Generator sizing & System data logging

Rossman · November 2013

Hi folks,

Had a couple more questions was hoping to get some feedback on.

1) How do you correctly size a generator for a system? Does it need to be able to output C/10 amps (at what, bank voltage)? I'm a little unclear on exactly how you size a generator to fit your system, and while the "Choosing a good generator" thread has lots of good info, I didn't see this addressed. Also is there a difference in sizing if you have an inverter generator vs. your standard gennies?

2) We are considering going with the Magnum MS4448PAE inverter. I saw there is some cool "web logging" module for this inverter but it requires an "always on" internet connection which may or may not be possible in my location. Has anyone "hacked" this to log data locally? Or is there perhaps a simpler data logging module which you can just hook up to a local PC?

Thanks, happy Friday!
mark

jcheil · November 2013

Re: Generator sizing & System data logging

How large of a battery bank do you have and at what voltage?

Blackcherry04 · November 2013

Re: Generator sizing & System data logging

There are two main reasons to have a Generator 1) to charge, 2) to run loads. Sized correctly you can do both with the same one and it requires less intervention because it'll have more bells and whistles. Having two requires more intervention, but you also have a emergency backup for your backup. Until you nail down everything about you system it's hard to say. Fuel is another big consideration to think about. Two way to look at short use with higher power output and more fuel or longer and slower at less fuel and lower output. At 48 Volts you need to figure what is a significant recharge rate that you can live with. Lately I have found that the longer and slower route works better for me. Generators have gotten better and with the advent of Inverter Generators it's a no brainer to do.

Rossman · November 2013

Re: Generator sizing & System data logging

jcheil wrote: »

How large of a battery bank do you have and at what voltage?

It's not 100% final but it's looking like a 1300Ah @ 48VDC bank. Still in the planning stages, just want to make sure I got stuff straight in my head before getting too committed one way or the other, or buying something that won't serve my purpose.

Thank you.

jcheil · November 2013

Re: Generator sizing & System data logging

Well, let's see if my math is right from what I have learned here in the past month of so (please correct me if I am not):

Assuming, 48v battery bank, Charging voltage at 58.8v, Battery Charger Efficency of 80%, Battery Charger Power factor of 0.65 and a max loading of the generator at 80%:

Charge Charger Gen
Rate % Size(A) Size(w)

0.05 66 9,329
0.10 132 18,658
0.13 172 24,255
0.20 264 37,315
0.25 330 46,644

Rossman · November 2013

Re: Generator sizing & System data logging

Wow, 18kW eh? Substantial...

jcheil · November 2013

Re: Generator sizing & System data logging

Rossman wrote: »

Wow, 18kW eh? Substantial...

That's a VERY large battery bank you need to charge.

Blackcherry04 · November 2013

Re: Generator sizing & System data logging

You have to add you solar contribution generally a back up generator is used to bulk up and make up for shortfalls. Having a C/10 is probably way overkill if you have a Inverter Generator with a variable throttle the 80% loading is some antique notion , gone like the buggy whip. Winter in Ontario is brutal, panels are cheap and fuel is expensive, size your array accordingly.
.

Sun Dog · November 2013

Re: Generator sizing & System data logging

Your battery is 2X mine and I have a 10kW diesel generator. If I was doing it again I would go with a 5kW or 6kW diesel. With your charger set for C/10 you are looking at 130A x 60V = 7800 watts. Then add whatever your typical load at the house will be plus a little headroom for something like a fridge to kick in.

I charge my bank regularly, at this time of the year my generator is on for 1-3 hours almost every day. So for me, the bulk cycle only lasts 30-45 minutes after which my charging amps start to drop off. Not to discourage you from the Magnum, but an inverter/charger with generator support further lowers your generator size requirement and subsequently the amount of fuel you will burn over the years. Charging batteries is a function of current and time. It is no fun watching some big generator churn away during the long slow end of the charge cycle.

How much solar do you think you will need, something like 6-8kW? If you put in X amount of solar I think X-2X is a good guideline for a generator.

jcheil · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

With your charger set for C/10 you are looking at 130A x 60V = 7800 watts.

From what I have learned here it is not that simple. You need to adjust for the the charger efficency rating, the power factor rating of the charger (which makes a big difference) and not exceeding 80% load on the generator. You'll never even come close to running a 130A 48v charger on a 7800 watt generator.

Rossman · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

I charge my bank regularly, at this time of the year my generator is on for 1-3 hours almost every day. So for me, the bulk cycle only lasts 30-45 minutes after which my charging amps start to drop off. Not to discourage you from the Magnum, but an inverter/charger with generator support further lowers your generator size requirement and subsequently the amount of fuel you will burn over the years. Charging batteries is a function of current and time. It is no fun watching some big generator churn away during the long slow end of the charge cycle.

Which inverter/chargers have generator support? I think the new Magnum's have this? While I understand the principle (combined battery supply + generator supply = total load) I'm not sure I understand how this would allow you to have a smaller generator for charging? Unless you are saying that, by going gen support route you can have smaller battery bank and therefore a smaller gennie?

Sun Dog wrote: »

How much solar do you think you will need, something like 6-8kW? If you put in X amount of solar I think X-2X is a good guideline for a generator.

Yeah, around 8kW seems like is the going amount in Ontario, if you don't want to live life like Ed Begley, Jr.

NorthGuy · November 2013

Re: Generator sizing & System data logging

It all depends on how you use it. Possibilities are endless and generator must be sized accordingly.

It is incredibly more efficient to feed your loads directly from the generator rather than chgarging batteries and feeding loads from batteries. So, you may decide to run your generator when you have some loads to feed.

You can decide to charge fast (if you don't like the noise), up to C/5, or you may decide to charge slow (to pick up more loads along the way).

So, you design a clever method to run your generator, and then you size it.

Sizing is easy. Generator load consists of

a) battery charging (e.g. you want 130A for your batteries. 130x55 = 7.2kW / 0.85 (inverter efficiency) = 8.4 kW for your batteries

b) max house loads fed by the generator at the time of charging (e.g. 5kW)

Then you sum this up and come up with (8.4 + 5) = 13.4 x 1.2 (margin) = 16kW. Numbers are totally out of the blue, just to illustrate calculations. The exact numbers depend on your method of starting your generator.

Then you look at your 16kW generator and figure that you need to run above 50% (8kW). People say that running below 50% is bad for the generator and inefficient. In the example above, this is not a danger.

Sun Dog · November 2013

Re: Generator sizing & System data logging

jcheil wrote: »

From what I have learned here it is not that simple. You need to adjust for the the charger efficency rating, the power factor rating of the charger (which makes a big difference) and not exceeding 80% load on the generator. You'll never even come close to running a 130A 48v charger on a 7800 watt generator.

Agreed, if a charging current of 130A is going to be used then I would suggest a 10-12kW diesel generator. My system is set at 135A and 29.8V and I often turn on 3000 watts of electric heaters, and use the microwave during the bulk cycle. All this plus the regular loads adds up to ~8500 watts off of my 10kW generator. If I were to half the load, remove the heaters and the microwave, could I not get away with a generator half the size?

Rossman · November 2013

Re: Generator sizing & System data logging

Too bad the Honda EU series doesn't go that big ;(

Looks like a Generac propane model might be more suitable for us as we will have a large propane tank on site anyway, then I don't have to fuss about with diesel...

Sun Dog · November 2013

Re: Generator sizing & System data logging

Rossman wrote: »

Which inverter/chargers have generator support? I think the new Magnum's have this? While I understand the principle (combined battery supply + generator supply = total load) I'm not sure I understand how this would allow you to have a smaller generator for charging? Unless you are saying that, by going gen support route you can have smaller battery bank and therefore a smaller gennie?

I know the Schneider XWs have it but I'm not up to speed on other manufacturers. A smaller generator, not a smaller battery. It allows the flexibility to get extra power from the batteries if more is required than the generator can supply. I wouldn't design a system with the intention of using this on a regular basis but it does allow you to run a generator closer to it's max because you don't have to worry about kicking it out when a water pump kicks in, or someone turns on a blow drier during the bulk stage.

NorthGuy · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

I know the Schneider XWs have it but I'm not up to speed on other manufacturers. A smaller generator, not a smaller battery. It allows the flexibility to get extra power from the batteries if more is required than the generator can supply. I wouldn't design a system with the intention of using this on a regular basis but it does allow you to run a generator closer to it's max because you don't have to worry about kicking it out when a water pump kicks in, or someone turns on a blow drier during the bulk stage.

But even XW6048 will not give you 130A of charging current. It can do 100A, but it is not very good to push equipment to the limit. I successfully use 80A.

Outback Radian is probably the bggest inverter/charger.

Sun Dog · November 2013

Re: Generator sizing & System data logging

Rossman wrote: »

Too bad the Honda EU series doesn't go that big ;(

Looks like a Generac propane model might be more suitable for us as we will have a large propane tank on site anyway, then I don't have to fuss about with diesel...

Well you wouldn't be able to charge at 130A but you could get away with the EU6500 (flame away

), though if it was me I would go bigger. Diesel isn't so bad, just have a dedicated tank for it. Be sure to install temperature regulated intake and exhaust vents and use a thimble in the wall for the exhaust.

Sun Dog · November 2013

Re: Generator sizing & System data logging

NorthGuy wrote: »

But even XW6048 will not give you 130A of charging current. It can do 100A, but it is not very good to push equipment to the limit. I successfully use 80A.

Outback Radian is probably the bggest inverter/charger.

Good point. I think the MS4448PAE is only rated for a max of 60A. It would be really good if you could get your loads down, which would allow for a smaller battery. What do you expect your worse case (winter I imagine) daily loads to be? Have you lived off-grid before, are you off-grid now?

Rossman · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

Good point. I think the MS4448PAE is only rated for a max of 60A. It would be really good if you could get your loads down, which would allow for a smaller battery. What do you expect your worse case (winter I imagine) daily loads to be? Have you lived off-grid before, are you off-grid now?

Never lived off-grid, and not sure what the worst case winter loads will be.

We are building a new house next year on our lot (with no hydro) so there are a whole lot of unknowns! Kind of intimidating.

Also yeah the MS4448PAE is 60A but I will have two paired which I guess is 120A combined?

westbranch · November 2013

Re: Generator sizing & System data logging

as a theoretical exercise including SWMBO:-)... look at what a battery of the same Ahr rating but only 24v would now be able to power for you and compare that result to what you have currently planned....
something will have to go or give, ie smaller size, that should indicate to you what you NEED and what you really just WANT.

It is amazing just finding out which of life's pleasures you like vs want...vs NEED.
enjoy!:cool:

Sun Dog · November 2013

Re: Generator sizing & System data logging

Rossman wrote: »

Never lived off-grid, and not sure what the worst case winter loads will be.

We are building a new house next year on our lot (with no hydro) so there are a whole lot of unknowns! Kind of intimidating.

Also yeah the MS4448PAE is 60A but I will have two paired which I guess is 120A combined?

120A, yes but as previously mentioned you don't want to design the system based on using the max. Intimidating...absolutely, but 100% possible.

What did you base your battery bank size on? Roughly where about in Ontario will it be? PVWatts is a great tool and has 10 different Ontario locations to compare to.
http://rredc.nrel.gov/solar/calculators/pvwatts/version1/

System design starts with knowing what your loads will be. Based on that you determine your battery size which in turn tells you how much solar/wind you will need.

jcheil · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

Agreed, if a charging current of 130A is going to be used then I would suggest a 10-12kW diesel generator. My system is set at 135A and 29.8V...

Yes, but that is a 130a (24v) charger. A 130a (48v) charger is going to need (roughly) twice the power from the generator to run it. Which is why, with all the system deratings, it would need around an 18k generator.

Blackcherry04 · November 2013

Re: Generator sizing & System data logging

More Panels, More Panels, More Panels, 25 years of no care, no maintenance, no fuel and the best ROI out there these days.

Rossman · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

120A, yes but as previously mentioned you don't want to design the system based on using the max. Intimidating...absolutely, but 100% possible.

Agreed, I definitely believe it's possible.

Sun Dog wrote: »

What did you base your battery bank size on? Roughly where about in Ontario will it be? PVWatts is a great tool and has 10 different Ontario locations to compare to.
http://rredc.nrel.gov/solar/calculators/pvwatts/version1/

Ottawa, ON, is like 45min north of my location.

Sun Dog wrote: »

System design starts with knowing what your loads will be. Based on that you determine your battery size which in turn tells you how much solar/wind you will need.

While I agree that this is ideal, for me there are some things which are "unknowable" to a large degree. I have never had a well pump, nor do I know what depth I'll have to pump from as the well hasn't been drilled yet. So there are some things which I'm not going to know and so what I am trying to do is buy the max system my budget will allow and work within whatever that affords. From what I have been able to determine, and this is guessing a couple of the unknown loads, I'm in around 10-12kWh/day. There are a few areas where I can still save energy so I'm using that range as my upper usage (worst of winter).

Rossman · November 2013

Re: Generator sizing & System data logging

westbranch wrote: »

as a theoretical exercise including SWMBO:-)... look at what a battery of the same Ahr rating but only 24v would now be able to power for you and compare that result to what you have currently planned....
something will have to go or give, ie smaller size, that should indicate to you what you NEED and what you really just WANT.

It is amazing just finding out which of life's pleasures you like vs want...vs NEED.
enjoy!:cool:

I am not a young man anymore, I already understand lifes wants and needs. I don't want to live like Ed Begley Jr, though I recognize that other people do and wish them well with it.

westbranch · November 2013

Re: Generator sizing & System data logging

Agree, we are both retired, so know what we have and what we need, however there are some places to shave power use such as a single slice toaster rather than the 2 or 4 slice ones, still looking for a single one or will have to modify a 4 slice on so that when 1/2 piece is needed, that is all the power yo need to use.
Another is LED's vs CFL's I planned for all 9 watt CFL's knowing that I will switch out to LED's when the price drops a bit more.

Most important , to me at least, is the coffee pot in the AM, so we located its home as close to the bedroom as possible. Coffee gets made in a french press, so we use a thermos to hold the coffee in till a second/third/fourth cup is needed, rather than a hot plate type.
It is amazing how the little things add up, especially lights on during our short winter 'days'.

For the well use an exaggerated 300 gals per day,(the old number was 100 gls per person per day avg if you are a heavy user, new plumbing will use a lot less) see how much the pump delivers and at 110v and 15 amps figure out the time needed to pump that much water. Those values are above what will actually be needed but will fudge for the number of starts that will actually occur. should be close.

HTH

Sun Dog · November 2013

Re: Generator sizing & System data logging

jcheil wrote: »

Yes, but that is a 130a (24v) charger. A 130a (48v) charger is going to need (roughly) twice the power from the generator to run it. Which is why, with all the system deratings, it would need around an 18k generator.

My backup generator is a 5kW gasoline powered Champion and it is able to charge my system at 135A and run the 100-200 watts at the house. When using the Champion I ensure that no other big loads come on during bulk (unplug the fridge etc). I haven't bothered to enable gen support on my inverter but if I did (and yes I should) then even loads like the fridge shouldn't be a problem.

Not trying to dismiss efficiency and PF ratings, just sharing my own experience.

Rossman · November 2013

Re: Generator sizing & System data logging

So, no takers on the Magnum inverter monitoring question?

Ideally I could just connect my PC to it via serial cable or ethernet and capture all the stats/data?

jcheil · November 2013

Re: Generator sizing & System data logging

Sun Dog wrote: »

My backup generator is a 5kW gasoline powered Champion and it is able to charge my system at 135A and run the 100-200 watts at the house. When using the Champion I ensure that no other big loads come on during bulk (unplug the fridge etc). I haven't bothered to enable gen support on my inverter but if I did (and yes I should) then even loads like the fridge shouldn't be a problem.

Not trying to dismiss efficiency and PF ratings, just sharing my own experience.

You must be running at 12v - Yes/No? Because mathematically, unless you have a near 100% efficient charger and perfect 1.0 power factor, I don't see how mathematically you could get a 135a charger at 48v to run on a 5kw generator. Even a 24v charger that would be doubtful at a full 135a and if did work would be pushing the generator to the limits.

mike95490 · November 2013

Re: Generator sizing & System data logging

For your loads, what is your monthly electric bill now ? A new, efficient house, should not be worse.

Well pump considerations.
a) conventional pump and motors ($400) have a pretty high starting surge requirement, and will need a large inverter to
start it. Repair parts are common. 3 wire units (starting cap above ground at the pump controller)

b) there are some soft start pumps, but they cost more, and harder to find repair parts for.

c) Solar / DC pumps have their own MPPT controller, DC/AC drive controller, and pump & motor. Some models, it's all
underground in the well. Pretty expensive.

d) pressure tank (pump cycles on and off as you use water) or gravity feed from a elevated tank. only 1 or 2 pump cycles a day.

I've got the Xantrex gear, and it took a while to get the Gen support fine tuned, but my 3KW diesel can power everything, and charge a 48V bank at about 30 amps. And if more loads kick in, the inverter dials back the charging, till it actually begins helping the generator. Just remember to allow the generator to cool down when finished, don't just shut it off, or the alternator windings will overheat from residual heat and no cooling fan.

Sun Dog · November 2013

Re: Generator sizing & System data logging

jcheil wrote: »

You must be running at 12v - Yes/No? Because mathematically, unless you have a near 100% efficient charger and perfect 1.0 power factor, I don't see how mathematically you could get a 135a charger at 48v to run on a 5kw generator. Even a 24v charger that would be doubtful at a full 135a and if did work would be pushing the generator to the limits.

24V. It is getting close to it's limit but only for 30-45 minutes until the charger goes into absorb. Clearly it is a choice that everyone has to make but all generators have a max and lower continuous rating. I am comfortable running close to the limit of the continuous rating for a relatively short time. By the end of the charge cycle the charger is only asking for 13A@29.8V =~400 watts and that isn't particularly healthy for my 10kW diesel either.

Generator sizing & System data logging

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Generator sizing &amp; System data logging

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Generator sizing & System data logging