Off Grid System with Propane Generater

System · May 2007

Hi Everyone,

I want to go off grid, but I cannot afford a full fledged solar or wind system. As a way of 'testing the waters', I want to know if a propane generator based system is practical. What I have in mind is the following:

- A 3 kw propane generator
- A battery charger
- 225 Amp hr 12 Volt batteries (between 2 to 4 of them)
- A 12 volt lighting and ceiling fan system connected directly to the batteries
- An invertor to power the 120 Volt system
- Possibly a system monitor and switch to automatically control the generator.
- Possibly a solar cell to reduce demand on the generator.

The generator would run 3-5 hours per day to charge the batteries and the house would get power from the batteries. Additionally, a some appliances, such as heating, would run directly from propane. I estimate that it would cost around $2,500 for start-up and about $2 per day to power the house. As time goes on and I start to save on energy costs, I would invest the savings in either solar cells or wind turbines.

Has anyone tried this or a similar system? Does anyone see any obvious problems or shortcomings?

Thanks,
Ezra

BB. · May 2007

Re: Off Grid System with Propane Generater

First, I would humbly suggest that you get a good handle on how much (kWhrs) of electricity you will need--and if it matters, season by season (AC, water pumping for garden, etc.). Also, if you draw much power from the batteries, there is a pretty good sized penalty for conversion of electricity, charging, and inverterting back (ex: 85% charger, 80% typ. lead acid battery charge, 85% inverter -> 0.8*0.8*0.85= 54% efficiency--or you will need about 2x the kWhrs if using the batteries).

Next, a battery system that can manage your loads for 3 days or so (draining, at most, to 50% of battery capacity) is a good place to start. More batteries are expensive, fewer batteries make it more difficult to ride through solar power "blackouts" (cloudy weather, etc.) and fewer batteries is more charge/discharge cycles, and higher currents (to charge and discharge)--which makes the batteries less efficient and wear out sooner. Also, if you can only run your generator 1 day out of three--you probably would enjoy the quiet.

Next, would be to look at a battery charger for a 3kW generator--for any good sized charger, you will need to make sure that it plays well with the generator--3 kW sounds a bit small if you have a larger charger.

Lastly, conservation of electricity will be the goal... I am guessing, but just in fuel charges you are talking about ~$0.25 to $0.50 per kWhr--so anything you do to conserve power, is worthwhile.

There are things here and there that you can weigh the cost/benefits for... Example, AGM batteries cost more but are 90% efficient to charge vs 80% efficient for flooded cell--plus there is less maintenance (watering, cleaning)--but they are more prone to damaged if overcharged.

I agree with the premise of a small generator--can be more efficient to operate a small one at 80% load than a large one at 20%.

There are folks here that can recommend combo-inverter/chargers/generator controllers and such that work well and pretty much are automatic. But a sine wave inverter is not cheap (probably more on the line of $1 a watt vs the $0.10 per watt of those modified sine wave inverters you see everywhere). And, if you are running a home (motors, electronics, etc.), you will want (perhaps need) to get the true sine wave inverter to keep your appliances happy.

But, until you better describe your loads and needs--it is pretty difficult to give you cost effective answers. I would guess that a "nice" 3 kW generator/inverter/charger/battery/controller setup is going to be several times your $2,500 cost unless your "true" electrical needs are small (or you want to start with a piecemeal system). Remember that batteries are a weak spot in most systems--if you accidentally deep discharge them (or have poor maintenance), they can quickly be killed. If you are not the one always watching charge levels, starting, and maintaining the batteries/generator (wife, kids) for a manual system--it is easy to see early battery replacement down the road.

Where are you doing this? Do you have sun/wind available with little shade/wind obstructions for panels/turbines? Snow/Hot weather?

-Bill

System2 · May 2007

Re: Off Grid System with Propane Generater

If you are already on grid, I can't imagine how you could save any money at all switching to a generator when all on going expenses are considered, including maintenance and replacement of the generator as it wears out. Unless you get your propane at one heck of an awesome price, I can only see this system costing you more for electricity, not less. With Solar, once the PVs are paid for, they just sit there and produce "free" power for the next 20 or 30 years. A generator will not do that, ever.
That's my opinion, now we'll see what "the experts" say.
All the best
Wayne
PS I see that BB posted while I was making my post. I consider him one of the "experts".

niel · May 2007

Re: Off Grid System with Propane Generater

wayne,
you don't think you're an expert yet?
ezpwr,
being on the grid at this point would be a plus as you can design a system that takes advantage of the grid like a battery would be. you generate an excess to the grid during the sunny periods and during other times you use it from the grid more sparingly. this is a batterlyless gridtie system. you can use batteries for backup power by reworking the system, but this cuts back on the overall efficiency of the system so more power is needed to be generated for status quo and would cost more. generators should be used sparingly and only for backup to a system unless it is a prime longrun genny and even then sparingly. in any case you need lots more bucks to be off of the grid altogether unless you can conserve extremely well.

BB. · May 2007

Re: Off Grid System with Propane Generater

Ezra--

Don't let Wayne "shine you on" (little solar joke there

)--He (IIRC) and a few others here actually either live off-grid or have off-grid capable systems and many years of experience.

Little old me--I life "on-grid" in the middle of a metropolitan area (south of San Francisco, CA) where going off-grid (other than for storms/earthquakes) is not reasonable or cost effective (cheap power, been 5+ decades where power has been off more than an hour or two).

So--if you are trying something to live off-grid in a city. Probably not worth it. If you are creating an emergency backup for grid--again, your best bet is just a small generator and a good fuel source (propane probably being on of the better ones--if you already have it available). If you are building a small system for off-grid installation at a cabin--go for it--but realize that it is not cheap to go "cheap".

I think that almost everyone here has spent some good money on a "starter" system (mod-square wave inverter, a 5 kW Coleman type gas generator, and some trolling batteries). Very quickly you find that almost each and everyone one of these type components have short lives and consume fuel like it is going out of style.

If you are trying to lead a "greener life"--and have Grid power already (or an existing generator system)--first spend the bucks on measurement equipment (like a Kill-A-Watt meter and DVM) and see what you current are using and where you can start saving...

Whether it is new appliances, turning off wall warts/home entertainment systems, converting away from filiment lights, replacing windows with vinyl double pane, new high-efficiency heaters, etc... Doing the conservation dance will make whatever you do down stream that much easier and cheaper.

I may be preaching to the choir here--but it is much better to go solar at 100 kWhrs per month than 1,000 kWhrs per month. The $2 a day in fuel costs seems to indicate that you are in the ~100 kWhr per month range--which if this is your normal "full" consumption is a very good place to be starting from (Best I can do with my home is around 175-200 kWhrs per month--very low for my area compared to a normal house around here--but certainly not great if I was off grid).

And, in the end, if you have grid power available, a Grid-Tied Solar system (like Niel just mentioned) with a simple Honda eu2000i or eu3000i type generator (or propane equivalents--would be better) for backup power is a much better deal (assuming you have net metering available--mostly limited to those living the US).

-Bill

mike95490 · May 2007

Re: Off Grid System with Propane Generater

Batteryless Grid-Tie system is the way to go, if you have reliable grid power.
I think this year (about 7 months into my solar ownership) I will only have a small ( < $200 /year ) electric bill. The last week, my electric meter has been unwinding my winter months usage. ( I hope the billing computer programming algorithm can interpret that correctly)

BB. · May 2007

Re: Off Grid System with Propane Generater

I am assuming that Ez really does want Off-Grid since that was the tittle of his thread--but it will be best to wait for his answer(s).

-Bill

System2 · May 2007

Re: Off Grid System with Propane Generater

My number one goal is to save money. My number two goal is to quickly get a few basic appliances running with little hassle.

The house is in New Orleans, La. It flooded up to the ceiling after the levee failures. We have it gutted to the studs and are about to rebuild it. Right now, there is no electricity there. FEMA installed a temp pole and Entergy N.O. connected it to the grid, but FEMA refuses to issue a permit for me to get the juice turned on. In the short term, we either live without electricity or we rig the pole. In the long term, it seems that electricity bills are going to rise at least 10% a year for the next ten years. All in all, it seems like a good time to consider inexpensive alternatives to the electric company.

In terms of usage, last months electricity bill (for my apartment in Baton Rouge) says I used 195 kWh. If I did my math right, that converts to about 55 amp hrs per day. However, I did a separate calculation based on the power consumption and estimated hours used of the individual items (refrig, lights, washer, Xbox, etc.) and came up with 212 amp hrs a day. (This calculation accounted to the advertized 90% efficiency of the inverter.) I then multiplied this by two to get around 450 amp hrs per day as a number to use in planning the system.

So, I think (but I can be wrong) that if I can just find a cost effective way to charge the two batteries on a daily basis then I am in business. I called one company that carries both chargers and batteries to ask about charge time for a combination of AGM batters and a charger that they list. They told me it would take 3-5 hrs to charge the batteries using a 3 kw generator. The generator consumes about a half gallon of propane per hour and propane costs about a dollar per gallon, so I estimate about $2-3 per day.

In comparison, the neighbors that have rebuild their homes and are living in them pay over $200 a month for the electric bill.

Thanks everyone for the consideration and advice.

BB. · May 2007

Re: Off Grid System with Propane Generater

Regarding the amount of power you are using--at 200 kWhrs/month, that would only be about $25 per month in my area (SF CA).

The price for propane looks to be a good deal for you--For my BBQ, I probably pay almost 5x that price.

I am not quite sure of your math and conversion to amp*hours... Trying it this way (assumptions: 195 kWhrs per month, 12 volt battery bank, 85% efficient charger, 90% efficient AGM battery, 85% efficient inverter):

195,000 watts per month / 30 days per month =6,5000 watt*hours per day (6.5 kWhrs per day)

(6,500 watt*hours/12 vdc of battery) / (0.85 eff of charger * 0.90 eff of AGM bat * 0.85 eff of inverter) * 1/0.50 depth of discharge of battery = 1,666 amp*hours at 12 VDC of rated battery capacity (about the size of 16 car batteries).

Assuming that your running 100% of your power through your batteries/inverter, that would indicate:

6.5 kWhrs per day * 1/(0.85*0.90*0.85) = 10 kWhrs per day (from generator).

Assuming you could run your generator at an average of 80% of rated capacity (genernally, the 3 kW rating is maximum short term, most small generator seem to rate at 80-90% of nameplate for long term use), that would suggest you would have to run your generator for:

10kWhrs/(3kW * 0.80) = 4.17 hours per day.

Are you sure about your price for propane? I would guess it would run closer to the price of gasoline. Assuming $3.00 per gallon:

4.17 hours * 1/2 gph * $3pg = $6.255 per 6.5 kWhrs per day (gen->battery->inverter->AC load)

Or, about, $188 per month just for propane (assuming I am anywhere near the true price for propane in your area).

This is basically $1 per kWhr. Generally the worst I have seen the price of electricity in a wired city in the US was around $0.25 to $0.35 per kWhr. And normal prices in the range of $0.07 to $0.12 in the range of electricity that you are using today.

Even if the propane were only $1 per gallon, just the fuel cost of your system (excluding the price of gen/bat/inverter and long term maintenance--batteries only last ~3-10 years) would still be ~$0.33 per kWhr.

Of course, if the "price" to get power from your pole to your home is excessive (or impossible because of "G" men), then you are stuck with the generator system for now as the best bang for your buck.

However, to reduce both your capital and fuel costs, I would suggest that you group all of your loads into 1) loads that must run 24 hours per day or from battery--fridge, lights, laptop computer and; 2) loads that you can run only when your generator is turned on like A/C, washer drier, large tools, etc.

If you can choose when to run your generator and heavy loads, that will allow you to massively down size your battery bank. Those batteries I suggested would only power you for 1 day--For the recommended three days (reducing the peak battery current increases efficiency, less deep discharges and less frequent charging increase life), you would be looking at three times the AMP*Hour rating of batteries.

Also, you can pick an inverter that has an internal transfer switch... No AC input, it inverts power from your battery bank. With stable AC input, an internal switch brings the AC input to AC output (no inverter losses) and its internal charger will begin to charge the batteries.

Also, if you are trying to use batteries for this large of system, you should be looking at 24 to 48 volt inverter/charger systems. Will make it much easier and cheaper to wire (4x the voltage, 1/4 the current through the wires/battery bus).

I am very sorry to hear what you went through in NOL--and I am even more sorry to hear about how they are making it more difficult for you to rebuild.

Comments, questions?

-Bill

niel · May 2007

Re: Off Grid System with Propane Generater

ezpwr,
know that the $ amount you wish to start with is not going to get you a whole lot more than maybe a quality sinewave inverter/charger and some batteries. that would leave out the generator and pv system. alternative energy is anything but cheap, unfortunately. you can check out the costs of the pvs alone on our host's webstore. look around at inverters, controllers, and batteries along with other needed accessories like wire, fuses, electrical boxes, mounts, etc. while you are at it. http://store.solar-electric.com/solarpanels.html
taking a good guess by your 195kwh/month that around 70-80ah of solar per hr at 24v would work with about 3 and a half hours of sun per day average. double the ahs for 12v.

i modified this when bb pointed out my error as i meant it to be per hr with some leadway on the number of hours of sun on average.

BB. · May 2007

Re: Off Grid System with Propane Generater

Niel,

Maybe I am missing something:

195,000 watt*hours per month / 30 days per month = 6,500 watt*hours per day (or 6.5 kWhr per day)

80 Amp*Hours * 24 Volts = 1,920 Watt*Hours (per day?)... or 57.6 kWhrs per month

Off by a factor of ~3.3 (excluding inefficiencies in charger/battery/inverter system)... Or are you looking at EzP's 212 Amp*Hours and assuming that was at 12 volts?

EzP,

I guess my confusion is your mixing of Amp*Hours and kWatt hours. Unless one knows the voltage, Amp*Hours is an incomplete description of the work being done...

Watts = Amp * Volts
Watt*Hours = Amp*Hours * Volts

195,000 watt*hours per month / (30 days * 120 VAC) = 54 Amp * Hours (close to your 55 amp*hour number)

But that is at a 120 Volts (AC or DC does not matter at this point)... If this was at 12 VDC, it would be about 540 Amp*Hours (all of this based on 195kWhrs per month average usage divided by 30 day per month).

When you said you calculated 212 Amp*Hours a day--I am at a loss--was this at 120 Volts, 12 volts, or something else? Normally, Amp*Hours is used for battery systems where the voltage is known (12, 24, 48 vdc, etc. and it makes sense because battery voltage changes between charging and discharging, but Coulombs of current in or out is relatively consistent measurement of capacity). But when somebody works with 120 VAC appliances, generally it is better to just stick with watts (or kWatts) and Time (in hours).

-Bill

mike95490 · May 2007

Re: Off Grid System with Propane Generater

EzPower wrote:

- A 3 kw propane generator

What engine style is this, diesel, or sparkplug ignition? I ask, because engine runtime will likely be a factor, with the more rugged diesel needing less overall maintenance over a several year period. (yes there are diesel that run propane/natural gas) If it's only used as emergency power, once or twice a month, it won't matter much. Propane is nice because it burns so clean, and does not go "stale" like liquid fuel does.

System2 · May 2007

Re: Off Grid System with Propane Generater

Below is the link to the invertor that I had in mind. While it is expensive, it not beyond my means. What do you folks think about? Can it run a refigerator, mircoware, washer, etc? OR do I need to spend many more times the price?

http://www.theinverterstore.com/the-inverter-store-product.php?model=pwrinv5000w-front-rgb

Thanks Everyone.

BB. · May 2007

Re: Off Grid System with Propane Generater

That is a BIG / inexpensive Inverter... Meaning, it is a modified sine wave type inverter. About 80% of your appliances will work fine on it. 10% will run hot and may die before their time. And another 10% will die within the week. You experiment with your appliances and let us know which ones die so that we don't have use ours.

I am probably overstating the problem. But, motors and some electronic devices really don't like "modified square wave" inverters. Modified Square wave (or Mod. Sine Wave) inverters have a lot of energy in the higher frequency ranges (harmonics). Those are what create the edges on the square wave. A sine wave has only 1 harmonic--the fundamental frequency (in the US, that is 60 Hz).\

Most equipment is designed around the assumption that they will be feed a sine wave--and when presented with something that is not a sine wave, the higher harmonics, typically, are converted into heat (and sometimes an annoying buzzing sound).

Motors are really susceptible to running hot with MSW inverters. Many other devices work just fine--and a few electronic devices will also overheat.

Also note that MSW inverters may have good efficiency numbers (input to output power conversion), but in reality quite a bit of the power is in the higher harmonics--which is usually converted to waste heat. And on battery/solar based systems, you don't want to waste energy.

If you have your heart set on running a microwave oven from your battery bank... I would suggest that you get a very good quality pure (or true) sine wave inverter, sized to be no more than twice your average worst case load, and run your fridge, TV, etc. from that one most of the time. Oversizing the inverter will just waste energy. Some of the inverters have intelligent standby--they draw very little power when everything is shutdown. Then when a switch is closed (or a mechanical thermal switch in a fridge is closed), the inverter will start generating AC power.

If you have some big (and less expensive appliances, like a microwave) get a 1,500-2,000 MSW inverter and only run the Microwave and hand tools (drill, saws, etc.) with it. When running the tools, you will always be there to see if they start overheating or acting funny--and microwaves are under a hundred bucks now so it won't kill you if it blows.

Now, back to that 5,000 watt inverter with 12 vdc input... How much current? Glad you asked. Assuming that the inverter will run down to 10 volts (probably 10.5 volts, but use 10 volts for even numbers):

I = P/V = 5,000 watts / 10 volts = 500 amps.

500 amps is a real plate warping current for most smaller batteries (ones that you can pickup by yourself without using friends and/or a crane). You would have to place quite a few batteries in parallel, and use very heavy cabling to get everything to run.

It is certainly doable... But if this was a system/battery bank you wanted to use for years into the future, and you wanted 5kW peak output, you should probably pick 48 VDC battery bus voltage and inverter. That will reduce the current down to 100 amp +/- which is a much easier amount of current to bus/fuse/and use.

To read about MSW vs SW inverters, read here:

http://www.solar-electric.com/solar_inverters/inverters_for_solar_electric.htm

To see an assortment of good quality / good pricing for inverters (as a starting point):

http://store.solar-electric.com/inverters.html

So, my recommendations would be to size/classify your loads. How long you need each to run (or how many kWhrs per day do they take), starting and running load. Then use that information to size your inverter (or inverters). If you want 240 VAC, there are inverters that you can rig-up in pairs. Each produces 120 VAC, but when connected together with a control wire, they will switch in sync and provide both 120 and 240 VAC.

Also, as you classify your loads, you may find that some are not very energy efficient and it may be cheaper in the long run for you to toss that old fridge/freezer and get a new one (perhaps a bit smaller than you are used to) that will allow you to use smaller batteries, smaller inverter, and less fuel to charge the batteries.

Is this helpful to you? Sorry, there is no easy answers, and many times there are no inexpensive answers either.

-Bill

System2 · May 2007

Re: Off Grid System with Propane Generater

I run my water pump off a dedicated 3000 watt modified square wave inverter. The motor draws about 20% more power than on pure sine, but since it only runs for about 30 seconds, the extra power isn't a big problem and the motor doesn't get a chance to get hot.
I also run my "Chest Fridge" (converted freezer) on another dedicated MSW inverter, this one 1000 watts. Again, the motor draws that extra 20% power, but again, the compressor only runs for 6 minutes, then rests for 45 minutes, so doesn't get more then slightly warm. The freezer however is another matter. Since it runs for extended times, it's compressor gets way too hot on MSW, and that extra 20%power really adds up. So, no MSW for the freezer, or a "standard" fridge for me. Also, the low voltage power supply that runs the control circuits in home appliances from washing machines, to bread makers, often use a capacitor/resistor/diode type circuit to supply that needed lower voltage, instead of using the traditional transformer. With these capacitor type power supplies, because the capacitor "passes" the higher frequencies which are present in MSW, will overheat and burn out, sometimes in as little as 30 seconds. Voice of experience. Usually the "fuse resistor" in the circuit burns up, protecting the rest of the circuit from the excess currents and fire. Replacing that fuse resistor with an exact replacement, will repair the appliance which can then be used as usual on PURE SINE power ONLY.
Even chargers for cordless drills and cameras will either not charge properly, or quickly burn out if used on MSW. Yet other things will have no problems.
This is just to give you an idea of what you're up against, using an MSW inverter.
All the best
Wayne

Off Grid System with Propane Generater

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