Generator sizing for new system

rbell

We have a new system installed in our off grid home in Baja. It consist of the following.

5.5kw of solar panels

Outback fp1 inverter

3x outback mx80 80 amp chargers

4 x 845 amp hour hup solar one batteries wired for 24 volts.

We consume around 10-12 kw of power per day

I now need to order a generator and have no idea what size. I am thinking a 14kw Kholer but not sure if that's to small or two big. Any input would be greatly appreciated. Thanks

BB.

Re: Generator sizing for new system

Welcome to the forum Rbell!

rbell wrote: »

4 x 845 amp hour hup solar one batteries wired for 24 volts.

What voltage are those batteries? 6/12/??? ... Or how many parallel battery strings do you have (1/2?). Trying to figure out your battery bank AH capacity at 24 volts.

What fuel (propane, diesel, gasoline, natural) choices do you have? Any fuel cheaper or more expensive than another (taxes, local availability)?

Are you looking for pure battery charging, backup AC power, running a shop with larger machine tools/welders?

-Bill

rbell

Re: Generator sizing for new system

Thanks for the reply.

The batteries are 12 volt rated at 845 amp each. I have 4 batteries in total to make up my 24v system. So it's 1690 amp hours capacity.

I would prefer to go with propane for fuel as that gets delivered weekly to my house. The generator will be for backup power if the system should fail and for when we have cloudy days. Some time in the future I might install an AC unit or two but for the most power I don't see my power needs changing from the current 10-12 kw per day.

zoneblue

Re: Generator sizing for new system

This is a kind of BTW. Is it too late to reconfigure to 48V? For a system that size it will be better.
For the batterys they can all go in series, ideal.
For the inverter, probably have to replace it?
For the CCs you only need 2. (1 and a half), or you could add 2x PV
Cable loss reductions by factor 4 everywhere.
better surge support.

Just thinking our loud. You didnt ask about that! And maybe you have some specific reason for 24V...

Cariboocoot

Re: Generator sizing for new system

You have an Outback Flexpower 1 system. These are usually wired with the FX3048 inverter(s) but you have a 24 Volt system so I'm going to assume you have an FX3524 or possibly two? If two, are they stacked for 3.5kW @ 240 VAC or for 7kW @ 120 VAC?

The inverter(s) will limit the amount of charging available to the batteries. With 1690 Amp hours @ 24 Volts you could utilize the full capacity of two of these FX units and would probably want to. There's roughly 4kW of power right there.

The next question would be what sort of loads do you average? Basically you want the largest gen capacity to be able to supply maximum charging and maximum loads, but you can scale down from there. Your averaged power consumption is low, so you need to think about what will be switched on while you're trying to charge with the generator.

rbell

Re: Generator sizing for new system

I am new to solar and relied on the contractor to spec the system. I started with two batteries and it was not enough storage capacity so two more where just installed. It is kind of late to redo now unless there was a big benifit. Thanks for the reply.

rbell

Re: Generator sizing for new system

Just confirmed I have one 3524 inverter. As far as loads go I think the highest I have seen at one time is 4kw. Probably my pool pump is running, my wife running the washing machine and usual items running like the fridge. I have no AC yet. On average my daily consumption is 10kw.

Cariboocoot

Re: Generator sizing for new system

rbell wrote: »

Just confirmed I have one 3524 inverter. As far as loads go I think the highest I have seen at one time is 4kw. Probably my pool pump is running, my wife running the washing machine and usual items running like the fridge. I have no AC yet. On average my daily consumption is 10kw.

Ow. Asking a 3.5kW inverter to supply 4kW is asking for trouble. No 240 VAC loads? Or is there an autotransformer in there somewhere?

That one inverter-charger is going to have a very hard time charging that big battery bank.

I think you should consider zoneblue's suggestion of trading up to 48 Volts. With the loads being at 4kW+ you really need two 3048's in parallel. One will be in standby mode and only fire up when needed. It will reduce current all around, improve system efficiency by a couple percent, and make it easier to recharge that massive battery bank (which will become 845 Amp hours @ 48 Volts).

Somebody didn't run the number right to begin with here.

marineadvection

Re: Generator sizing for new system

14k is ok.do it if u want, but if u r stuck at 24v i would take the $ and buy more pv first if the spot is good in the winter and u have the space for panels. 3 cc handle 1980w nec rating each so you got the 5940w potential filled well with the 5.5... already overkill for most. you would need another fmx80 cc for more panels. these are worthit!! add one more cc and 2kw in panels and u will be stoked. delete the generator idea!! i have a 24v and 48v,and ive been offgrid since 81. or get message the other guy said which is sell your 24v ob stuff used and get some 48v action. then your cc's will handle double the incoming pv each, to so about 4k each or 12k pv total with 3 cc's. that is a better idea. whoever burns fuel loses.period.

marineadvection

Re: Generator sizing for new system

i agree with coot, either your contractor is fired or he/she built the system for you...before... and not after your wife, kids, washer and pool, make sense?

rbell

Re: Generator sizing for new system

I meant to ask what is the benefit of changing to 48 volts. My amp hours would go down from 1690 to 845 so does that not mean my batteries would deplete sooner. Sorry this may be stupid question but as I stated earlier I am knew and relying on what a contractor is telling me. Thanks

Cariboocoot

Re: Generator sizing for new system

rbell wrote: »

I meant to ask what is the benefit of changing to 48 volts. My amp hours would go down from 1690 to 845 so does that not mean my batteries would deplete sooner. Sorry this may be stupid question but as I stated earlier I am knew and relying on what a contractor is telling me. Thanks

No, they will not deplete sooner: the net power is relatively the same. A 48 Volt system is slightly more efficient because more of the power is delivered as Voltage rather than current. Current = heat, and that means energy that is being wasted not used for work.

A somewhat relevant thread: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

rbell

Re: Generator sizing for new system

I am pretty sure I don't have any 240 volt equipment at the moment but if I install AC there will be. All my solar equipment is brand new and only installed in the last few months. It replaced a much smaller system that had to run the generator every day for a couple of hours. Not sure what I would have to upgrade in order to make it 48 volts but I will ask. I will order another inverter for sure though. In southern Baja we have on average 350 days a year of sunshine so its ideal for solar. Having said that the most kw's I have made in a day is around 21. I keep asking why a 5.5 kw system only makes 21kw of power in a day and have not got a straight answer yet. Does that seem to low for the amount of panels I have?

rbell

Re: Generator sizing for new system

Unfortunately I have maxed out my roof space for more panels so if I really need more racks will need to be installed on the ground which will be a challenge. With my current setup and landscaping. The only thing I could do is replace the 16 x 100 watt panels and upgrade them to 240's. which would help.

Photowhit

Re: Generator sizing for new system

rbell wrote: »

I keep asking why a 5.5 kw system only makes 21kw of power in a day and have not got a straight answer yet. Does that seem to low for the amount of panels I have?

It 'can' make more, it just doesn't need to. As the batteries charge they require less amperage to charge and reduce the rate they are taking in current. Off grid systems need to have more panels/power than they need, to insure they reach a full charge regularly.

If you average 350 days of sun a year, I'd forgo the generator too, just be aware on those rare days when the clouds come you might want to shut down your bigger loads, pool pump, water heater?, save those loads of laundry for when the sun shines again.

Photowhit

Re: Generator sizing for new system

On the A/C front, I'd look into mini split units, sounds like your place was built with out duct work so they would likely work out better for you any way and are as efficient as central and more if you choose to just A/C parts of the house.

Cariboocoot

Re: Generator sizing for new system

Do not order another inverter until you decide on whether or not to up the system Voltage.

The change would be the inverter(s) and re-wiring the battery bank, plus checking all circuit protection on the DC side to be sure it meets specs for the higher Voltage system (current will be lower, Voltage will be higher).

On a battery system you will not get full array output capacity because if there is no place for the power to go (batteries full, no loads on) there's no place for it to go. Typically you could expect:
5500 Watts * hours of equivalent good sun * 0.52 over-all efficiency. So 12 kW hours would be on the low side whereas 17 kW hours would be on the high side. Efficiency is improved if you can make use of power that would otherwise go unrealized by turning on loads once the batteries are full.

With a grid-tie system all the power the panels can provide has someplace to go; either to household loads or to the grid.

rbell

Re: Generator sizing for new system

Yes we don't have any central ventilation system. I would probably only install a unit in the bedroom so you are correct split units would be required. I am not sure we need them anyway as we are not typically there in the summer months when its really humid. But I have not spent an entire season there yet so I don't know.

rbell

Re: Generator sizing for new system

So the one inverter I have would be fine with my current system at 24 volts? Thanks for the explanation on why I am not seeing more power being produced. I did not know that is how it works.

So I would need two new inverters and the rewire the batteries to go to 48 volts. I might be able to get the contractor to swap my current inverter out so that would not be too bad. I am grateful for all your help. Thanks.

Even though there is lots of sunshine I have had trouble with running an eq cycle at time and had several days in a row with clouds and only a bit of direct sunshine so that is why I need a generator.

mike95490

Re: Generator sizing for new system

rbell wrote: »

..... Having said that the most kw's I have made in a day is around 21. I keep asking why a 5.5 kw system only makes 21kw of power in a day and have not got a straight answer yet. Does that seem to low for the amount of panels I have?

then you may have some issue with panel angle or orientation. I have a 3KW array, and made (after a couple cloudy days and the batteries were low) 20,090 watts - all MPPT charging - no absorb or float . (7/26/2013)

On a more normal summer day, I harvest about 17KWh, and spend 240 minutes in float. That all changes in winter. 48V system. But I'd hold out for a smallish generator, in the 3Kw - 6Kw range. If you get a couple cloudy days, you need to keep the batteries alive, or if you run them flat down, you destroy them. A generator (even manual start, $600 5KW screamer) will save a $5,000 battery bank. Or if a inverter dies, you can power critical loads off the generator till a new inverter is installed. Flexibility and redundancy is the word when off grid.

BB.

Re: Generator sizing for new system

And, it is not unusual for solar power controllers to only have a 5-10% measurement accuracy. Your neighbor may see 20 kWH and you may see 18 kWH from your displays--And yet you are both generating the same ~19 kWH per day +/- 5%.

-Bill

rbell

Re: Generator sizing for new system

Thanks for the explanation. I thought I would need a generator for days of cloud or to run an eq cycle or for emergencies. For about the cost of a generator I could replace my 16 x 100 watt panels with 250 or 290 watts and that would give me 3000 more watts for a total of 8.5 kw which is almost my entire average days use. Maybe I could then get away with just a small honda 2 or 3 kw for topping up the batteries if I have two or more days of could. Not sure if that small of a generator would be able to charge my batteries though.

rbell

Re: Generator sizing for new system

Ok thanks for your input.

Photowhit

Re: Generator sizing for new system

FWIW - If you measured 21 Kwh of production in one day, you likely used70-80% of that, that day, so your use figures might be off, or it was a high use day. Adding an A/C will be a large additional load, window units will use around 500watts while running ... if you didn't have duct work installed for A/C how about insulation? Some things to think about.

rbell

Re: Generator sizing for new system

So I decided to go with the 14kw Kholer for my own comfort. Outback also recommended go with twice my Inverters capacity which will soon be 7kw with second inverter. The question now is it still beneficial for me to go up to 48 volts or just add one more 24 volt inverter. Does it make a difference for the generator with having the second inverter? Would it have to run less? Thanks

Cariboocoot

Re: Generator sizing for new system

rbell wrote: »

Does it make a difference for the generator with having the second inverter? Would it have to run less? Thanks

Not really.

Going to 48 Volts would hinge mainly on stored capacity requirements. Yours is on the largish side, so it could be a good idea (few Amp hour @ higher Voltage for same Watt hours = lower current to battery bank, easier handling by charge controllers).

The two or one inverter issue is a matter of how many Watt need to be put out at any given time. I believe you said you were drawing 4kW sometimes? On a 3.5 kW inverter that's in the surge zone and not anything to be done for extended periods. If your load demands are consistently up there the dual inverters will make this easier to manage as they won't be overloaded.

Outbacks advice on having 2X inverter capacity for generator size is rather simplistic as it is based on supplying full charge power and full load power from the gen. This is not always necessary; rarely would you be running inverter at full power all the time your trying to charge from the gen. Outback inverters will nicely reduce charging to accommodate loads as needed, and ramp it back up again when the power is available: you set an AC in max and a charge max and it is then fairly difficult to overload the gen; only a sudden high demand will do it.

I use a Honda 1600 Watt inverter on my 3524 with a 232 Amp hour battery bank. Since the inverter never runs at full capacity it is no problem. But the water pumps are an issue and thought has to be given to operating them from the system; you don't just let them come on whenever. It is better to run them from the inverter, then use the gen afterwards to replenish if need be.

What I'm saying here is that with careful attention to loads the gen can be much smaller than that 2X recommendation. Frankly that's quite large and may not be very economical at all. If the gen isn't kept 'loaded' more than 50% of its output capacity you're burning a lot of fuel for a few Watts.

rbell

Re: Generator sizing for new system

Ok I just figured it was better for the larger size and the smallest kholer makes is 14kw. How small do you think I could of went with the one or two inverters? If smaller is better in this case I will change it. I was looking to match what I have and really hope I never really to need to run it much anyway. It will be for only when I need to top up the batteries on cloudy days and in emergencies.

vtmaps

Re: Generator sizing for new system

rbell wrote: »

Ok I just figured it was better for the larger size and the smallest kholer makes is 14kw. How small do you think I could of went with the one or two inverters? If smaller is better in this case I will change it. I was looking to match what I have and really hope I never really to need to run it much anyway.

Your battery bank could handle 5200 watts of charging power during bulk charging. Considering the efficiency of your inverter/chargers, a 6kw generator would be the right size.

The smaller generator will cost you MUCH MUCH less to run.

Don't worry about your loads... if you turn on too many loads, the Outback will cut back on the charging so as to not overload the generator. Outback's advice to buy a 14kw generator is a good way for them to give you advice that is guaranteed to work, but it comes at a great cost to you, not them. btw, if you were to use a diesel generator, their advice would be a disaster for you. Diesels really don't like to run at low power... you will develop 'wet stacking'.

rbell wrote: »

It will be for only when I need to top up the batteries on cloudy days and in emergencies.

Topping up the batteries is very inefficient (by solar or by generator) As batteries fill up (absorb stage of charging) the current into them tapers off... you will be running a big generator and only using a few hundred watts for charging. You have already discovered this effect with your panels... when the battery is full your panels stop producing power because there is nowhere for it to go.

The smartest strategy in cloudy weather is to run the generator in the morning to bulk up the batteries. Then turn off the generator and let the solar panels take over for topping up the batteries. Even on a cloudy day you will be producing power. Absorb charging is inefficient. Better to use the sunshine inefficiently than the propane.

--vtMaps

rbell

Re: Generator sizing for new system

So my supplier has a honda eu6500 inverter series that runs at 23 amps. It would be super quiet and cheap to run. It cost more than the 14kw kholer but its also portable. Is 23 amps enough to run my chargers. I would not need a customer enclosure for this one either as its super quiet and comes on wheels.

vtmaps

Re: Generator sizing for new system

rbell wrote: »

So my supplier has a honda eu6500 inverter series that runs at 23 amps. It would be super quiet and cheap to run. It cost more than the 14kw kholer but its also portable. Is 23 amps enough to run my chargers. I would not need a customer enclosure for this one either as its super quiet and comes on wheels.

That's 23 amps at 240 volts AC. You would need a transformer to step it down to the 120 volts (at up to 46 amps) that your inverter needs.

Note: that generator can make 46 amps at 120 volts, but only by using two or more of its outlets. The most amps you can get out of one outlet at 120 volts is 30 amps. That's about 3600 watts.

btw, the honda inverter generators are fantastic generators. I used one to build my house and was able to use its full power at 120 volts... but we had our saws, air compressors, etc plugged into two of its outlets.

--vtMaps

rbell

Re: Generator sizing for new system

Ok so this one is not a good choice then or it is and I just need the transformer? I the inverter series the right choice? That explains why my current Coleman 6500 which is now broke always set off the breaker.