatively new with electronic speed regulation you should be fine on 60 Hz accuracy.

    Only problem I have sometimes with Generac 15kW is the throttle stepper motor increment resolution when generator has no load causes the speed to wobble a little bit. Sometimes the SW+'s have a little trouble locking on. The wobble is eliminated with just a small load on the generator.

    The wobble is less then +/- 1 Hz but because it is changing at a couple seconds rate it causes a problem with the initial phase locking on the SW+'s. It only occurs once in a while and putting a 60 watt light bulb on generator 120v AC socket eliminates the wobble.
  • PhilSPhilS Solar Expert Posts: 370 ✭✭✭
    Re: Generator / charger sizing

    I, too, think you'd be fine with a 10K genset. Our backup is 10KW propane charging through an SW2512 (and limited to 90adc charge) and a Magnum MS2812 that charges at 100adc and there's an automotive charger "always on" that charges another 20adc.

    I've never had a situation where that generator wasn't enough.

    What Mike said about 1800rpm generators being better is very true. Also the mention of diesel gensets needing to run under load is true.

    Propane generators run fine with any load, tho more efficiently with larger than smaller, but they won't carbon up. Also, propane can be delivered to you (maybe diesel can too?). Carrying gas cans isn't fun. BUT I've never seen a propane generator that runs at 1800rpm. Probably something to do with the torque of a diesel.

    I've had a few different gensets over the years with the 10kw being the largest. This one is Kohler powered. I've had a couple of Hondas that lasted a long time (one old gas 4kw is the backup for the 10kw). I cheaped out one time and bought a tri-fuel Briggs and Stratton. More problems with that one than any other, ever.

    Just buy quality would be my main recomendation.

    Phil
  • dagr51dagr51 Solar Expert Posts: 72 ✭✭✭✭
    Re: Generator / charger sizing
    PhilS wrote: »
    BUT I've never seen a propane generator that runs at 1800rpm.

    Phil

    Onan green, baby! My 1800 rpm thumper will happily run for days and days on propane.
  • RCinFLARCinFLA Solar Expert Posts: 1,397 ✭✭✭✭
    Re: Generator / charger sizing

    A gasoline or propane engine running at 1800 rpm's is not efficient from a fuel consumption perspective. The engine horsepower peaks in the 3200 to 3800 rpm range with the lower compression ratio on these engines. The engine size must be increased to produce the needed h.p. at 1800 rpm's.

    The engines will last longer at 1800 rpm's.

    Propane is most expensive from a fuel cost perspective. Clean and relatively safe.
    Diesel is best for fuel cost. Messy, very safe.
    Gasoline in large storage quantity can be a safety problem.

    It depends on your use case. If running every day for several hours then diesel is probably the way to go. If you have utility line natural gas then that may be best solution. If you already have hot water heater & cook with propane, and only need gen for emergency backup then that may be best solution.
  • Ralph DayRalph Day Solar Expert Posts: 952 ✭✭✭✭
    Re: Generator / charger sizing

    I agree with diesel being the safest choice. A little fuel leak and you have a stinky oily puddle to clean up...a little leak of gasoline propane or natural gas and you might have to clean up the remains of a building.

    Ralph
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Generator / charger sizing

    Well--I think we had one person here who lit off his PV out building with a Diesel genset...

    Separating your major items. Diesel genset in one shed, fuel in another location (with appropriate shutoffs/safety devices), and solar panels+batteries+inverters in another will help limit the damages if the worst case happens.

    From the 911 event, one of the problems that they had with the NYC emergency center (as I understand the reports)--The diesel tanks were in the basement and the gensets elsewhere (on the roof?). Once the planes hit, debris severed the diesel lines between the tanks and the gensets, and the electric feed pumps in the basement kept pumping fuel into the middle of the building--Not a good thing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Ralph DayRalph Day Solar Expert Posts: 952 ✭✭✭✭
    Re: Generator / charger sizing

    My bad...Backroad from Bagdad had that misfortune. Diesel the cause or just lost with the rest?

    Ralph
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Generator / charger sizing

    I don't like to bring up past bad karma--but here is the first post:
    backroad wrote: »
    I haven't posted here in a while cuz my solar has been doing so well lately......til last Sunday evening. All my solar shed, including panels, controllers, inverter and batteries were caught up in a rather large fire. Also, took out three generators and about 400 gallons of fuel. It was either started by my running generator or arson. The Bagdad fire marshal said there was just too much fire and heat to be sure.

    What did I learn from this? My generator, solar equipment and fuel storage will be kept in 3 completely different areas. At least if sometimes goes up in smoke, it won't be a complete loss. Lesson #2 is not to wait to cover the solar with insurance.

    I did have six of my panels on a pole mount that were not engulfed in flames, just lots of heat. Cables were melted on 4 of them and 2 of the panels are complete devoid of any kind of backing. 4 of the best panels are showing 17-19 volts which is correct and the 2 with the missing backing are still showing a little over 16 volts.

    Because of the totality of the loss, I'm going to have to maintain the use of the 6 panels that are left. Anyone know what they use for the film they use on the back side? I was thinking that it might bring the voltage up a little.

    Life is good and God will supply and strengthen. Just going to take some time to rebuild. One step at a time. Currently running on a 3000 watt generator for the refrigerator, freezer, propane heater and fish tank. Still, thanks to all on this site who have helped me learn a bunch on the old system. I know the new stuff will be better and cleaner.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Generator / charger sizing

    Bottom line, S*%T can happen in any system, gas, diesel, L/P nat gas. Additionally electrical fires. Bottom line, design and build trying to anticipate the worst. The best solution is one that fits your needs the best. Full time, part time, standby, shops etc all might have potentially different answers. From a stability of fuel, Nat gas or L/P clearly wins, followed by fuel oil, lastly gasoline. From a leak equation, I would probably rate diesel as safest from a fire perspective, but worst from an potential environmental disaster in the event of a major leak. Gasoline in the middle on both counts, and L/P-Nat gas as the safest from an environmental leak stand point some what more dangerous than gasoline simple because it is pressurized.

    Tony
  • PhilSPhilS Solar Expert Posts: 370 ✭✭✭
    Re: Generator / charger sizing
    icarus wrote: »
    Bottom line, S*%T can happen in any system, gas, diesel, L/P nat gas. Additionally electrical fires. Bottom line, design and build trying to anticipate the worst.... Tony

    Something I installed at least 20 years ago:

    a sprinkler system.

    Only one sprinkler head, in the building with my battery bank and inverters (and also the rarely used backup 4KW Honda genset). I put one in with a higher-than-normal pop-off temp (240 IIRC). The propane genset is a few feet behind the building.

    Of course, if it sprays that means there's gonna be some electrical events (water + electricity = events).

    But IF it sprays, that also means something has gotten VERY hot, so the electrical event would likely be the smaller of the problems.

    Thankfully, this system has never been put to use.

    Phil
  • System2System2 Posts: 6,290 admin
    Re: Generator / charger sizing

    RCinFLA - What will be the result if your AC2 in settings are higher than your generator can produce?
  • RCinFLARCinFLA Solar Expert Posts: 1,397 ✭✭✭✭
    Re: Generator / charger sizing
    jhog wrote: »
    RCinFLA - What will be the result if your AC2 in settings are higher than your generator can produce?

    You will pop breaker on generator.

    The charger current setting is a subset of the ACin setting. If you set Charger AC amps higher then ACin max AC amps the charger will limit at ACin setting. ACout load takes priority so if output load is high, inverter will back off on charging current to avoid exceeding ACin amps max setting.

    You should have the ACin2 limit set slightly below the generator max output. If you have a load surge from a motor/compressor, lowering ACin max can help keep the inverter from disconnecting from generator during the startup surge.

    The disconnect occurs if the generator bogs down in rpm due to the surge current and the inverter may not be able to follow it down in frequency fast enough.

    If you are using an old mechanical governor generator you may have to lower ACin max because generator voltage or frequency may fall out of inverter spec limits when generator is heavily loaded. I had an old Generac 4 kW that had AC voltage dropped to about 105 vac at 3.5 kW load on generator.

    It takes a few cycles for the inverter to react and suppliment the generator current for ACout current demand.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Generator / charger sizing

    Phil,

    I could tell some funny stories about sprinkler heads. Like the time I was working building out a store in a mall. The painters had stuffed all the can lights with masking paper. We came in the next morning, turned the lights on via the breakers, and started two work. In a few minutes, flaming balls of kraft paper were falling from the ceiling. The smoldering paper trigged a couple of heads. The system was designed such that if any one head triggered, you had about 5 minutes to get to the sprinkler control room, unlock it and shut down the system. The result of being to slow was the sprinkler surge pump firing up with enough pressure to blow ALL the heads in that sector, which was about 20 stores! So while someone went in search of a key, others were stomping out the balls of fire, and others were corraling the water from the tripped heads! (We got to the pump before it triggered,, I still don't know how!)

    The other issue I encountered quite often were wet sprinkler heads in ceilings over covered walkways that would freeze, since there was no heat, causing lots of damage.

    Tony
  • backroadbackroad Solar Expert Posts: 185 ✭✭
    Re: Generator / charger sizing

    BB, no problem with bad karma or old issues...

    The generator that I believe was the cause of my fire was a gas powered B&S, but when it caught fire it also ignited my diesel supply. What a mess it has been! I'm now tottally on diesel, bit stiil have the panels, generator and the invertervv/batteries very well separeted.

    Many lessons learned by past experiences...
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