Demonstration of Generator Support

Re: Demonstration of Generator Support

You're welcome.

I would bet that if you did a random survey and asked people what the most expensive thing is about living off-grid that you would not get the right answer from most folks. It is batteries. Not solar panels, or inverters, or controllers, or generators, or fuel in the generators. Batteries.

Sandia Labs did a study a few years back on off-grid deep-cycle batteries related to DoD and battery life using Rolls-Surrette batteries. They determined that the amount of usable energy gotten out of a battery over its life is greater if you deep cycle it vs shallow cycle it. I'm sure the study could be googled up for those who are interested. So people who buy extra battery capacity to prevent cycling them below 70-80% SOC are actually spending more money over the long term on batteries than those who deep cycle them to 50% and replace them more often.

Where generator support in an off-grid inverter comes in, is allowing you to buy the bare minimum in battery capacity to run your normal daily loads and let the generator help with the big stuff instead of just throwing more batteries and solar panels at the problem. Many folks consider it the "pinnacle" of off-grid to not have to run their generator. In reality, that generator fuel is a lot cheaper than the extra battery (and RE generating) capacity it takes to never run the generator.

In most off-grid homes the normal loads are pretty light, and the heavy draw stuff is very intermittent. Instead of spending the extra money on batteries and multiple inverters to meet the peak demand, generator support is a lot cheaper. So it not only saves you money on batteries, it also saves you money on extra (or bigger) inverter equipment.

And the last thing it allows you to do is save money on generator fuel. Too many off-grid people have a 10, 12 or 14 (and some even over 20) kW generator on a XW6048 inverter or similar. This is ridiculous. The generator should not be any bigger than required to get either maximum charging amps from the inverter, or C/10 charge rate for your battery bank, whichever is smaller. Running a 8 or 10 kW generator to charge batteries with a 6 kW inverter is like using a semi to haul an armload of wood. Inverters that only have generator pass-thru support are limited to either what the inverter can run, or what the generator can run. And that means if you want to power a 6 kW load with a 4 kW inverter you need a 6 kW generator, and you have to run that 6 kW generator for charging when you only need 3,500 watts to max out the charger in the inverter. With gen support you can save the gen fuel and run your 3,500 watt generator for battery charging, and still power that 6 kW load with ease.

These are all the reasons why, to my way of thinking, an inverter that does not have generator support in it is not a real off-grid inverter. I know way too many off-grid people who have over-sized their generator because they got Outback FX-series inverters and need to power an occasional heavy load that only the generator can run.
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Chris

Re: Demonstration of Generator Support

In that whitepaper study that I read that one time they didn't really have any cost analysis in there. It was just a study of cycling on deep-cycle batteries to see what the life was vs cycle depth. The life is shorter when you deep cycle them. But the amount of energy was better with a deep cycle because of the inefficiency of getting from 80% SOC to 100%. So if you deep cycle your batteries to 50%, over the long run you'll get better return on amp-hours in vs amp-hours out vs shallow cycling them, for a tradeoff in shorter life (measured in years) and number of cycles.

They didn't say so in the study - Sandia just presented the data. But one can read into it that if you buy less batteries and work them harder you'll get better return on your investment over the long haul than buying more batteries and shallow cycling them. We could actually use more batteries because ours spend more time from 50-70% than they do from 70-100% SOC, and we can catch them up easily on a good day. But I'm not going to buy any more because if they last me 7 years they're already costing me $100/month.

I figured it out once and we'd have to increase our bank capacity by easily double to run the stuff we run with inverter, that we presently run with the generator. And then I wouldn't have enough RE capacity to charge the confounded thing. At present, during the worst months of the year we might spend more than $100 a month on generator fuel. But over the year's time, it is about $50 a month for generator fuel on average. So buying more batteries, plus the stuff to charge them, would never pay because the batteries are going to go bad eventually no matter what you do, and then you take it in the shorts with a $17,000 bill (at today's prices) to replace a big battery bank.
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Chris

Re: Demonstration of Generator Support

vtmaps wrote: »

Is it worse to be dependent on generator fuel than to be dependent on LP? --vtMaps

I wouldn't call it worse or better - just having the option of using RE power for some things that most off-grid folks will tend to use LP gas for. On a good day with plenty of incoming power we can operate the oven in that range without generator support - it does not overload the inverter unless we have some other larger loads going.

In my demonstration video, even with 700-750 watts load on in the house, it was barely on the edge of the threshold that causes the generator to start. If the house loads would've been 2 amps instead of 6 amps before I turned the range oven on, it would not have started the generator. And that would not be the end of the world because it only takes roughly 5-7 minutes to get the range up to 425 degrees to cook with, then the element cycles off. The battery bank can easily handle that if it's above 60% SOC.

If the Load Amps doesn't start the generator, then one of the voltage sag timers (either the 2 hour or 15 minute) will start it if a heavy load that does not reach the Load Amps threshold persists. The only reason it started on Load Amps in that video is because the load amps was at the threshold where I like to see the generator come online to help out. And learning where that threshold is requires some experience with your system.

If you have a LP fired range you do not have that option. You can have a nice day with all sorts of incoming power, the bank floating - you go to cook something and instead of using what you make with solar panels and wind turbines to "fuel" your range - you're burning a fossil fuel. That's why I've grown to like the electric appliances, despite all the advice I've gotten over the years that for off-grid you should use LP gas. The electric appliances are "dual fuel".
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Chris

Re: Demonstration of Generator Support

It's less energy efficient by BTU input to the generator when you factor in engine thermal efficiency and electrical losses in the generator and wiring. But it's about the same cost as LP because LP appliances are typically only about 50% efficient where electric is >90%.

What makes it pencil out even better is that the cost/BTU of LP is higher than gasoline or diesel fuel. Delivered propane cost here right now is $2.72/gallon for 91,700 BTU. Winter blend gasoline is 110,800 BTU/gallon and the cost is currently $3.09/gallon here. So LPG figures out to 3 cents/thousand BTU. Gasoline is 2.8 cents/thousand BTU.

LPG is usually highest priced during the winter home heating season and it drops in price during summer.

Gasoline goes the other way and usually get more expensive in summer due to more people driving and higher demand. But winter time is when we use this feature the most because we can't use the clothes line to dry clothes outside, and we tend to do more cooking with the range where in summer we will use the charcoal grille outside to do the majority of our cooking. So for most of the summer our generator is in retirement and the inverter starts it about once every two weeks for 30 minutes to exercise it. In the long run, according to the figures I've come up with on it, it competes very favorably with using LPG appliances in actual out-of-pocket cost.
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Chris

Re: Demonstration of Generator Support

Like I explained in another thread here on the forum, the cost/kWh doesn't really matter to us. It's fun to try to put a pencil to it and figure it out. But every kWh we consume easily costs us 3x what people on grid power are going to pay for electricity. It's the part about being independent that we enjoy. We selected our place where we live because we like it and we're not going to spend $168,000 to get a single phase line run to it. After living here for 11 years this coming June without utility power we enjoy what we have built over time, we enjoy where we live, and we would not trade it for someplace with grid power. So whatever it costs, it costs. Living cheap is not what we were after. But we DO want to live comfortable and we have spent a LOT of money to get there.

And also like I said in that other thread, nothing really irks me more than somebody that doesn't know us that comes to our place and sees what we got, then thinks we "got it made". My reply that I've come up is "yeah - spend fifty thousand and you can 'have it made' too."
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Chris

Re: Demonstration of Generator Support

Dave Sparks wrote: »

I can see your point Chris and I can also see Bill's point in asking for a rough dollar per KWH metric. As a business person I sometimes have get to number with some clients.

I've always told people that if they are trying to save money by generating their own power with solar and wind that they're barking up the wrong tree. Hydro power, I think (according to the numbers I've come with on it), is different and you can actually beat grid power cost with a well designed hydro power system. The only thing that has made solar or wind power viable for a lot of folks is government subsidies or income tax credits, etc.. Without those, you are not saving any money.

To me, this photo embodies what off-grid living is all about, and you can do it with the most basic system that gets you by or by going extravagant and living in style - all it takes is money. But you have control over it, do not send a check to a smoke-belching power plant someplace every month, and you are independent and own all your equipment.

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Chris

Re: Demonstration of Generator Support

inetdog wrote: »

I like that analysis! :-)
Just less efficient when running on the LP fuel that a single fuel appliance would be, but you hopefully make up for that when running on electric.

The other thing my wife uses a lot is her 120V slow cooker. She's cooking our supper right now at lunch time - chicken and it sure looks good

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I really think those slow cookers are quite efficient. I think the thing draws about 1350 watts until it gets up to temperature and then it cycles on and off thru the day to maintain the temp. And the cycle-on time to maintain temperature in it is very very short - usually less than a minute at a time. Our system doesn't even appear to know it's there but it will cook supper for us this afternoon and I'll bet it don't use more than 1.5 kWh of electricity to do it.

For whatever reason, this is another thing that I've told other off-grid folks that we know, that we use, and they can't believe it. They have the mistaken notion that their system could never run such a thing. But we used to use the slow cooker even with our old low-powered 12V MSW system that we had for years, and it never seemed to affect it much on a windy sunny day. Batteries still got charged up and we had supper cooked at the same time, many many days.

I've tried to convince folks of this for years - if you're off-grid use electric stuff for pete's sake instead of gas. Your equipment doesn't make gas - it makes electricity. So use it :cool:
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Chris