Alternative energy advice from Ask Jeff Yago. Volume #28

Ask Jeff Yago
Solar & Energy-Related Issues

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

Sorry. Jeff no longer answers questions online

Temperature setback and May/June article

Dear Jeff,

I just read your May/June “Energy Class, Part 1” article in Backwoods Home.

While I understand you were trying to educate folks that space heaters are not energy savers, I think you may have done your readers a disservice.

That’s because the article sort of implies that temperature setback doesn’t save energy:

Will lowering your thermostat save on your heating bill? Of course, but only for those hours you maintain a lower temperature.

While I know you were making a different point, the article as a whole doesn’t really communicate that lowering overnight temperatures can provide big savings.

Instead of writing against “magic box” space heaters, you could have written to recommend instead installing a programmable setback thermostat!

You never really do come out and say that in a positive way in the article, which I think was a missed opportunity.

As I recall from my days at Xenergy (a leading provider of energy audit software in the 80s), just about the only energy-saving technologies with a payback measured in months was a setback thermostat.

Now these thermostats cost $25 or less and save 5-10% of a homeowner’s oil bill. At today’s oil prices, that $25 investment would pay for itself in 1 or 2 winter months.

I enjoy your articles, but I think you missed a chance here.

But maybe the punchline is coming in Part II.

Best Regards,

Craig Davis
Keene, NH


You forgot to quote where I said “Now for those of you yelling at me that it’s cheaper to let your home cool down and only heat ………….” I realized those readers who understand this subject already may disagree with my teaching method, but stay tuned. Remember, some readers may not even have a central heating system or a wall thermostat to adjust. We have many readers living on tropical islands, or heating totally with a wood stove. Don’t put the cart before the horse.

You cannot believe the money people are throwing away each winter on these junk heaters and all the older retired folks that can’t pay their heating bills now, who get ripped off believing these outrageous ads claiming these heaters will heat your home for pennies a day.

This is a 3-part article. My first task was to show readers how energy follows specific rules of heat transfer which you cannot void. Part 2 will teach readers how to read their own electric bills for indicators where there are problems and how to attach a cost to each of these energy flows.

Part 3 will provide a list of very specific and low cost things each reader can do to lower these electric costs. Yes, for those with central systems we will be recommending a setback thermostat. But there are many many other things they can do and which improvements they take on will depend on understanding Part 1 and Part 2 first.

Thanks for taking the time to write, but please wait until I am finished and then you can complain if I have not provided accurate information.

Jeff Yago

Solar back up water system

I am in the gathering phase of designing a back up water system for my home. I want to utilize solar panels to charge a battery bank, as I would operate my pump on battery power. I do not have enough experience to know what I can and cannot do, so my question is this:

How many batteries would I need?

And if I only used it once a day to fill a 75 gallon holding tank, what wattage solar panel would I need?

Thank you,

Kevin Pitts
Granbury, TX


We cannot provide specific sizing assistance on any project since there is far more data and information we would need to make this kind of determination. For example, to size any solar pump, you need to know how many gallons per minute and per day you need, what is depth of well, what is pumping head pressure, what is elevation of tank in reference to top of well, what will be the well draw-down, what is length and size of pipe, is this a DC pump or AC pump in which case you will need an inverter, will you fill this tank every day or will there be many days between times the tank will be used, how big is the solar array charging the batteries, and on and on. As you see, we cannot provide this level of design assistance on a free web site.

However, I have written many articles on this subject, including articles on off-grid solar cabins, solar water pumping, and solar powered pumps to fill storage tanks and all are listed on the Backwoods Home website. I suggest you read these articles and see if any are close to what you are trying to do.

Good luck,

Jeff Yago

Battery Charging Rates


I would like to know what amp/hour(ah) charging rates are optimal for different types of 12V and 6V batteries used in solar applications. For instance, do marine deep cycle batteries (heavier plates) require a slower re-charge rate as opposed to a 6V golf cart battery vs a car battery (lighter plates)? What would be a typical amp break point between what is considered a slow charge vs a fast charge (2-7,10, 20 amps?)

I currently use a 12V 70ah wet lead acid battery and have been trickle charging it with a 1.5 amp charger when on a power grid (ac) and I currently have 20 watts of solar panels on my pickup shell (1-15w, 1-5w) producing, theroetically in optimal conditions, about 1.3 amps with a controller. I typically use my system just for DC lights, a car radio and a couple of DC charging units for my cell phone and my laptop and want to be able to keep it topped off in a 24 hour cycle.

This is my original system and I am looking now to upgrade my battery bank to 2-6V golf cart sealed batteries and maybe add an additional panel to beef up the charging rate.

I have been on the road for a year and have another four before I settle down into “old age.” My needs and wants are few and I’m just enjoying the journey.



Sounds like you have been trying some interesting things with your vehicle.

For most charging applications using non-sealed lead-acid type deep-cycle batteries, most battery and charger manufacturers recommend a maximum charge rate of C/5 for a battery almost discharged until it reaches about 80 to 85% charged, then back down to a C/10 rate. This means you take the total amp-hour rating of the battery, say 220 amp-hours (typical golf cart battery) and divide by 5 giving 44 amps for a maximum charge rate to start with. This limit is an “average” and takes into account how a battery will get very hot if charged too fast, while trying to reduce the re-charge time as much as possible. Since your RV type battery will have a lower C value, its charge rate will be lower. Of course if you are using a trickle-type charge with a very low charging rate, it will never be able to reach these maximum charging rates for either battery type.

Hope this helps,

Jeff Yago

Solar Power Battery Bank

Mr. Yago:

I recently setup a small off-grid solar power system in my residence. I am using three 110w, ~6amp Mitsubishi 12v panels wired in parallel, charging two 12v MK/ DEKA Gel-Cell batteries with a 184 aHr capacity (at a 10 hour rate) wired in parallel. The controller in use is a Xantrex C40. The wires in use are all oversized (#2/0 AWG as between batteries and inverter, #10 AWG as between panels and controller and controller and batteries).

I have been running the system for about a month now. Is it too late to expand my battery bank capacity? I have received advice to the effect that once you initiate a battery bank’s cycles you can’t then expand the bank later on.

My problem is that I am discharging the bank too quickly. I have only a chest type deep freezer drawing power, and when the compressor is running it draws 10 amps. The compressor runs 10 minutes out of every hour. My bank and panels produce/ store enough electricity to run the freezer for a 24 hour period, but after a nighttime of no sunlight my battery state of charge is about 12.4 volts.

It is difficult for me to imagine that load that requires 10 amps an hour will defeat a battery bank that can produce 30 (183aHr x 2 = 363/ 12 hours = about 30 amps) amps an hour in a single night, but it has been.

In sum, may I add two more 12v batteries to double my bank’s size at this point? Also, does my energy consumption and battery depletion look accurate to you or is something amiss within the system?

Thank you for your time, and amazing articles.


Matt Houghton


You have several possible problems to deal with. First, you could have the worlds largest battery bank, but if your freezer removes each day more stored energy than you are putting back from the solar, it will only take a few days for this system to run down and never catch up. I would first determine if this load is more than you thought and it may pay you in the long run to buy a more efficient refrigerator.

For example, If you review my recent article about building a solar trailer, you will see eight (8) 350 amp-hour deep cycle L-16 batteries being charged by a 600 watt array. The array can actually be extended to 1000 watts when we set it up as a display, but when sitting next to my home we only use 600 watts of array. This system will keep the battery charged for months at a time and the only load is the refrigerator-freezer. However, it is $3,000 SunFrost refrigerator-freezer which I think still holds the world’s record for being the most energy efficient. Also, it is being powered by 600 watts of solar, not the 300 watts you are trying to use.

If you really want to do this and not need to increase your battery and solar array size, I would consider switching to a SunDanzer Model #DCF225 12/24 volt DC top load freezer and then you will not need the inverter. It will operate straight from the battery and I think the 300 watt solar array you have should easily power one of these unless you live in the extreme North.

Next, since your batteries are still new enough, I do not think you would get that much of a “mis-match” if you did add more batteries, but the problem is you are operating everything at only 12 volts and this is requiring you to wire everything in parallel which is not good. We sometimes wire batteries with two parallel strings, but when you increase the number of strings you can get all kinds of imbalance and even the risk of one battery string discharging into the other if it has a weak cell. If you do decide to increase the battery bank size and you stay with 12 volts, I would switch to 6-volt deep cycle golf cart batteries which would more than double your amp-hour capacity without having to make four parallel strings.

Good Luck!

Jeff Yago

Wood fired hot water

We have a wood fired boiler… it is for hot water only " it needs replaced. Any idea where I can get a new one? The one we have is about 20 years old… it has a fire box, then the water reservoir on top. It is not used for anything other than heating water.




I addressed this question several weeks ago and I am afraid the answer has not changed. Wood-fired domestic hot water heaters are very popular in Mexico but as far as I know, none of these meet our plumbing and boiler codes for safety. I am not saying they are not safe, but there are very strict regulations on the construction, testing, and quality control that must be met and I do not believe any of these have gone through this certification process for (legal) sale in the US. I visited a manufacturer located in Eureka California about 10 years ago who was making a really good unit, but he moved his operation to Mexico years ago.

The following is a link to unit hand-made by the Amish, but pay attention to the disclaimer note on the ad:

Hope this helps.

Jeff Yago

Steam from Solar

I read the article Is steam power in your future? and have a question.

Has anyone developed a solar powered steam engine?

I’ve been paying 2/3 of my electric bills for the last 5 years from a photovoltaic system (which has now paid for itself) and just installed a solar hot water heater. Here in Southern California, they have to put temperature limiters on the solar. That got me wondering if we could build a big reflector and produce steam for a small engine.

I remember being in the boiler room at the college my father worked at – he was a steamfitter by trade – and being impressed by the complexity, but the drawings make it seem simple, even to a physicist/electronicker like me!

So can it work?


Jim Hayes
Fallbrook, CA


I am impressed that you are already using solar power to the extent that you are. However, using solar to make steam requires industrial type equipment not suitable for home construction. We have had all types of solar-to-steam plants for many years, but most are located in desert areas as they require a very strong sun and very clear sky. Unlike solar thermal panels and solar photovoltaic modules that will still produce energy and partly cloudy days, making steam requires high temperatures only achievable with polished mirror reflectors and these do not work unless the sky is clear.

As far as a solar powered engine, there have been several designs using the Sterling cycle, but again, this still requires polished mirrors and lots of ground area for construction. Keep in mind that regardless of the heat source being the sun, fuel oil, or gas, any steam system can be very dangerous and steam piping and boilers require special steels, special welding, and must meet strict national pressure vessel codes which require a yearly inspection. This alone can introduce problems that many potential builders cannot meet.

Although several companies have tried making solar concentrator “tubes” that have a horizontal pipe with the bottom half wrapped with a mirror surface that “tracks” the sun, these have had limited success in a small-scale application and are very expensive. In addition, unless you are in the dry desert, the typical air pollution, grit, and grim of populated areas require constant cleaning of the reflectors.

For my money, if I were you, I would invest in more solar pv panels and provide 100% of your home’s energy the less costly way.

Good luck,

Jeff Yago

Charging Flashlight batteries

Hi Jeff,

I would like to find a solar system that will charge the various rechargeable flashlight batteries I have. From AAA to D, and 9 volt size, nicad, NiMH, plus whatever other batteries I might buy. It needs to be portable, and fully dependable for function. (Heavy duty preferred.)I would also like it to be strong enough to charge a laptop computer as well. Do you know of anything designed to do this? Or, what I would need to assemble the pieces myself? I was thinking of using PVC as a portable frame and stand to hold the panels.

I’m new to solar panels and am unfamiliar with the system.


Jeff P.
Fairbanks, AK


C. Crane is a great company and has the product you need.

Good Luck,

Jeff Yago


Dear Jeff,

I’ve read recently about ultra-capacitors which can store and deliver large amounts of energy quickly. Can they be useful in a solar system battery bank? I’m specifically interested in whether large sudden loads degrade the batteries and if these caps can extend battery life.




I may be a little rusty on my electronics, but as I understand it, most of these high-power storage capacitors are used for very fast discharge and slow re-charge applications, and are very large and very expensive. In addition, many of the larger units can store a lethal jolt of electricity for long periods without any electrical connection and must be handled carefully. Think of the capacitors used to charge up the heart stopping jolt they release in a heart defibulator.

I have seen a few smaller sized projects using a capacitor for power storage to power very low power loads like LED lights overnight, but these have been small applications. Most likely what you are hearing about are applications requiring very fast response times like a computer system UPS power supply to carry the loads for a few seconds until a backup generator can be started and up to speed.

There is lots of research going on in these areas and I am sure we will be seeing some amazing technology in the next few years.

Hope this helps,

Jeff Yago

Reverse Polarity Damage

Dear Jeff:

Last winter, with snow falling at a foot a day rate, I was forced to witch out my batteries. Due to the weather conditions, worried about the delivery man getting stuck in his two wheel drive, I did notice the small note on my Vanner Battery Equalizer Voltmaster Model 60-20A (run a 24 volt step-down to a 12 volt system) , suggesting to avoid Reverse Polarity damage I would need to disconnect the Ground Terminal first. I you likely surmised I failed to do so and the result is that I lost my DC service.

My question to you is: what damage did do the whole system (Equalizer, Inverter, and Power Center) and what do I need to do to repair it?


Brooks Wilson


Hard to say what may have been damaged, but I do not think it is as bad as you expect. There is the possibility that the downstream 12 volt appliances may have been connected with 24 volt power, but most inverters have fuses or other safety circuits to shut them down if this they are connected to the wrong voltage or reversed polarity.

I would however, expect that any 12 volt lights may be toast. Bottom line, you will need to check everthing out before starting up again.

Let us know how it turned out,

Jeff Yago

Inverter Question

Hi Jeff,

I need some help with a inverter question. first the dc power requirements really confuse me, I’m looking for a 4000-6000 watt inverter. some companies state 30-50vdc (xantrex 3000model) vs. others will state dc inputs from 150vdc to 500vdc. (fronius 5100)

Any help understanding these vast dc input differences would be great.


Mark from Greensboro, NC


You are comparing inverters for totally different types of solar systems (comparing apples and oranges).

An inverter that indicates it has a solar power input of 150 to 450 volts DC or in that general range is called a “grid-tie” inverter and is not designed to be installed in a battery-based system. The output is 240 VAC and connects directly into your home’s main circuit breaker panel and “turns the meter backwards” when operating. The high voltage solar input allows using much smaller wire since the current is low.

Inverters designed to provide backup power when the grid fails or in an “off-grid” application with no utility grid are usually designed for a 12, 24, or 48 volt battery bank. Usually a 12 volt battery inverter would be under 1000 watts output, while 48 volt DC inverters are usually in the 3,600 to 4000 watt AC output range.

Lower cost solar charge controllers are usually designed to have the same voltage solar array as the battery. For example, a 48 volt solar array connected to a 48 volt battery. However, when the solar array is a long distance from the inverter and battery bank, there is a large voltage drop due to the long wire. One way to off-set is to use a “power-point” charge controller, which allows a mis-match between the solar array and the battery and inverter. For example, we may design the solar array to have a 72 volt DC output less a few volts loss going into the charge controller, with a 48 volt output going to the battery and inverter.

Hope this helps,

Jeff Yago

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