Alternative energy advice from Ask Jeff Yago. Volume #13

Ask Jeff Yago
Solar & Energy-Related Issues

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

Sorry. Jeff no longer answers questions online

220 to 110

Dear Jeff:

I have a washer dryer stackables is there any way to change the power cord from 220 volts into 2 110 volts cords


Not unless it is two totally separate 110 volt appliances that were “tied” together so each load is applied separately to each “leg” of a 110/220 volt braker panel to reduce the current on the cable and outlet.

If this is not the case, you can purchase a 110 to 220 volt (or 120 to 240 volt) transformer. Xantrex and Outback both offer this device to allow powering a 240 volt well pump from a single 120 volt output inverter. These transformers and very rugged and will almost never “wear out”.

Good Luck,

Jeff Yago



I need to get plans to build my own biofuel system. We are starting a greenlife store and we want to offer an alternative fuel for Tennessee. can you find me the plans for such without having to buy their equipment? I have most of the things we would need.

Thank you

James Fisk


Looks like you are not a subscriber. In Issue #101, Sept/Oct 2006 I had a very detailed article on making your own bio-diesel. The article included photos, piping diagram, and internet links to lots of do-it-yourself web sites.

Jeff Yago

Calculating House Loads for Batteries

Hi Jeff,

Your articles are very informative and interesting reading.

I’m trying to figure (and a little lost) out how to calculate the rating for my home service (300Amps) into DC and then the number of batteries needed to supply that much peak load.

Also, is there a way to convert kWh to what a battery bank could supply?

The application I am thinking of is an off the grid house with a standby propane generator and a battery bank normal operation. Using the generator to periodically charge up the battery bank or during peak load times.


Frank Schoof


You don’t have enough money to buy or room to store a battery bank that would be sized using this procedure! Your 300 amp house panel was sized for the expected maximum peak load that could EVER occur on this service for one instant of time, so the service is large enough to handle it, even if it never happens. In other words, just because the power pole and wiring leading to your house is bigger than the pole leading to another house, this has nothing to do with the actual usage of each home so don’t waste your time.

Take a pad of paper and make two columns down page. List EVERY light, appliance, refrigerator, doorbell, cell phone charger, electric clock, TV, blender, hot water heater, furnace, electric garage door motor, EVERYTHING!!. Put all items in either the “must run” column during a power outage or in the not critical column. You also need to consider actual run-time for each. For example, a micro-wave oven is a big load, but it only operates a few minutes each day. From this list you can see what loads your generator or solar back-up system MUST power at the same time. Some of these loads are not constant, so they may not all be operating at the same time and you can down-size some from the total (system diversity).

This is your starting point. There are still many things to consider when sizing systems but at least this should get your thinking in the right direction.

Good Luck,

Jeff Yago

Powering Computers

Dear Jeff,

Your articles make a very good read. I am encouraged to think in the following direction.

[Brief Background]

I am currently delivering a community-wide area network in my Rural area. This setup consists of A home site using all AC devices and a cell site using all DC devices with AC adapters.

I am hoping to power the same 2 computers, 1 cisco switch/router, 1 Wifi radio, a Dell server and some lighting. All totalling about 2.5KVA – An addition of the Power ratings from manufacturers datasheet.

[The Problem]

I hope to build a battery backup for a locally made inverter which is currently using 3 nos. 12V accumulator/acid batteries used in generators and big trucks. The inverter maker says his inverter is a 36V, 3KVA inverter, what ever that means. And Pure sine. I have no way of knowing as we dont have any oscilloscope or so.

[Before, All was well]

In the city, the same inverter gave 6 hours on the 3 x 12V Lead Acid batteries in series but this has deteroriated to barely 1 hour. Washing and Changing the battery acid gave an extra 15 minutes. That’s no good at all. I have just read about battery care and 50% discharge and effect on battery life cycle. Thanks

[The Direction to a Solution]

The only batteries that we found in the shops – seeming sealed deep cycle/Marine looking batteries, has 12V/200 AMps written on it.

Public power and Diesel Generator supply are usually 230V/13Amps.

Please, I need to know ;

1. How to obtain the AmpHour (Ah) ratings from this battery information.

2. How to achieve 20hrs from this battery-inverter system.

3. Is it OK to assume that;

3a.Total Current requirement of equipmntis 13Amps x n. Where n is No. of devices and the

3b. So, the Total AC Power of Devices (PWR) = 240V x Total Current = 2500VA/240V = 10.41A

4. Can I Power the Cell sites by DC as all devices there use 12, 24 and 48Volts only.

What I gather for now is;

A row of 3nos. 12V/200Amp deep cycle batteries in series -> 36V and 200A. Two extra duplicate rows connected in parallel yield 36V and 600A

My whole project team looks up to me to provide a solution because I read Physics in school, but honestly I am lost here, I know more of networks now.

[A new thought]

I figured it would be smart powering the cell site with a 1.5KVA DC Rectifier (another problem. I just read of their existence over the internet)

Can I send you a JPEG picture of my power design? Due to Inverter transfer delay and failures, I shall put 1200 VA UPS on a fault tolerant line via something called Auto Changerovers I hope to import over the internet.

I may make this more readable upon request as I get clear on the ideas, I am aware it will be read by the public.

Thank you for your kind assistance.


Garvey Ndukwe


You have a real problem that is byond the scope of this short free help-desk service. I can give you a few suggestions, but not sure you will like them. You are expanding into something you need some real engineering design to sort all this out for you. You are expanding and adding equipment to existing equipment that may be better served by starting over completely instead of just adding more devices. For example, if you have some sites where all equipment can run on DC power, then run all the equipment on DC and don’t have the issues with an inverter. If some equipment is different DC voltages, you still run everything on same battery bank and use a DC to DC converter to raise or lower the voltage for that specific device. Keeping it all DC will increase reliability and reduce line “noise” problems with the AC side.

For the equipment on the inverter that must stay 120 volts AC, please understand that a 36 volt inverter is less common voltage as most are 12, 24, or 48 volts, and the higher the voltage, the less current and smaller wire sizes can be used and the easier it will be to find a common voltage for all other devices. Maybe its time you stopped buying car and truck batteries and purchased real deep cycle batteries for this service – like “L-16” size 6 volt cells that weigh about 110 pounds each. These can really take a fast generator charge and can be discharged 50% every single day. You would need six (6) wired in series for a 36 volt inverter, but if this does not give you enough run time you may need two rows of six.

Next, I never ever heard of changing the acid in a battery. Battery acid does not go away. Even in a brand new battery, as soon as the battery is dischaged the acid specific gravity changes to that of just water, but as soon as you re-charge the battery the acid becomes back full strength, it is not “used up”. The only battery I have ever heard of doing this was the old “Edison” akaline batery cells that required mixing up an akaline mixture that was replaced every few years but this was not battery acid! If a battery no longer will hold its charge, its not the acid level, its the lead plates that have “calcified” or “flaked” off most of their surface material which has dropped to the bottom and shortted out some of the cells. Also, when a battery is left at a low charge level for an extended time, a buildup occurs on the lead plates which acts like an insulator and keeps the acid from making contact with the lead, which is like making the battery smaller since its the surface area in contact with the acid that provides the power.

I think you should re-plan where you are going and stop just adding more odds and ends. Look at your future expansion needs, standardize on a system voltage and a battery voltage and then only purchase equipment that matches these. Finding an inverter with a high battery charging section will reduce generator run time and the savings in fuel and generator repairs will easily justify the cost of a better inverter.

Good Luck,

Jeff Yago

What did the retreat cost?

How much did your entire “battery powered weekend retreat” cost you without the appliances, just the system itself? Thanks a bunch. We’re going off-grid!



Not an easy answer to give due to many variables. You said not including appliances, but its the appliances that drive the system size (and cost). You are working backwords. First, determine what lights and appliances you need. Then see if you can find them in very efficient models. For example, a washing machine costs $300. But a standard washer will require a very large 4 kW inverter and batteries to power through a cycle, plus, it will use 3 to 4 times the water which must be pumped and heated. A Staber clothes washer costs over $1100, but it will operate on a 1kW inverter, and require about one-third the hot water and cold water as the lower cost washer. Same for lights, stereo, micro-wave, TV, well pump, and refrigerator. Find what your power needs are first, then you can get a “guesstimate” on the size of the inverter, batteries, and solar array. If you have nothing to go on, try about $8.00 to $9.00 per watt of load. Note – I said per watt, not per killowatt. A 100 watt incandescent light will require about $900 of solar system to power. Now you know why we use only compact fluorescent lights!!!

Buy the best and largest inverter you can afford first, as this is the main part of your system. You can always go small on the solar array now to save costs, and add more solar modules later as you have more money, but you will not want to keep replacing a too small inverter with a larger model. Just as a rough guess, you will need about $5000 for an inverter and related electrical switchgear – minimum. Figure another $2,000 in industrial batteries, and about $12,000 in the solar array and mounting hardware to start. You will need about $4,000 for super efficient refrigerator/freezer, well pump, washer, propane stove, and special light fixtures. Almost every client I have ever helped that was off-grid always came back in 2 years to “Double” the size of their solar array which then reduced the need to run their backup generator. Oh yes, you will need a good 8 to 12 kW propane or diesel generator which adds another $3,500.

Can you do this for less – yes, but you will be doing what we call “camping out” and not living off-grid.

Good luck,

Jeff Yago

Building an off-grid cabin


I have written you before concerning solar power. Thank you for your repsonse..very helpful!

I was wondering if you had any insight or reference material for my eventual cabin.

Currently I am designing a cabin for my property in Utah, under 1000 sq ft.

It is located in Hamblin Valley, approximately 20 miles north of Modena, Utah. As you may know, the climate is quite warm in the summer. It is similar to a high desert climate. Some snow in the winter. Hot in the summer.

There are no services, so this will be entirley off the grid. My concerns are overlooking the obvious things that will make this place livable year round. Such as water storage (in ground or above ground), water collection, solar power and/or electricity generation.

Do you know af any good reading material for the “first timer” or have any insight to share?

Sincere thanks,

Brent Toft


I am very familiar with your area and spent some time there doing research for an upcoming book. I found the people in your state willing and able to take care of themselves and I am sure nobody in Utah will be waiting around for the government to save them if they have a crisis.

Your main concern in that area is a good water supply, and hopefully you can have a well. If a drilled well is not possible, you can use surface water and have your own storage tank and water purification system. Refer to my article in the Sept 2001 issue (#71) that describes how to build one for an off-grid cabin.

I am afraid my solar book originally published in 1998 is getting some age on it, but there is a new book out titled The Renewable Energy Handbook which I reviewed and can highly recommend. You can find it on the Backwoods Home Bookstore.

Good luck with your project.

Jeff Yago

Dawn to Dusk photo cell


I am curious do you any plans or know where I can get plans to make a PHOTO CELL dusk to dawn unit capable of 20-1500 Watt 12-36 volt DC application in a small size ( 8-9mm deep x 15 mm in diameter circular) this size would fit into my existing round cup.



For many years this was a big issue as all photocell controls on the market were 120 V AC. However, about 5 years ago several manufacturer’s started making DC powered photocell controls. Moringstar makes a solar charge controller that uses the output of the solar module to tell it when the sun goes down and cones up. Several models include an adjustable timer dial so you can have a DC lighting circuit start at sundown then time off 1,2,4, or 6 hours after sunset, or even stay on until morning. Real Goods offers several 12 volt DC digital 7 day timer controls. They are not photocell, but you can program an on and off time each day.

If these are not heavy duty enough you can have them control a relay that has higher amp rated contacts.

Jeff Yago

DC generator

Hi Jeff,

There are many 3000-5000 watt portable generators available, but one with DC output (72vdc-108vdc). What are my options?

1) Get a regular generator and modify it to be DC. I don’t know how are the run of the mill portable generator designed.

1a) What type of generator head are used in these?… # of poles, brushed/less, etc.

1b) How can they generate 60Hz AC when driven by a 3600 rpm motor? Are they geared down by chain drive?

1c) How do they control their frequency and voltage? Mechanical cetrifugal weights or electronics?

1d) Can I use a full bridge rectifier to get DC with off the shelf parts?

1e) How might I modify the control mechanisms to get 72-100vdc desired?

2) I saw some “higher end” generator using inverters to produce the AC (Honda has the EU1000i). For these type of generators, I presume they will have a DC generator.

2a) What is the nominal DC voltage?

2b) Is it a good assumption that if I tap the DC directly but leave the AC inverter intact, the throttle and control mechanisms will still work if there is a varying DC load?

2c) How can I reverse engineer the throttle control mechanism so I can “adjust” it for the 72 to 100 vdc I want (probably not practical unless supported by original design).

3) Another approach is to use an automotive alternator. From the booklet “Alternator Secrets”, it is clear that a modern car alternator can generate up to 3600 watts if the regulators are bypassed and 12 volts are fed into the rotors to get 120vdc output. However, to get 120vdc, the alternator needs to be spinning at about 5000rpm. I was thinking of direct coupling for the best efficiency and simplicity.

3a) “Alternator Secrets” mentioned $25 control box kits that consists of a four-pole double-throw switch, a 30 amp fuse, an outlet, and 0-150 volt DC meter. Do you know sources to buy this kit or other good sources for such parts?

Danny Wang


Lots of questions!

First, you did not say why you need a 72 to 108 Volt DC generator, so I will assume you are trying to charge a large battery bank. After WW2, there were many DC airplane engine starters available on the surplus market and these made great high amperage 36 to 48 volt DC generators by just driving them with a motor, even though they were designed as engine starters. They were very rugged and made great welders when driven by a motor instead of connecting to a DC power supply. You can still find these from surplus dealers, although the voltage may be lower than you need. I purchased a steam engine-driven 120 volt DC generator on the surplus market which was designed to provide standby power on WW2 ships. This thing only turned 400 RPM and would last forever, but it weighed over 6000 pounds!

You can NOT alter an AC generator to make it DC because a DC generator requires brushes and a commutator to produce a constant voltage output. There are no brushes and no commutator on an AC generator, as the voltage output is a sinewave going from zero to 169 volts over 60 cycles per second. The 120 volts you read with a standard voltmeter is the RMS average (root-mean-square) voltage of a sinewave having 169 volts peak to peak.

The high end generators you refer to like the Honda “i” series include an “inverter” circuit in their output to hold the voltage rock solid as the load increases. Most lower cost generators start to drop their voltage output as the load increases to keep from over-loading the generator. The “i” series provide a much better control of their output, but they are still 120/240 volt AC generators.

A special note – ALL battery chargers use only the “peak” part of the 60 cycle wave form to produce their DC output for battery charging. If a low cost generator gets loaded down and the output voltage drops, a battery charger will not put out any charge. I do not mean the battery charger lowers its output, I mean it might stop charging altogether when connected to an under-sized or low cost generator.

I would not recommend driving an alternator as fast as you suggested or you may start throwing metal parts! You may get a higher voltage output as you increase RPM, but all mechanical devices are designed for a specific RPM range and when you exceed these specifications bad things happen.

I would never drive a generator with a chain drive as you suggested due to high speeds, lubrication, and noise issues. If you can operate at the same RPM, then a direct drive flexible coupling is best. If you need to increase or decrease generator RPM from a engine RPM you can use a pulley and belt drive, but beware, some generators do not have large end shaft bearings that can handle a large side load from a pulling belt drive. Many generators are only designed for shaft to shaft coupling connection which has no side loading on the bearings and this does not allow changing speeds.

There are several small firms making 5 HP gasoline motor-driven automotive alternators that are well designed and really put out a good 12 to 24 volt DC battery charging current. However, I do not know of any being made in the higher DC voltage you are looking for.

Keep in mind the reason we are getting away from DC generators and DC motors is the high maintenance factor for the brushes that are required on most DC equipment. I had a 24 volt DC attic fan and I have to take the motor apart every spring, sand down the commutator, clean out a pile of powdered carbon dust that is nasty, and replace the carbon brushes. I finally replaced this DC fan with a standard AC fan and never had another problem.

Generators are designed to generate power very efficiently, and battery chargers are designed to provide a “controlled” battery charging process. Why don’t you buy a really good quality 120/240 V AC generator, and a really good quality battery charger that you plug into the generator. You can order a battery charger for any voltage and current, and an AC generator can be used to AC power tools, lights, and appliances during a power outage, something you cannot do with a DC generator.

Good luck!

Jeff Yago

Illinois Power

Dear Jeff,

We are planning a new home on 5 acres in northeast St. Clair County.

I have been looking into an off grid system for all our power needs except a propane hot water heater. We are planning 2800 SQFT above grade with 2000 SQFT additional in a walk out basement. Based on our similar existing on grid home we estimate a daily average KWH load at 85 with the peak at 115. We have a preliminary budget of 45K can this be done at this price ? Are there prepackaged combo wind/solar/batt systems that you would consider ? We already have a backup 16000 watt propane generator (still in box made by Guardian) that we were hoping to use with the system. The wind maps list us as fair avg about 8 mph. We are going to include the cost in our construction loan/mortgage so we won’t be at the mercy of big power. By the way I am a building contractor so I have quite a bit of capability to complete this project, I just don’t have these special resources.

Thanks Jeff

Steve Harris
Shiloh, Illinois


As I keep telling everyone who sends in their questions, we do not design solar systems based on the sq. ft. size of a house. How much electricity does a storage building require that is 50,000 sq.ft. if it only has 25 light bulbs at 100 watts each? How much electricity does a 400 sq. ft. apartment require that is full of computer equipment for a home office, has tons of audio-video equipment, an all electric kitchen, a electric hot water heater, and electric heating?

You said you average 85 kWh per day with a 115 kWh peak. This is over 2-1/2 times the national average for a single family residence and SIX times what my 3,400 sq.ft. solar home requires. The average cost today for solar pv systems is around $10 per watt installed for grid tie systems (no battery backup), and from $12 to $14 per watt for off grid systems depending on system size.

This means without any change in lifestyle or your home’s power requirements, you will need a 17 kW solar array to generate 85 kWh per day based on 5 hours of sun per day(85 kWh / 5 hr = 17 kW). Keep in mind this is very optimistic since your peak solar hours is probably less than 4 hours per day in the winter where you live. This means a solar system for your house will cost at least $170,000.00 (17,000 watts X $10 per watt) for grid tie without battery backup, which could add many thousands of dollars more.

You think maybe its time to review your present electric usage?

Your 16 kW Guardian generator is very popular for residential backup power use during the typical once or twice per year storm outage lasting only a day or two. However, many of these “residential backup” type generators are only intended to operate under load a few hours or a few days per year. If you plan to be off grid, you will need to move up to a more heavy duty low RPM type generator designed to operate 200 to 300 hours per year which is more typical for off grid applications. Otherwise, you will be spending a ton of money on generator re-builds. Check your owners manual to see what they recommend for run time per year or time between major overhauls. You will pay over $9,500 for a low RPM cast iron engine 16 kW generator designed for regular use applications. I am betting you paid less than $3600 for the generator you have in the box. Nothing wrong with what you have if used for emergency backup, just not intended for off grid type applications.

If you have lots of wind then maybe that would be something to consider. Wind maps are a start, but wind is so different for every area, and is strongly affected by elevation, height of site, height of nearby mountains and obstructions, and height of wind turbine. Basic rule – the higher the better, but make sure this is not a problem with neighbors as they can get noisy. A 1,200 to 1,800 watt unit might fit your needs and cost under $5000.00 with tower.

My advice – When you can get your electric bill down to 1000 kWh per month you can start thinking about solar. When you get it down to 750 kWh per month then its time to get serious. Otherwise, your solar system is going to cost more than your house. Hate to be so hard, but just trying to give you some advice that many others would never tell you. Best bet – Take $300 and head down to a lighting supply house (not a builders supply) and ask for “warm” color compact fluorescent lights in the 15 to 25 watt range and toss out every incandescent bulb you own.

A lighting distributor will have much better quality lamps than what most builders supply carry and yes, they will cost more. We built our solar home in 1994 and every light is fluorescent or halogen. It is now 2007 and so far we have not replaced any lights after 13 years. No doubt we soon are going to have to replace a whole bunch of fluorescent lamps at the same time, but you do get what you pay for. How many light bulbs do you buy in 13 years? I have purchased a few of the “cheap” compact fluorescent lamps from the local builder supply and big box stores and several have burned out in less then six months. Ask for some 45 watt halogen PAR 30 or PAR 38 (depending on fixture size) for any recessed lighting in color critical areas like bathrooms and over work spaces.

Just changing all of your light bulbs will save you about $10,000.00 of solar system cost if you do decide to go solar, but I realize this is not as exciting as showing off your pv array to the neighbors. Now go replace some lights!

Good luck and let us know your results,

Jeff Yago

Books about solar electricity?

Hi. I am wanting to learn more about designing and building a small solar power generator so that I may experiment with it and see what it is capable of. Can you recommend any good books on this subject?

Thanks for your time.



I’m not sure what you are mean by “solar power generator” or “experiment”.

If you are asking where to buy a single “solar module”, a device that converts sunlight to DC electricity, you can buy these from any of the solar suppliers that advertise in these pages. They come in all sizes and wattages, but most dealers sell these in larger quantities as part of back-up or grid-tie pv systems and do not usually sell just a single module so expect a high price.

If “experiment” means you want to build your own solar module, then the only book I know that shows how to actually build a solar module is “Practical Photovoltaics” by Richard Komp.

You indicated you just wanted “to see what it is capable of”. If that is all you want to do, save yourself several hundred dollars and just read the nameplate data or specifications for any module off the internet sites of the manufacturers. All of this data is available on line for free, and will tell you how much current and at what voltage any module will produce.

Good Luck,

Jeff Yago

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