Alternative energy advice from Ask Jeff Yago. Volume #29

Ask Jeff Yago
Solar & Energy-Related Issues

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

Sorry. Jeff no longer answers questions online

Woodgas Stove

We live on a smallholding with lots of invasive alien trees : Port Jackson ( which the government wants to pay me to remove). I would like to build a woodgas stove with the following criteria:

  • it must be aesthetically pleasing.
  • it must not be made of old tins, or used toilets
  • I would like to cook on it
  • I would like to use it to heat the house in winter
  • i would like to use it to heat the hot water

I imagine something build out of bricks?!

Am I expecting the totally unattainable??


(South Africa)


I thought most woodgas stoves used very small branches and tiny pieces of wood. Sounds like you have far more trees than you can burn up in even a large woodgas stove. Yes, you can add a copper coil located above or at the rear of the stove and heat water, but I would pipe the coil to an un-pressurized tank located above the stove so you will not need pumps.

Here is a link to a web site that shows how to build a small version of a clay woodgas stove.

Good luck,
Jeff Yago


Dear Sir:

I am currently building a house, and read your article on generators used with a battery/inverter system to provide off grid power. This intrigued me.

My question is whether or not it is better to get an LP powered generator, versus a diesel because of the price of fuel. The price difference in my area is almost $2.00 per G. I realize that diesel motors usually have greater longevity than other types, but Guardian has introduced an 1800 rpm liquid cooled 18KW generator, and I’m wondering if that might not be more cost effective.

I would also appreciate any info you could provide that would allow my builder to do the install correctly for a future battery /inverter system.

Thank you.


Mike Jamison


My selection of generator type has to do with how often it will be running each week, month, or year. For example, if I want a generator for backup power for a home on the grid that only suffers a power outage for a few hours or days each year, I would select a propane generator as the fuel (propane) does not go stale or get old, so it can sit in the tank for years until needed. Of course you should run the generator every few weeks to keep everything lubricated and operational.

If I was designing an off-grid solar home and the solar system was large enough that the generator would only be needed to run a few hours each week, I would also select the propane fueled generator for the same reason.

If I was designing an off-grid system that needed a generator to run many hours each week I would select the diesel generator as the fuel costs will be lower and would also allow making my own bio-diesel if needed. Since this generator would go through a fuel tank much faster than an occasional use generator, there is less chance that the diesel fuel would go old. Of course you can add additives to the diesel to make it store better, but most of these additives are expensive and another thing to take care off.

If the home’s location is not accessible for filling the propane tank with a propane delivery service truck, you may have to select the diesel, as it will be easier to deliver the diesel fuel in 5 gallon hand-carried fuel cans.

As you see, all designs are have trade-offs and there is not always a perfect answer.

Hope this helps,

Jeff Yago

Connecting a PTO generator to wind turbines


In searching the net I have not been able to find anyone or any information that describes how (if able) to connect a PTO generator to wind turbines.

I am looking to purchase a 12kw PTO generator from Nothern Tool and attempt to build a turbine and see what I can make happen. The PTO unit requires 24 horsepower to operate.

Do you have any knowledge or know of anyone who has done this in the past and also can you tell me where (if possible) I may find a conversion chart for wind mph/turbine diameter/horsepower???

Thank you in advance for your time and help.

God Bless,



What you are trying to do is not impossible, but may be far more work and cost than you realize. First, to generate 24 horsepower will require a very large set of balanced wind turbine blades. Next, they will most likely turn much much slower than the high RPM power-take-off generator, which means a custom designed gear box plus a very heavy duty hub to hold the blades. Finally, all this is very heavy and will require a major effort in building the tall tower.

If you are still interested, why not start a little smaller and work your way up. There have been many home-made wind driven truck alternators. Maybe you should build this first and get a feel for the size of everything? There are lots of plans on the Internet for these,

Good Luck,

Jeff Yago

L16 batteries


We are looking to install a “PacWind” wind turbine in South Florida on a +/- 30 pole. We are told by Pac-Wind that their (4) L16 batteries (the batteries they sell with their wind turbines) are solid batteries with good performance, recent technology, and good storage capacity.

Can you tell us your honest opinion towards these batteries and life spans. Are we buying into 1990’s technology?


David M. Hawke, RA


There are several things to consider. No matter how good the battery type and brand, if the battery bank is undersized in reference to the output of any wind or solar input by having too few batteries, it will not be able to absorb a given day’s worth of wind or solar charging, and will not be able to carry the load until the next charging process starts. If a battery bank is oversized with too many batteries regardless of type or size, your wind or solar system will never be able to get them back to a full charge once they have been discharged, and each day the batteries will get lower and lower until totally drained. The number of batteries they are proposing (total amp-hr rating) is almost as important as the brand or model.

Although I believe Trojan Battery Company was the first to produce a model “L-16” battery, this is now considered a generic physical size of battery just like we say “Jello” when we refer to almost any brand of gelatin. All industrial battery companies now make an “L-16” size battery, but they do not all have the same amp-hour charge capacity or discharge rating. Each manufacturer will use a different mix of Lead-Antimony or Lead-Calcium in their plates and each combination will result in a different battery performance. While one combination may result in longer life, it may have a faster self-discharge or more out-gassing during charging. Another combination may result in stronger plates or a higher energy density, but take longer to re-charge or require higher charging rates that some wind or solar systems cannot maintain.

In other words, I would say in general almost any “L-16” battery would be a good battery for a small residential size wind or solar power system, but these batteries can very greatly in amp-hour capacity, re-charge time, out-gassing, and rated life from one manufacturer to another. I have had very good experience with Trojan, Decca, and Concord deep cycle batteries, although I am sure there are others just as reliable.

Good Luck,

Jeff Yago

20 foot ceilings and energy help

Hi Jeff,

I recently stumbled onto your website. It is very helpful! You have great articles, and therefore, seemed like a person I could send my energy “situation” to for advice. Here it is:

My wife and I live in a large (4500 sq ft), 3-story house in northern Maryland. Our two energy sources are electricity and propane gas for heating. The main shape of the house is square. But from that body it has two “wings”. The first wing is a 2-car garage, with a room above it. The other wing is a large bathroom and walk-in closet.

In the summer the trees that surround the house aid in keeping the entire house relatively cool in the summer. I recently added ceiling fans to the first floor (study and living room), the second floor (3 bedrooms and a movie room) and the third level (the roof was designed with trusses, allowing an arts and craft room). The fans keep the air flowing –and the house cool enough–where air conditioning is not necessary except for days where it is 100š or higher.

The problem is heating the house in the colder months (primarily late November through early March). I have a 1000 gallon propane tank and can go through 250 -300 gallons of propane a month during this time. At roughly $2 a gallon, this amounts to $500 – $600 a month.

Beyond the wings of the house, which are very cold in the winter, the other challenge is that the central part of the house (the 20′ x 20′ living room) has 20′ cathedral ceilings. To cut our heating costs, my wife and I are considering a wood stove for this room, We are also considering buying a tankless instant propane hot water heater.

I’m looking into solar hot water system, but it doesn’t appear to be a viable option, especially given the cost to install it, and the low return on heated water in the winter when we would really need it.

Any further suggestions on cutting down on my heating bills?

Thank you in advance.

Phoenix, MD


You have a very large home and it costs more money heat and cool than a small home, no matter what you do. Your cooling costs are low because the high ceilings allow space for the hotter air to rise, and most likely lots of cracks around windows or open windows to provide good ventilation. I have visited many very high end large homes that the owners just could not afford to heat. Most put up barriers and sealed off all but the main rooms they lived in during the winter. They even had plastic sheeting at all doorways inside you had to push aside to go from one room to another. I could not live like this and feel very strongly that if you cannot afford to heat the home you have, maybe its time to move.

It sounds like you need to either get a wood stove for your large living room, or maybe one of those outside wood boilers that can heat your entire house, assuming you are able to keep it loaded with wood all winter. Just for the record, solar is out of the question for your situation due to the very high cost and high energy demand.

Good luck,

Jeff Yago

Cheaper low PV panel count grid tie?

Is it possible to run one of the less expensive high voltage grid tie inverters (e.g. the Sunny Boy 1200U needs 130-400 VDC input) with just a few 12 volt panels through the intermediary of a DC to DC converter (hopefully the rectified output of a cheap 12 volt inverter)?

I’d like my three 75 watt panels to do something useful when they are finished charging batteries and irrigating the garden.

David Kopf


The SunnyBoy and similar grid-tie inverters need a solar input well over 240 volts DC or they cannot put out 240 volts AC to grid, and normally have solar inputs above 300 volts.

Yes, it is possible to custom design and build some kind of DC to DC converter that could take the 36 volt solar array you have and convert this to 240 volts AC, but the resulting current amps would be very low and may not be enough to overcome the efficiency losses involved. Nobody I know makes anything like this and there is a good reason for that.

In real world terms, your 75 watt solar array is so small it would best be left doing what you are already doing – charging batteries and running small pumps or fountains.

Hope this helps,

Jeff Yago

Solar powered pump


I would like to purchase a solar powered pump to move excess water from a native plant rain garden on my property, uphill approximately 100 feet to a soaker hose on my turf grass. The rain garden is about 8 ft X 60 ft and about 6-8 inches deep.

What system would you recommend?

Joanne Bartel
Eden Prairie, MN


You have not provided much information for me to give a specific system sizing answer, but I have several suggestions.

I would go with a straight solar pump system with no batteries. When the sun is up the pump pumps, when it goes down the pump stops and no batteries required. You will need a pump controller in addition to the solar modules and the DC pump. The pump controller adjusts the voltage and current going to the pump from the solar array for maximum pumping power. During the early and late hours the current output may be too low to run the pump so it just stalls with voltage still passing through it. This is very hard on the pump so the pump controller can convert excess voltage to more amps of current and avoid having the pump stalled during these conditions. In addition, a pump controller will have several extra electrical terminals to connect float switches. You can have one float switch at the lower rain garden to shut off the pump if this garden water level gets too low. You can have a second float switch at an upper tank if you want and have it shut off the pump when this tank is full. Although they make 12 volt DC pumps, I think you will get better pump life if you go with a 24 volt pump and 2 solar modules @ 12 volts in series for 24 volt to match the pump. I would make this a fixed pole mount and locate the solar array on the pole as near the pump as possible. The pump controller can mount on the pole at eye level, and the solar modules should be about 8 feet above ground level unless you have large animal or cattle issues. They like to rub against this equipment and can bend things if too low.

I doubt that you will be able to have enough pump head to use soaker hoses as they require high water pressure to equalize the drip along the entire length. Most likely the low flow and low pressure pumped water will dribble out the first few feet of this hose. I suggest pumping up to a storage tank at a higher elevation, then hooking this to your watering hose. When the tank is full you will have water pressure to water using almost any type sprinkler or hose until the tank is drained. You would then shut off the watering until the end of the next day when the tank is re-filled.

There are several suppliers of solar pumps in your state:

Good luck,

Jeff Yago

Good used car


I lost a good paying job and as a result I now need to let my escalade go. I have about $3500 to spend on a replacement used car. I live in Ohio and I have five children so snow and space are issues. Also obviously since my cash flow right now is very poor, milage is very important. Any ideas as to what make and model would be my best bet?


Dennis in Dayton


With todays cost of gas almost all of us are in the same boat. However there are some good used cars out there that get great mileage. The Volkswagen made a diesel Rabbit in the 70’s that got over 50 MPG, and their newer Jetta and Golf diesel models of the 80’s and 90’s also got great mileage. A diesel engine has a higher up front cost on any new car as it is heavier construction and much longer life. Also, since a diesel engine does not have spark plugs or ignition system, there are less things to go wrong.

If I had limited funds and wanted highest mileage, I would try and find any used Volkswagen Golf or Jetta. They have better mileage than the new hybrid models and are far less expensive. I think most Rabbits are long gone but it may still be possible to find one and the parts are still available. My best friend had a diesel Rabbit in Ohio many years ago and it would go through the snow like a snow plow. He loved that car although he did have to plug in the oil heater on cold winter nights to make it easier to start.

Regardless of the high cost of diesel fuel, it is about to level out while gasoline has been lower but will soon pass diesel again like it was until this mess started.

I am sorry you are hurting as I know many others in same situation. Hang in there as things will get better,

Good Luck,

Jeff Yago

Solar problem – parallel or series

We have a 1 kw off grid system. The actual electrical use is very low, but the system constantly fails- that is, it gets so low we have to shut everything down and run the generator. I think it may be the design. The house is in Kauai. There are 8 L16 6 volt batteries, all wired in series. I do not know how the panels are wired, but we are told that they total 1050 watts- I suspecty they are also all in series. We use flourescent lights- 4 in all, plus propane refrigerator and stove and solar hot water.

Other than lights, all that is powered off the system are a cd player, charging cellphones, a hairdryer in the morning for about 3-5 minutes, and a shop vac during the day occasionally for about 15-20 minutes. We cannot run the washer w/o the generator.

Surely there has to be a better way to live. We have called in two solar guys. We have replaced batteries. We have added panels.

I have done a great deal of reading and it seems that there should be some batteries in parallel, plus the panels need to be wired some in parallel. Before I challenge the solar guy, I wanted some advice. Unfotunately, I do not think that the experts on Kauai really analyse load, etc. Noone has asked us what we need; they simply tell us what the system will be.

Thank you for any advice or direction you can give me. We are desperate.

Gail Stebbins


There is no difference in total battery capacity regardless of wiring in series, parallel, or series-parallel. Now before all those other “experts” out there read this and start sending nasty email back (yes, it happens all the time even knowing this is a free site!), I need to explain.

Batteries absorb energy measured in amp-hours, and they supply your loads by sending these amp-hours back out. A 6-volt deep cycle battery will have the same storage capacity regardless of how it is wired. It makes no difference. Lets use an example, a typical L-16 battery will have 350 to 425 amp-hours depending on weight (lead) and manufacturer. Lets say 400 amp-hrs to keep it simple.

If we wire 8 batteries in series, we will have a 48 volt battery with a total amp-hour capacity of — 400 amp-hours! In series, the amp-hours do not change. This will have a total capacity to power ( watts = amps X volts) = 19,200 watt-hours ( 48 X 400).

Now lets take the same batteries and re-wire as a 24 volt battery. This will be two separate strings of 4 batteries to make 24 volts each, then wired in parallel. Again, watts = amps X volts = 24 X 400 = 9600 watts capacity per EACH parallel string, so in this case the amp-hrs are added, or 2 X 9600 = 19,200 watt-hrs.

The same would be true if we re-wired them for 12 volts, you will still have the same total power capacity. Having proven that fact this does not mean they are not real differences with each layout. We always try to use the highest voltage battery as possible, as this results in the lowest amp current flow, which allows using the smallest wire size, and inverters designed for higher voltage can use smaller components as the currents will be lower.

Another problem in wiring batteries for smaller voltages is you need multiple series strings. Like our example above, it would take 4 separate “strings” to wire your 8 batteries if you wanted a 12 volt battery bank. The problem is, the more strings you have, the more risk there is of un-equal charging between each parallel string. For example, if one battery in one string is starting to fail, or just does not charge as well as the others, this will cause that entire string to receive less than its share of the charging current. It is also possible for a single battery to fail, then all the other strings will “see” this as a load and start moving massive currents into that string, just like they were chargers. This can even cause the failed battery to get so hot it will melt down, so less strings is better.

Again, things are never simple as you think, because our desire to have only one battery string and a higher voltage may not match your loads. Maybe your inverter can only operate on 24 volts, or you have a bunch of 12 volt lights. Several manufactures for DC refrigerators only offer a 12 volt or 24 volt model, not 48 volts, so this is usually why some systems must have lower voltage batteries.

They say if you ask an engineer what time it is, they will tell you how to build a watch. I hope I have not done that with your question, but we need to first dispel this battle you are in about battery voltage. If your inverter and other DC loads are 24 volts DC, then your battery must be wired for 24 volts DC. If you have a few 12 volt DC loads like lights, you can still use a 24 volt battery and install a DC to DC converter. These are low cost and will allow an equal loading across all individual batteries that would not be the case if you wired them to only part of the large voltage battery bank. Due to high cost, your inverter voltage will determine the battery voltage.

Solar Array – Most solar modules under 100 watts are 17 volts output (ideal voltage for charging a 12 volt battery plus some efficiency loss). This means the they are wired just like the batteries in the battery bank, you wire them to give the output voltage to match the battery voltage, and this may require several separate strings. The recent introduction of MPPT solar charge controllers now allow using a higher voltage solar array than the battery bank as it will adjust the voltage and current as required for the best charging output. However, this advantage was not intended to allow using a high voltage solar array to charge a 24 volt battery. Most people use a “slightly” higher voltage solar array when there is a very long wire run between the solar array and the batteries to off-set the voltage loss of the long wire.

Again, there are many technical reasons for using different voltage arrays, different voltage battery banks, and different voltage loads, but you can’t say one way will have much less capacity than another as long as you are comparing the same components and just wiring them differently.

Your Case – You said your batteries were replaced, but If I were there, I would first check each battery to make sure they are all good. If you have 7 new batteries and 1 dead battery, the battery bank will never charge right. You can use a digital volt meter for this, but these readings need to be taken when the battery is under a modest load. A much better way is to use a hydrometer. If you are adding water to your batteries more than once every 3 or 4 months, either your charger is programmed to high or your batteries are getting old. Frequent watering indicates failing batteries if the charger is working correctly.

I would next dis-connect the solar array from the batteries and check their output voltage, then check again after they are connected to the charger. Not connected, most 12 volt nominal solar modules will have an open circuit voltage around 21 volts. If this array is wired for a 24 volt battery, you should see 42 volts, and for a 48 volt battery you should read 84 volts. After re-connecting to the battery, it will be more like 27 to 29 volts into a 24 volt battery, and 54 to 59 volts into a 48 volt battery. Your solar charge controller may have a meter to provide this information.

I would next check charging set-points. You appear to be in a warm climate, and this requires “lower” charging set-points for both your solar charge controller and your inverter charger set-points when using the generator.

The last thing I would check is the charging current when using the generator. I have found that most “contractor” type portable generators do not work well with inverters when charging batteries, its just they way they are designed to control their output voltage. If there is even a slight drop in generator output voltage your inverter will not just have a lower charging output, it may not charge at all. I have watched low cost generators run for hours and not do any charging at all. Assuming you have the worlds greatest generator, if your inverter is not programmed wrong it will take for ever to re-charge.Assuming you have the worlds greatest generator, if your inverter is programmed wrong it will take for ever to re-charge. When using an inverter that has programmed charging set-points, I try to increase the charging current amps up to the point I almost stall the generator, then back off a few amps. Generators use almost the same fuel at half load as almost idle, so loading them up provides the best fuel economy, fastest re-charge time, and less run hours.

If all this checks out just fine, then you may have the classic solar home problem I see every day – the small system was designed to run a few lights and appliances, then each year you add more load or use them longer hours each day and pretty soon you start complaining the system just does not provide the power it did when first installed. If this is the case adding more batteries may not help if your solar array is too small for your loads.

Sorry you are having so many problems, it should not be that way,

I hope this helps.

Jeff Yago

Solar panel on car

Hi Jeff,

I find your articles on 12 volt interesting. I’ve often thought about cars having eg. a roof that was a solar panel which would be connected to a rechargable ni-cad battery and then to an air conditioner / heater which would keep the cabin of the car cool or warm.

Any thoughts?


Frank Savoy


A solar module is not magic, it does not make energy, it just collects the sunlight energy available and converts this from solar photons to electricity electrons, and even the best are only about 10 to 15% efficient. Using your example, lets put a solar module about the size of a car skylight. What ever solar energy is striking the solar module, you only get 10 to 15% of that as electricity, which we wire up to your example electric car heater. Now if we remove that solar module, and let the same amount of sunlight pass on through into the cars interior, almost all of that energy will be converted to thermal heat, not 10%.

In other words, unless you enlarge your car’s roof to mount the 400 square foot solar array you will need to power a small AC unit or electric heater, the only thing a normal size solar module will do on a car roof is keep the battery charged if you like to park for hours and operate the radio or laptop computer!

Hope that helps,

Jeff Yago

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