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Ask Jeff Yago
Solar & Energy-Related Issues

By Jeffrey Yago, P.E., CEM
Jeff Yago


Sine wave inverters

I have read about modified square wave inverters which I take are an older style of inverter. Then I have read about sine wave inverters. I saw at harbor freight an inverter that just says it is modified sine wave inverters. What type inverter would this be. I have heard that there are some appliances that won't run on modified square wave inverters. I am starting out small and thinking of buying their 45 watt kit but will have to buy an inverter.

Thanks for your time.

Bones

In the solar world, modified square wave and modified sine wave usually refer to the same inverter type, although there can be some differences. A modified square wave inverter usually is just reversing a 120 volt DC voltage source on and off at 60 cycles per second. It looks like squares above and below the zero voltage line on a scope. Later inverter models increased the switching in several "steps" of increasing voltage and looks like a stair step on a scope.

These were the first inverter designs for RVs and boats, and were later used for off grid homes, but are by no means obsolete. There are still several inverter brands making this type of inverter because they are rugged and cost much to make due to fewer parts.

Be careful however, as some models also keep the price down by not including good over-current protection for the internal components. Higher cost models usually protect their circuits from abuse and are not damaged by plugging in more loads than they were designed for.

I am not sure what you mean about a 45 watt kit. If you mean a 45 watt inverter, that will not power much more than a laptop computer! It takes at least a 1000 watt inverter to power most small power tools and A/V equipment, and a quality 2500 watt inverter to power household appliances.

You are also correct that some loads do not work well with modified sine wave inverters. A partial list (but not all!) of electrical loads that either will not work or can be damaged by a modified sine waver inverter include:

  • Wall dimmers - will be damaged
  • Micro-wave - will take much longer cook time
  • Photo-copier - don't even try!
  • Laser printer - don't even try!
  • AM radios - high background noise
  • X-10 remote control devices - will melt!
  • Fluorescent lamps - some will and some will not
  • Cell and pager battery chargers - most will not charge
  • Telephones and answering machines - Phones have reversed grounds -lots of hum!

What will work (usually!) on a modified sine waver inverter:

  • Any kind of incandescent light
  • Most computers (monitor may have smaller display image)
  • Most electric tools, drills, saws, grinders
  • Most stereo components
  • Most televisions
  • Most dish-washers and clothes washers - if inverter is large enough

Final note - if you are not sure, check with the manufacturer.

Good luck!

Jeff Yago


Small pumps

Can you recommend a small 12 v pump that can pump enough water from a reservoir to shower or wash dishes but won't kill three 12 deep cycle batteries.

I have a 1 room cabin which I heat with wood. I can make hot water on the wood stove and put it in a bucket. looking for something to push it to a shower head.

Thanks,

Sal

Since you did not indicate how you are keeping these batteries charged, if there are other electrical loads, or how much "lift" the pump will be required to pump, the answer will need to be qualified. Your deep cycle batteries will power these pumps, but I would suggest using two or four 6 volt golf cart batteries which have a heavier plate and will last longer that the batteries you are now using.

The easiest solution and lowest cost would be a 12 V.DC "demand" pressure pump sold for RV trailers and boats. However, these are not designed to be out in the weather or submerged.

They have a built in pressure switch which starts the pump as soon as you turn on a faucet. You do not a pressure tank with this pump. However, these are designed for minimum "suction", so you would need a water storage tank or barrel nearby or mounted just below the pump.

You can find a quality demand pump here: http://www.pplmotorhomes.com/parts/rv-pumps-water/rv-water-pump.htm

You could also use a "slow" type solar pump which cost in the $400 to $600 range. They use minimum electricity, can be submerged in a deep well or pond, and can lift water over 200 feet. This would allow pumping about 1 GPM all afternoon up to a storage tank above your cabin, then allowing the higher elevation and stored up larger volume to provide the "free" pressure and high flow down to your cabin. Our first summer home had this elevated tank type system fed from a spring. That was REALLY cold water!

You can order this pump from: http://www.dankoffsolar.com/

Good luck!

Jeff Yago


4-wheel bike batteries

I have a 4-wheel bike I use a commuter bike to and from work. I have two 50 watt automotive lights replacing the lamps from 1 Million watt spot lights you can find at any home depot or auto parts store for $10-$20. I read your article on Solar power and have a dilemma.

I am using a 75 watt solar panel to recharge the battery during the day (automotive battery that I will be changing to a golf cart battery today) and when I put on the lights the max they will last is maybe 45-1 hr.

I know now that I am using the wrong battery. My question is can I use a golf cart battery to run head lights (two 50 watt automotive bulbs) and DOT trailer tail lights with the original bulbs (you can pick up a prepackage set at any automotive store under trailer tail lights) and do I need to be aware of any other connections other then connecting the wires directly to the battery? For example: Do I need some type of device to regulate the amount of current going out of the battery to the lights, etc...?

I would appreciate any help you can give me. If you saw the way people drive here is south Florida you would want to make sure your lights don't die out on you 45 min into a 2 hr ride home in the dark! ;)

Q Fortune

I am not sure about the lights you are trying to use, but if they are those advertised "million candle-power" 12 volt type hand spotlights, I can tell you they really use the power. I had one that melted the cigarette lighter plug it got so hot! 50 watts each, times 2 equals 100 watts.

100 watts divided by 12 volts is about 9 amps current draw, not counting wiring losses. Your 75 watt solar module probably is putting about 40 amp-hours per day charging into the battery (average), but this could be more or less depending on local weather conditions. If you put in 40 amp-hours each day, you can only take out 40 amp-hours each day, or you would soon deplete the battery down to "dead".

Now 40 amp-hours each day divided by 9 amps lighting load, means your headlights would operate 4 hours with a quality battery. However, you also said you have two standard 12 volt tail lights plus we need to add the battery charging and wiring losses, which means you may barely reach 2 to 3 hours of operation with a quality deep cycle battery. Remember, at 12 volts D.C., your current will be 10 times what the same wattage light would draw at 120 volts AC.

Warning - the car battery you are now using will not take this abuse and that is why you are losing charge after one hour. However, the golf cart batteries you are planning to use are either 6 volt or 8 volt each. I have used both with 12 volt automotive lights and there color is a soft orange due to the lower voltage. I bet two batteries to get the right voltage will be too heavy for your cart. You may want to try a really good single 12 volt marine "trolling" motor. It should give you the run time and hold up to your daily charge-discharge cycling.

Forget the headlights and taillights you are trying to use, as these are too big a power drain for the system you have described, unless you want to supplement the solar charging with a 120 volt AC powered charger. I would buy the new LED style tail, brake light, and even side running lights. They are expensive, but are bright and use almost no power at all!

I would try the new low-wattage halogen fog lights they are now making for pick-up trucks. These guys are really bright and will be a lower wattage than what you now plan to use. This could double your run time. I have two on my golf cart which I drive up and down a rural gravel road to pick up the mail each night and you can see my coming for miles! Pep Boys, Auto-Zone, or a Nappa dealer should have the correct lights you want.

You did not mention if you have a solar charge controller. If you are connecting the 75 watt solar module directly to your battery - look out! That means 9 amps will be going into the battery as long as you have good sun. If you parked it over the weekend, this could "boil" all the water out of your battery since there is nothing to lower the charging level after the battery is full.

A good solar charge controller works like the voltage regulator on your car. It monitors the battery charge level and lowers the charging output from the solar module as the battery starts to reach its full state. You will need at least a 12 amp model, but might get by with a 10 amp unit. Cheap ones will run about $25, but the better quality units will be around $75.

The more expensive units have "MPPT" which will give much better charging performance from your limited solar array, and is well worth the price. Some models also have little digital display that will give you a voltage readout of your battery charge level. (like a gas gauge!)

Good luck with your project and be sure to add one of those tall flags!

Jeff Yago


Solar water power

Hello,

Do you know of anyone who is using solar power to pump water then using the stored water to power an electrical generator for home use?

I live on a lake with plenty of water and sun. It would be ideal to be able to generate electricity without using batteries.

James Uselton

I knew a man that told me he could lift 150 pounds, but when I asked him to grab his own ankles and lift himself off the ground he couldn't do it! That's something like what you are asking me.

Yes, it is possible, but not very practical. There are two lakes near each other here in Virginia, but one lake is several hundred feet up a mountain from the other. The lakes are located near a nuclear power plant, also owned by the same company that owns the lakes. During the night and when excess power is available, part of the nuclear generated power is used to operate huge motor driven pumps to move all the water from the lower lake to the upper lake. Then the next day during afternoon peak loads on the power grid, they open the upper valves and the water starts back down to fill the lower lake again.

As the water passes (backwards) through the pumps, they turn the motors which are wired to act as generators. It takes several days to re-fill the upper lake, and it can only takes about 3 hours of generating to drain it back. This multi-million dollar water merry-go-round only makes economic sense because of the very high cost difference between afternoon and evening electric rates and grid loads.

If I lived on a lake with lots of sun, I would enjoy the evening view, go fishing, and use solar modules on the roof to charge the batteries. Life is complex enough already!

Jeff Yago


Dry batteries

Jeff,

I bought four 6 volt batteries ( Trojan 105 ) and two solar panels about five years ago and never used them. I would now like to install this equipment and use it.

The problem I have is that I looked at the batteries and they are dry.

Can I just put water in them and re-charge them or do I need some special sulfuric acid mix?

Tony Williams

If these batteries were purchased dry (no acid or water) then you can add a bag of pre-mixed acid and water. If they had water and acid and have just evaporated out over the years then I do not recommend re-filling them. It is very unusual for all of the liquid in a wet cell battery to evaporate out in only a few years. My guess is they were stored in an area subject to freezing temperatures and froze which created a crack in the battery case, then they drained out after warming up. I see this often as a totally discharged battery contains more water than acid and will freeze at a higher temperature than a fully charged battery.

Once the tops of lead plates in any wet cell battery become exposed (due to low liquid level), they build up a very tough sulphate deposit on the plates that isolates that surface area from the acid after the battery is re-filled. Although the battery still works, the plate area exposed to the acid is now reduced so the battery has less capacity. Also, these damaged plate surfaces "flake off" creating a pile of "flakes" at the bottom of the battery cell that can actually short out the plates.

I do not recommend re-filling these batteries and trying to re-use them. Even if they could be re-charged, the odds are that they would not hold a charge for long and could become very hot during the re-charging. Time to buy new batteries!

Jeff Yago


Batteries

Hello Jeff,

We are building a +/-2500sq.ft. log home @ 10,000' in Colorado. We would like to run full solar, off the grid. I have a friend living off the grid with the following system: 28ea. L-16 batteries, 8ea. 24V/120W panels, 2ea. Sine Wave AC Inverters (24V/4000W) and a propane converted 4500 watt genset.

His home is larger than ours and occupied 24/7. I believe this same system will cover all our needs. How far away from the battery bank can I locate the Solar Panels? Ideal would be to put the panels on top of our steel building, but it is about 290 feet from the home.

Is it possible to charge the batteries from this distance? Could you help the situation by putting half of the battery array at each end of the 290' run? (i.e. 14ea. @ the building, near the panels, and 14ea. at the home, 290 ft away) I appreciate any input.

Steve Pettee
Fairplay, CO.

First, your friends system appears to depend more on the generator than solar as they have a large battery bank for the small solar array size. Of course its possible their daily electrical loads are very low even if their house is large. However, many people have fewer solar modules than needed due to budget or limited space for an array, and rely on the generator to charge the batteries more than the solar array.

Keep in mind you can power a 20,000 square foot totally from a single 20 watt solar module - as long as the only electric load in this large house is a single 20 watt light bulb! It's not the physical size of a house that determines how large your solar system needs to be, its what your electrical loads are and how many hours each day they will operate. I discuss these system load issues in much more detail in my book (available from this web site).

There are several ways to address your distance problem. First, system losses are much lower and wire sizes are much smaller for 120 volts AC than 24 or 48 volt DC. You should try to locate ALL of the batteries near the solar array if possible, and do not split up the battery bank. Of course this may require a heavy insulated battery room or enclosure if the batteries are not located in your house or garage since you are in a location with very low winter temperatures. I have run 48 volt DC cables underground from pole mounted solar arrays to homes up to 200 feet away, but this required very large (and expensive) cables and higher voltage arrays.

The new Outback MPPT 60-amp solar charge controller may solve your distance problem as they allow a "mis-match" between solar array voltage and battery voltage. For example, you could have a higher 60 volt DC solar array to reduce the circuit amps and wire size for a long wire run to where the charge controller and battery bank are located. This MPPT charge controller will then convert this higher array loop voltage to the correct lower battery voltage, and your line loss is reduced.

Also remember that any remote high mountaintop solar array with a long wire run to a house is a perfect lightning rod, so be sure this is addressed in how the system wiring is protected and grounded!

Good Luck!

Jeff Yago


Charge controllers

I'm looking at two charge controllers (to buy for my house) and I would like to get some objective critiques from some knowledgeable people.

John Martin
Texas/New Mexico

First I would suggest looking through the many solar articles that have been included in our back issues. Many can be viewed from our web page, and are available in our anthologies.

Since you did not indicate what you wanted to do with the charge controllers (there are many "house" system types), I can only provide general suggestions. Also, you did not indicate the wattage size of your system, system voltage, battery voltage, and why you wanted two (2) charge controllers, this also makes it difficult to know which product you need. For example, there are several really good charge controller manufacturers that only make units up to 10 amps in capacity, and only for 12 volt systems. Some manufacturers only make larger units in the 40 to 60 amp size for only 24 and 48 volt systems, and some manufacturers offer digital system status displays and field programmable setpoints and others do not.

Some manufacturers offer maximum power point tracking (MPPT) type solar controllers. These will cost much more than a non-MPPT model with the same system capacity, but they can really increase yearly battery amp-hour charging totals. If you have a large array, a MPPT charge controller is really worth the higher cost. I am not sure why you want two controllers, as you may be able to get by with one larger controller by changing how you wire the solar array (increasing array voltage reduces controller amp load).

For a general guideline, for systems under 30 amps I would recommend ProStar and SunSaver units. These are available with or without digital displays for 12 and 24 volt systems. In the 30 to 40 amp size range, its hard to beat the industry "standard" Trace (now Xantrex) C-35 and C-40 models, available for 12, 24, or 48 volt systems. Above 40 amps and up to 60 amps, its hard to beat the new Outback MPPT charge controllers, but these are pricey. These are field adjustable for 12, 24, 48, and 60 volt arrays. All of these products have good warranties and there are thousands of each in service.

Hope this helps.

Jeff Yago


Solar panels and batteries

Jeff ---

I appreciate your articles and I am looking forward to reading your future articles on solar powered systems for the home. I have read a number of articles and books on the subject, and it appears that most seem to ignore something that keeps coming up in my mind. Most of what I have read talks about the Amp-Hour capacity of the batteries and how to size them for the load and then they usually say that you have to put in what you take out during the recharge period. This seems reasonable enough, but unless I've misread most of what I've read, it seems as though they assume that no power will be used during the recharging period.

For an example, let's say I have a house that uses 4800kW-Hrs per day (just a number that divides well with 24) and that the loading is fairly continuous throughout the 24 hour period. Obviously the loading at night would be significantly lower than during the day, but for argument's sake the assumption will be that it can be averaged out through the day (perhaps this is a poor assumption, though). So, in any instantaneous point of time in the day, the house load is at 200W, or 1.67A at 120Vac-rms.

I don't know what the efficiency of the 120V inverters on the market are, but I'll make another assumption that between the battery bank and the loads, there is a 30% loss of power (70% efficiency). So, the batteries would see a continuous load of 286W, or 24A of continuous current for 20 hours (assuming only 4 hours of useable sunlight per day). This would translate to 480A-Hrs of discharge on the batteries.

Let's say that the battery pack consists of 12 L-16, 6V, 350A-Hr batteries wired for 12V and 2100A-Hrs. This set-up would allow the battery pack to run for almost 2 days straight (about 44 hours to 50%). In a single, 20 hour discharge period, the battery would have discharged by about 23%.

Now, I have to calculate the power output of the solar panels. If I calculate for only replacing 480A-Hrs with a 20% loss from the system and charging losses, then I would have to size the array to be 1800W at 12V. This, with the 20% loss would give 480A-Hrs in the 4 hour recharge time. This, however, neglects to take into account the load that is still being drawn by the house, which is 286W from the 12Vdc system. This would only leave 1154W for charging the batteries, which in 4 hours only gives 385A-Hrs, which is only 80% of the necessary A-Hrs required for a full charge. Doing this would cause the system to go into a losing cycle where the battery capacity is not being replenished adequately on a daily basis. This is where it seems that most articles and books are lacking, at least the ones that I have read (perhaps I need to read more of them).

To correct for this, I would have to size the solar array for 2200W, which allows for the 1440W needed for charging, the 286W needed for the load and the 20% loss in the system and the charging while giving about 34W of overhead power.

Is this the correct way to look at this? It seems that it is. I also don't know what all the estimated losses in a system can be. So, if I did size the array for 2200W, should I really be making it with more panels to account for low light days? Or is it unwise to have so much solar array power vs battery bank(i.e. - say there is no load during the 4hour recharge period and the total 2200W goes into the batteries - the charge controller regulates this?). I also understand that the instantaneous power consumption will not be equal throughout a 24hr period and perhaps that during the recharging period more conservative methods of power consumption could be undertaken. I'm just trying to get a well designed system on paper before committing money to such an expensive endeavor. Knowing all the pitfalls of losses, etc, would be a great help in getting there.

Thanks

D. Mattingly

Although you have provided lots of information related to your battery sizing question, you have "mixed" terms and values so I cannot give a specific answer. However, I agree that battery sizing is difficult and information is hard to find. First, you said your example home uses 4800 kWh per day. Since this would be a $11,520 per month electric bill (assuming $0.08/kWh), we are starting off on the wrong foot.

You are right that you must put in what you take out, and yes, there are energy conversion losses involved both ways. There is a conversion loss when converting sunlight to DC electrical power, an electric resistance loss in the DC wiring, an efficiency loss through the DC to DC charge controller, a conversion loss from DC electrical energy into chemical energy in the battery bank, another conversion loss from chemical energy back to DC electricity, another efficiency loss in the inverter when converting from DC to AC electricity, and the electrical resistance loss in the AC wiring to your electrical loads!

Although we could put lab test equipment on everything and measure these losses exactly for your example, you may find it much easier to keep it on a more "rule-of-thumb" design basis. As a start, you will have at least a 10% loss in the solar charging process, and another 20% loss in the battery to inverter process. Expect a minimum of 30% loss from the array DC rating to actual AC output, and remember, the nameplate ratings on the back of your solar modules are at "standard" conditions. This "standard" condition is very hard to meet unless your are in the desert at high noon! This could introduce another 10 to 20% in "missing" useful energy.

Most people end up sizing the battery bank and solar array based on the physical space they have available, or what they can afford! Their system will switch to the backup generator or grid power when the battery bank charge is low, and they know the more power they use, the more often the generator will operate when they exceed the battery charge level. Since your local weather conditions, cloud cover, your electrical loads, and the time these loads operate are constantly changing for every hour of the day, and for every day of the year, your system will never see the same input and output from one hour to the next, so don't expect your calculation projections to match actual real world values.

The only time this level of design is really critical, is when we design a totally off grid solar system that cannot have any other source of backup power. However, for these cases we keep very strict control over the connected electrical loads and the times these loads are allowed to operate. In other words, stay out of the tall grass!

Jeff Yago


Iron-nickel Batteries

Hello Jeff,

I remember reading in some older books about Iron-Nickel batteries. From what I remember, they claimed that these batteries had an impressive life cycle, sometimes lasting almost indefinitely. What can you tell me about these batteries and why don't we hear much about them anymore? Were there safety concerns or were they just not as good as people claimed? Thanks for your time.

Micah

The batteries you refer to are called "Edison" batteries as this was the battery design used by Thomas Edison for his DC lighting systems. Yes they will last forever, however they have many drawbacks that I do not like for alternative energy systems. You must "mix" your own alkaline mixture (used in place of acid) every year or two, which is a very caustic operation for most homeowners. They have half the voltage per cell as lead-acid batteries and a voltage drop that is twice as fast on discharge which reduces your battery power time.

We just replaced a set of 24 two volt Iron-Nickel cells for an off grid client with industrial lead acid batteries due to the above problems.

Jeff Yago


Solar home

I'm very interested in building an off grid solar home and I'm leaning toward the whole "earth ship" concept. the problem is, the mountain top I plan to build on has a fantastic view to the north. naturally this is where I would like to orient the front windows. how much will this hurt me in terms of energy efficiency? I live in the foothills of upstate south Carolina so heating is not a major factor, but cooling is.

George Wilkey

Put the windows where the view is! North facing windows do not add any cooling load but will have a much higher heat loss during the heating season. This can be offset by buying much better double or triple layer thermal insulated window units. If your house is facing North, this means you will have a east-west roof. This should give you a perfect South facing roof if you want to add any solar heating panels or solar photovoltaic modules.

Jeff Yago



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