Generator Sizing

cdre · May 2013

Wondering if anyone has considered running 2 generators of different sizes for their offgrid systems?

I'm thinking it would be best to have a larger generator capable of handling the house load as well as charging the battery bank when autostart kicks triggered by heavy loads or low SOC. BUT, I assume loads on the generator will be significantly smaller than that most of the time. With generators being terribly inefficient below 50% loading, I'm wondering if I'd run a smaller generator enough to justify investing in a good one.

My thoughts on installation - a manual transfer switch feeds the inverter system which triggers the large genset with autostart. The smaller generator would be manually started and manually switched. I'm thinking of a 8 - 10kw diesel genset for primary with a 3.5 - 5kw secondary. Likely running inverters capable of 6kw or so with worst case surges being water pump, refrigerator and washing machine all on.

Does this make any sense?

cdre · May 2013

Re: Generator Sizing

BTW, I already have two 3.5kw gas gensets which would be backups... But with diesel being lower cost than gasoline in Belize and with the better efficiency, wondering if it would be a good investment.

Cariboocoot · May 2013

Re: Generator Sizing

Brakes on.
Are you talking about running two gens together with their AC outputs coupled? That's a no-go.

Are you talking about having a large generator to use when occasionally needed and a small generator for charging/light loads on cloudy days? That's pretty much standard.

Most of us use quite small generators unless we have really big systems. You'd be amazed what you can do with a small gen and the right small inverter (use gen support to couple the two to act as one large power source, for instance).

Until you know what your loads and usage patterns are and get the rest of the system designed you can't really pick a gen. If you have to use a large gen a lot then the lower fuel cost will be a major factor. If you only need a small gen occasionally then it isn't.

mike95490 · May 2013

Re: Generator Sizing

3 generators here,
5,000w 3600 rpm screamer, that can fit into the truck and power the welder
2,500w 650 rpm diesel, main genset
2,000w inverter genset, auto throttle, for long, extended run times.
All can be connected to the main panel, and either run loads directly, or recharge via the AC2 input of the inverter. Also have an Iota 48V charger that can connect to batteries to recharge them.

cdre · May 2013

Re: Generator Sizing

Definitely not thinking of running 2 gens to feed the same load. The phases would never sync and I'm sure I'd let magic smoke out of everything connected. That's why I was thinking of a manual transfer switch to eliminate such a situation all together.

I'm going to have a big system and big loads. Likely around 11kwh daily with a 7kw array and leaning toward an Outback FP2 (6kw inverter). I'm starting to think that if the generator is triggered to come on, between charging and handling house loads, I'll probably be loading the generator pretty well. Especially since (AFAIK) the Outback inverters don't share the load when receiving AC - they pass it over to the gen and add to the gen's load with battery charging. (Am I completely wrong in my understanding of this?)

BB. · May 2013

Re: Generator Sizing

On thing to watch with some Outback inverters families... There are some designed for grid use and have very tight AC input tolerances (60 Hz +/- 1 Hz or tighter) and these will not sync up and charge with most gensets.

-Bill

Cariboocoot · May 2013

Re: Generator Sizing

Standard Outback off-grid inverter (VFX/FX) use a relay to transfer loads to the gen when they switch to charge mode.

The grid-tie versions (G prefix) can sync to the gen for gen support output to loads (this has recently been proven to work by a member of the forum, despite claims to the contrary). Whereas it is true they have tighter AC input tolerances (for obvious reasons) they can be programmed "looser". Using an invert-gen eliminates the problem altogether.

Do not confuse your daily Watt hour requirement with the maximum Watts drawn at any one time. The former is key for sizing battery bank & array, the latter key for sizing inverter & generator. Just because you need a 6kW inverter does not mean you need a 6kW (or larger) generator.

Four basic ways of using a generator off-grid:

1). Generator powers stand-alone battery charger, started as-needed. This is used for systems without a built-in charger. Normally it would be manual start but auto-start can be an option.

2). Generator connected to AC IN on inverter-charger. Manually or auto-start as needed. Generator handles loads and charges batteries.

3). Generator AC coupled to GT type inverter-charger. Auto-start as needed; gen can charge batteries and handle loads or work with the inverter to power large loads.

4). Generator connected via manual transfer switch to totally bypass inverter function and provide all AC. This may be used with a stand-along charger (as per #1) or not.

A few variations on the basic themes are possible of course.

westbranch · May 2013

Re: Generator Sizing

Just a thought about location and availability: would it be better to use 2 smaller inverters (stackable 3048 size) than 1 big (6048 ) and split your house wiring into 2 parts, I am assuming there will be 2 ,or more, A/C units, so this might be more versatile than the one big one needed when 2 units kick in at the same time...??

hth

Cariboocoot · May 2013

Re: Generator Sizing

westbranch wrote: »

Just a thought about location and availability: would it be better to use 2 smaller inverters (stackable 3048 size) than 1 big (6048 ) and split your house wiring into 2 parts, I am assuming there will be 2 ,or more, A/C units, so this might be more versatile than the one big one needed when 2 units kick in at the same time...??

hth

You wouldn't even have to split the wiring with two FX3048's; they can be stacked parallel with one idling (<6 Watts) until needed. But they would be more expensive than a single XW 6048.

cdre · May 2013

Re: Generator Sizing

Thats what I'm thinking - an FP2 (two 3048's). Guessing my generator situation would follow #2 above. "Generator connected to AC IN on inverter-charger. Manually or auto-start as needed. Generator handles loads and charges batteries."

cdre · May 2013

Re: Generator Sizing

Not sure what I'll do as yet with these calculations, but I was playing with forecasting daily loads and overlaying average PV generation based on PVWatt's estimates for my area. I'll need to find a way to really maximize natural ventilation or invest in a serious array. The input on these estimates assumes a 14kW array.

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cdre · May 2013

Re: Generator Sizing

Also, for my calcs I averaged each month's daily patterns and overlaid them in a graph. Not a huge difference across the seasons, so I averaged those curves together for my typical daily forecast.
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Cariboocoot · May 2013

Re: Generator Sizing

Do you think you might be over-thinking it a bit?

People who don't leave themselves wide margins for error invariably regret it.

cdre · May 2013

Re: Generator Sizing

Not really trying to design a system to those specific specs. I just like playing with excel (6 sigma guy in a previous life). Moreso curious as to what goes in the batteries and what's consumed... Forecasting my needs over the day has actually shown me where I expect my loads to grow. Better to figure that out now than after I've put a system in...

NorthGuy · May 2013

Re: Generator Sizing

cdre wrote: »

Also, for my calcs I averaged each month's daily patterns and overlaid them in a graph. Not a huge difference across the seasons, so I averaged those curves together for my typical daily forecast.

There will be a big difference between sunny and cloudy days, which (unless you're in Arizona) surpasses differences between seasons.

In your modeling, you cannot regard batteries as a place where you put energy and then take it back. Batteries require some structure in their charging - bulk, absorption, etc. Your day may go completely different if you strat from batteries that are discharged more/less than unsual.

Your water pump consumption is surprisingly low. But anyway, with solar it's better to run it during day time.

IMHO, if your solar is strong and loads are small, you'd be better off with a big generator that will charge batteries and support loads. It will not be underloaded because battery charging is a huge load. That's what I do. I also bought a small generator, but I don't use it. It is only for emergency.

However, if your solar is not that strong, and you anticipate occasional big loads (say cooking range or cloth dryer), it will be more beneficial to run a small generator for these big loads only. This gives you huge savings because supporting loads directly is much more efficient than charge battery and then support loads by discharging them. Chris Olson (who posts here) does this with great success.

stephendv · May 2013

Re: Generator Sizing

You could also consider splitting the array into SE and SW facing to flatten the PV production over the day and increase it early morning and late afternoon.

ChrisOlson · May 2013

Re: Generator Sizing

While we have two generators, we only use one. The other one is backup in case something happens to the primary one.

I see too many unknowns for your installation yet to figure out what size generator you need. With the Outback FlexPower 2 you don't have gen support so you're going to need a bigger generator which is going to burn more fuel - or like you say, buy two of them for different loads. With other inverters like the XW that have Generator Support you can use a smaller more fuel efficient generator instead of buying two for different sized loads.

You really need to have some concrete baselines set, like what inverter you got, what size is your battery bank, what are your average loads, peak loads, daily consumption, and ability to use Gen Support or not to meet peak loads. That all determines what size the generator is going to be. Whether it's fueled by gasoline, diesel fuel or LP gas doesn't make any difference.

Theory is fine to try to put a system together. But in theory there is no difference between theory and practice. In practice there is. So keep in mind that using theory to try to put a system together is more than likely going to end up being a disappointment.
--
Chris

Mangas · May 2013

Re: Generator Sizing

We run just one Cummins Onan 25 kw propane water cooled genset. Common fuel and LP storage shared with the house. Our philosophy was the system is big enough not to run the genset very often during the year (about 60 hours total) but enough to run everything when needed. Fuel use is small considering it runs so infrequently. The major cost is the actual investment in the genset but amortized over 20 years is really peanuts given the peace of mind it delivers.

Redundancy wise, we built out the house into two zones in the event we lose two stacked inverters on one side, we still have two stacked inverters and batteries on the other to run the A/C (compressor), heat (air handler), lights on the other half. Each zone has its own dedicated A/C compressor and air handler. Idea being genset is backed up in a way by splitting the system the house relies on.

Don't know if this thinking is right but so far it works well for us.

Cariboocoot · May 2013

Re: Generator Sizing

I'm glad I designed my system to be as much solar as possible and only run the gen when absolutely necessary: gasoline is now $1.35 per litre, or $5.13 gallon US. Solar still costs the same.

NorthGuy · May 2013

Re: Generator Sizing

Cariboocoot wrote: »

I'm glad I designed my system to be as much solar as possible and only run the gen when absolutely necessary: gasoline is now $1.35 per litre, or $5.13 gallon US. Solar still costs the same.

You're exaggerating. It's only $5.11 CAD/US gallon, or $5.06 USD/US gallon

Cariboocoot · May 2013

Re: Generator Sizing

NorthGuy wrote: »

You're exaggerating. It's only $5.11 CAD/US gallon, or $5.06 USD/US gallon

$1.359 * 3.8 litres per US gallon = $5.1642
I let the currency exchange go, as we tend to hover around par these days plus or minus a few pennies.

NorthGuy · May 2013

Re: Generator Sizing

Cariboocoot wrote: »

$1.359 * 3.8 litres per US gallon = $5.1642

$1.35 * 3.785 = 5.1098

I have heard that farmers in Alberta can buy red fuel for off-road use at much cheaper prices. Diesel for sure. I'm not sure about gasoline. May be you can do that too?

We now only use 91 grade for all our needs because that's the only one you still can find without ethanol. It would probably be around $6/gallon in BC.

northerner · May 2013

Re: Generator Sizing

It's much cheaper to run on natural gas than gasoline or diesel (if you have ng to your property) Almost everyone has ng here, even most of the farms throughout the PR region.

Cariboocoot · May 2013

Re: Generator Sizing

We have off-road fuel here, but it's not available everywhere and it isn't much cheaper than regular. You can only buy small amounts without a farm license. We also have the ethanol blends, but not available everywhere either.

Wouldn't use it in any small engine of course.

As such I don't store gasoline I "keep it on hand" as every piece of power equipment gets fed the same poison, including the vehicles.

ChrisOlson · May 2013

Re: Generator Sizing

NorthGuy wrote: »

We now only use 91 grade for all our needs because that's the only one you still can find without ethanol. It would probably be around $6/gallon in BC.

I use premium non-ethanol gasoline in the generators and lawnmower engines, and our 40hp outboard for our small boat. The non-ethanol premium is currently $3.91US per gallon here. The small engines seem to run too lean on E10 or E15 fuel.

Like Mangas, our generator only runs infrequently for peak load management. And that's not very often in the summer time because we don't use the clothes dryer, rarely use the oven in the range in the summer, and with the longer days our power consumption drops to only about 18-20 kWh/day. I don't even run the wind turbines in the summer unless we get a cloudy day and need the power from them.
--
Chris

techntrek · May 2013

Re: Generator Sizing

cdre wrote: »

Wondering if anyone has considered running 2 generators of different sizes for their offgrid systems?

I'm thinking it would be best to have a larger generator capable of handling the house load as well as charging the battery bank when autostart kicks triggered by heavy loads or low SOC. BUT, I assume loads on the generator will be significantly smaller than that most of the time. With generators being terribly inefficient below 50% loading, I'm wondering if I'd run a smaller generator enough to justify investing in a good one...

This is my plan at home (for emergency power). I have a 12 kw LPG for the water heater and well pump, but only run that 2 hours at night to get the water warm, and 2 hours in the morning during showers. The rest of the time I use the equivalent of an inverter-generator to handle base loads and normal stuff like TVs, lights, etc. I recently bought a Yamaha inverter-genset mostly for my camper but it is also my backup-on-backup.

Generator Sizing

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