The big experiement

[randallhilton]

I'm moving forward with taking my suburban homestead partially off grid. Yesterday I took delivery of 600w worth of panels and a xantrex prosine inverter/charger. My plan is to take our refrigerator and a few other gadgets off the grid. Since we have grid power, I can easily adjust the size of the load on the solar system so that I'm using all it will provide. It looks like I can run the system with four 6v golf cart batteries - providing for a couple of cloudy days. If the system seems to run my load and keep the batteries charged, I may add another pair of batteries.

In the event of a real disruption in the power grid, I would reduce the load to necessities only but until that time, I want to shift as much load to the system as possible so it can reduce the electric bill. The way I figure it, this pays better interest than the bank, is safer than the bank, I get to learn about the limitations and hassles of maintaining a self sufficient power system and -- if TSHTF, I have a back up power source. As always, I'm sincerely open for suggestions and wisdom since I'm pretty empty headed about this stuff. ;D

[Anon001]

Are you sure 4 batteries will be enough to run a refrigerator, especially for a couple days of no sun? Keep in mind that you can always add solar panels but you can't add batteries to a current battery bank. If you add batteries to a battery bank, they ALL have to be replaced at the same time. So, you may need to do some calculations to see if 4 batteries are really enough. I haven't done the calculations myself. But, it just seems like not much storage for a refrigerator and other gadgets at the same time.

[randallhilton]

Are you sure 4 batteries will be enough to run a refrigerator, especially for a couple days of no sun? *Keep in mind that you can always add solar panels but you can't add batteries to a current battery bank. *If you add batteries to a battery bank, they ALL have to be replaced at the same time. So, you may need to do some calculations to see if 4 batteries are really enough. *I haven't done the calculations myself. *But, it just seems like not much storage for a refrigerator and other gadgets at the same time.

Actually, I'm not at all sure about 4 batteries. Here's what I'm calculating:
According to the Watt minder, the fridge is using 1.3 kwh a day under normal usage (I'll re-check that during the summer time)

If I figure on 20% system inefficiency (inverter losses etc.) that would be equal to 1560 watt hours a day which, if I'm doing the formula correctly, that's 130 amp hours at 12v.

I think, but I'm not sure, that if I use 2 x 6volt batteries at 220ah each then I'll have 440 ah available. (or am I doing this math wrong?). If I have 4 batteries, I'll have 880ah available so 880 / 130 = 6.77 days of storage.

I could post an armed guard in front of the fridge to prevent my soccer kid from standing in front with both doors open. I could also disconnect the defroster heating element to drastically reduce the load (it pulls about 600 watts while heating).

But I'm unsure of the math I'm using so I'm eager for input. Thanks!

[AlchemyAcres]

Have you considered building a super high efficiency fridge with a DC compressor unit?

This is the fridge that i built.

http://www.boatelectric.com/Nova%20Box%20Building%20Plan%20for%20the%20LT200-F%20Kit.htm

Do a Google search for DC marine refrigeration unit.
You should find lots of info,

~Martin

[AlchemyAcres]

I think, but I'm not sure, that if I use 2 x 6volt batteries at 220ah each then I'll have 440 ah available. (or am I doing this math wrong?). If I have 4 batteries, I'll have 880ah available so 880 / 130 = 6.77 days of storage.


Series wiring (positive terminal of one battery to the negative terminal of the other) will increase volts, but not amps

Your four 6 volt 220 amp batteries, series wired for 12v, will give you a total of 440 amps.

~Martin

[randallhilton]

Series wiring (positive terminal of one battery to the negative terminal of the other) will increase volts, but not amps

Your four 6 volt 220 amp batteries, series wired for 12v, will give you a total of 440 amps.

~Martin


<slapping sound made by hand impacting forehead> Oh, duh. . .that could make a big difference! So, that will knock me back to 3 days worth of fridge (without neutering the defroster). Now I need to ponder whether to put more batteries on board now. I know it will be easier on the batteries so I should give that a high priority.

One of the things I want to find out is how much power is generated on a cloudy day. I already know these panels do a little something, even under a rain cloud so this part will be interesting.

RE: building a super efficient fridge -- I like what you've done! I'm not sure I need to go that route at this level but you have a good set up and it's well documented. Thanks!

[Anon001]

Martin is right. *Yout 2 6v batteries still only give you 220ah as a 12v bank. *However, with four batteries then it doubles to 440. *

The main thing to keep in mind is that you should never draw down your battery bank more than 20% to 25% of its total storage. Doing so will drastically shorten the life of the battery bank. *

Let me do it this way... Hopefully someone will correct me if I get it wrong! LOL

With 4 batteries, you have 440 ah. *Using only 25% at the MOST would give you 110ah to work with. *However, if you produced the full rated power of your panels, it would be 50 ah for the 4 hour peak. *200 ah. *See the problem yet? *You may or may not get 50ah out of your panels in full sun. *Depending on the manufacturer and the quality of the panels, they may only produce 45 ah in full sun. *So, if you have a cloudy day, the generator has to come on.

Hopefully someone will chime in that is better with the configurations than I am.... and correct my errors LOL.

This is why most people on alternative energy use a more efficient refrigerator that is either 12v or propane. I would bet that most go the propane route.

[Dave]

Paul,

I believe you are on the right track.

If you approach this with "best practices" (a term I have come to hate in the corporate world), you will want to have sufficient capacity in both your battery bank and solar panels.

I am not very experienced but if it were me I would plan to have 200% in manufacturer rated capacity in your solar panel array based on your daily estimated power consumption. This should account for cloudy days.

Additionally, based on the 25% usage rule (for optimal batter life) I would expand my array to allow two to three days of run time based on the 25% premise.

This may have added a little to your battery and solar panel costs but maybe not too much!

Dave

[Anon001]

Basically, when someone gets serious about designing an alternative energy system, there are worksheets that list everything you will use per day. It takes into account your "zone" and average sun and wind,etc. then shows you what you need for wattage in panels and amp-hour storage in batteries.

[randallhilton]

The main thing to keep in mind is that you should never draw down your battery bank more than 20% to 25% of its total storage. Doing so will drastically shorten the life of the battery bank. *

Being mindful of the fact that I'm certainly the novice on this venture, the 20% - 25% working limit makes me scratch my head. I've been referring to Rex Ewing's "Power With Nature" book and he seems to claim good success with a 50% threshold with golf cart batteries.

I certainly can't point to personal experience but somewhere there's a point at which getting the proper battery capacity would overwhelm the ability of the panels to properly charge the batteries.

In my particular situation, adding more panels is currently out of the budget but I could boost the battery bank up to 6 if it seems reasonable for 600 (rated) watts is sufficient to run the load as well as keep the batteries happy.

One thing I could do is gamble that the system could handle 6, then, if I find I can't keep 6 juiced up I could take two off line, charge them separately and keep a maintenance charge on them with a small, dedicated panel. Then I would have a day or so back up without having to worry too much about keeping the batteries matched.

If it helps any, we live in the "6 - 7 daily hours of decent sun" zone as described by Ewing.

Since this is an experimental rig, I could lower my expectations for cloudy run-time. In a short term grid outage I could closely manage how the fridge is used. In a longer term situation I'm pretty sure a fridge simply goes away in favor of "non-mechanical" food storage techniques.

(I'm not keen on a gas fired fridge simply because it would require purchasing fuel -- something that could be difficult to obtain once my supplies run out).

I know I'm sounding like a hard headed novice but I have to balance this project with other projects -- I wish I could fire up the presses like a certain treasury secretary I know of but it ain't a gonna happen right now. ;)

[12vman]

You "can" drain golf cart batteries down to 50% (I don't suggest it) BUT you had better have enough panel to bring them back to full charge plus 20% more to cover the normal internal losses..

We'll round to 400 aHr. with 4 batteries. At 50% discharge, you would need to replace 200 aHr. plus 40 more!

You stated that you have 600 watts of panel. During "Perfect" conditions they should deliver 50 amps of charge. Always remember, nothings perfect. Not even in Mr. Ewings world.

Many factors come into play here. Remember "Angle". The panels will only produce optimum power when the sun is at a right angle to the unit. If you plan to install a fix array, at 10 am. they start to produce decent power. At noon, they are at optimum and start to drop off in output until around 2 pm. Before and after these times, the output of the panel is just more or less a trickle charge.

Different areas can give different results but plain common sense will rule. It really doesn't matter where your location is. The angle of the sun to the panels and the conditions of the day are the important factors.

[randallhilton]

you had better have enough panel to bring them back to full charge plus 20% more to cover the normal internal losses..

Thanks a bunch for your sage advice! Just for clarification -- when you refer to 20% internal losses are you referring to things like losses from transforming from one voltage to another and things like that or are there other internal losses that I'm not aware of?

FYI: The loads I mentioned - the refrigerator for example -- include the 20% inverter loss from the get go.

Thanks!

[12vman]

Batteries, by nature, will not retain 100% of what you put into them. They only retain 80% of the charge..

Inverter losses are around 10-20%, if that much. Modern inverters are more efficient than older units..