Coping: With Solar Power

Funny how life works.

Yesterday morning, out of the blue, the power went off.

Even funnier?

I didn’t even know about it.

Until, that it, my lovely bride and brother-in-law wandered into my office and asked:  “Did you know the power was off?”

I gave a gesture around the room:  My super-computer (in the midst of an upgrade to an SSD) was still on.  So were all four monitors that display what’s going on via the two dual HDMI video cards.  The music was on.  Zeus the cat was laying half up against the 750-watt electric heater *(which was on low).  My halogen desk lamp was on.  The scanner was still displaying blue ready for scanning.  The duplex laser printer just had a page coming out of it.

No, how would I know the power’s off?”

This, my friend, is the joy of a huge, massively over-built power system.  I seem to remember there had been a single “beep” from the computer, router, and satellite uplink UPS power system a few minutes earlier.  But the light hadn’t gone out.

Still, even though my office was warm and cozy while the rest of the property was getting more than 3-inches of rain, I picked up the analog old-style wired phone and called the local power company and reported the outage, although what most people may not appreciate is that while yes, Smart Power Meters may slowly morph our genetics, the Smart Meter would also stop reporting and power companies know that when blocks of meters go off-line together, there’s an outage.

Checking for Dangerous Backfeed

I turned off the halogen lighting and the heater.  They’d been on since morning and I didn’t really need additional heat or light. and had Elaine follow me out to the power center.

I held up the 7-watt light in a plug-in light socket – one of those $2 parts that you can find at Lowes which is useful to see when an outlet is powered up, or not.

Even though I don’t need to do this, I do it anyway,” I said, as the light was plugged into an outlet box labeled “Inverter 1 grid side” and then into “Inverter 2 grid side.”

As it should, the light stayed out.

Do I need to know how to do that?”  Elaine wondered.

No, I just do it whenever we are at home to make sure the grid-interactive part of our system is still dropping the grid like it should.  “ A note to the system log followed.  “Tested system for no backfeed” and the date.

What’s backfeed and why does it scare the hell out of power company linemen?

Suppose you have a generator and the power goes down and you haven’t spent the money to put in a real transfer switch to physically disconnect your home  from the grid in event a wire goes down somewhere down the road a ways.

People often times will plug a generator into the closest outlet in their home and if they are lucky, that leg of the 240 VAC power coming in will bring up a TV and the fridge and maybe some lights in the house.

Few bother to think about the downed line.  A generator that’s plugged into house wiring will feed power back down the line to where it is broken.

It’s enough to kill an innocent lineman.

So apparently, Universe arranged our Thursday to make a major point about real back-up power systems.


Now, about that Real Emergency Power System…

Hers is the shopping list I would go with for a really simple system:

Renogy 100W Mono Starter Kit: 100W Solar Panel+20′ Solar Cable+30A PWM Charge Controller+Z Bracket Mounts   This will set you back $185 at Amazon.  For another $150, add a second RENOGY 100 Watt 100w Monocrystalline Photovoltaic PV Solar Panel Module 12V Battery Charging as their 30-amp charge controller should handle the power of both cells.

Then you just need a good to great deep cycle battery and an inverter: Power Bright APS1000-12 Pure Sine Power Inverter 1000 Watt continuous / 2000 watt Peak 12 Volt DC To 120 Volt AC which is about $280 bucks.

Inverter School 101

There are two kinds of inverters:  The cheapo kind are “modified sine wave” and the way these work, if you look at their output on a scope is it looks like those stepped pyramids in Mexico, rather than a smooth rise.  It’s a stair-step kind of thing.

The pure sine wave is a better choice if you are planning to use a television, microwave, or radio gear because those “steps” may cause microwaves to hum oddly, televisions to have oddities to their pictures, and ham gear to hear “hash” on low bands.

But they are less than half the price of pure sine wave and if the batter is fully charged, you can microwave popcorn with either variety.

Battery Capacity 102

Now we need to talk about your battery capacity.

The most important thing to know is scientifically called the Peukert Exponent.  Old Wilhelm (going from memory) came up with this law in the 1800’s.  What he discovered was simply this:  The faster you discharge a battery, the lower its effective capacity is. 

For the science nerds, the details are in Wikipedia:

Manufacturers rate the capacity of a battery with reference to a discharge time. For example, a battery might be rated at 100 A·h when discharged at a rate that will fully discharge the battery in 20 hours. In this example, the discharge current would be 5 amperes. If the battery is discharged in a shorter time, with a higher current, the delivered capacity is less. Peukert’s law describes a power relationship between the discharge current (normalized to some base rated current) and delivered capacity (normalized to the rated capacity) over some specified range of discharge currents. If the exponent constant k was one, the delivered capacity would be independent of the current. For a lead–acid battery, however, the value of k is typically between 1.1 and 1.3. It generally ranges from 1.05 to 1.15 for VRSLAB AGM batteries, from 1.1 to 1.25 for gel, and from 1.2 to 1.6 for flooded batteries.[1] The Peukert constant varies according to the age of the battery, generally increasing with age. Application at low discharge rates must take into account the battery self-discharge current. At very high currents, practical batteries will give even less capacity than predicted from a fixed exponent. The equation does not account for the effect of temperature on battery capacity.

Although the new Xantrex 84-2031-00 Link Pro Battery Monitor ($260) user guide doesn’t get deeply into a discussion of Peukert exponents, they are terribly important and the reason we built the forerunner product (the original eMeter at Cruising Equipment which was also branded the Link 10 for then sister company Heart Interface, before the whole shebang was rolled up under the Xantrex label) was to make the best battery “fuel gauge” possible.

The BIG lesson we wrote on page 25 of the old Link 10 manual (that you can still find here) is on page 25 where this handy-dandy chart appears.  (I wrote the manual and in this part was explaining that most battery capacities (marine and RV deep cycle type) are rated based on their 20-hour discharge rates.  So our typical Group 27 100 Amp-hour wet cell should support a 5 amp load for 20 hours before crossing the 10.5 volt threshold which is where a 12-Volt battery is considered dead.

imageNow, if you increase the load (like microwaving or running a high power ham radio) what happens if you eat way more than 5-amp loads?

Well, you can see that if you  are running a 100 amp load, the battery capacity is effective cut to less than half!

imageThere is so much more information about batteries in the old Link 10 manual (like our discussion of how to solve for Peukert’s exponent and do the calculations is on page 39 of the manual) with discussions like Charge Efficiency Factors and so forth, it really is worth a ready because it’s about the best battery course out there.

Still, I have to agree, somewhat, with the vastly simplified product documentation Xantrex provides on its newer product.  Do you really need to know how Peukert’s exponent is derived?  I mean like any more than you need to understand how signal strength of the RF envelope arriving at a cell tower’s receiver scores link quality and compares it to the same digital signature from nearby towers?

Sadly, our technology is leaving us behind and becoming magic to most people.

Nevertheless, one of these days, I will even be adding a Link Pro to my system because the Outback Power Systems MATE monitor on our system doesn’t tell us anything (except voltage and charge current) about our battery bank.

I wouldn’t be worth a damn as a technology business geek if I didn’t have this compulsive need to know…

The Professional Grade Battery Installation

A lot of readers will have problems with batteries if they start stacking up golf cart batteries (6V type) in order to build a bigger capacity battery bank.

You simply can’t screw up two six-volt batteries in series.  BUT if you have four, or more, you can screw it up and here’s why:

imageWhen you hook the batteries up this way:

What happens?

There is a wire from the + side to the other + terminal highlighted in yellow and a wire from the – terminal, too.

These wires are very small “resistors” electrically speaking, so the lower pair of batteries will do a bit less work than the top pair of batteries.  Rinse and repeat for 6 months and you will have four batteries will along the road to perdition.


By connecting your + and – connections at opposite corners, do you see what happens?

Both pairs of batteries has an equal amount of wire resistance in it.

The bottom string has the left yellow wire resistance while the top string has the right wire’s resistance.

Which *(if you make your cables equal length for these connections, which they should be) will result in vastly improved battery life.

I am amazed at how many people in the battery business don’t know this one…or, if they do, they don’t tell people about it.

But, then again, what do we say around here?  Oh, yes…Everything’s a Business Model.

Like I said, more in the Peoplenomics archives on solar, but this ought to at least get you buying by the watt, and if you are a dry-camper or anyone who uses lots of battery capacity (like transoceanic sailors) where life depends on batteries to run the water-maker, then the Link Pro of any of the kilowatt-hour integrating amp-hour/capacity meters we designed in the late 1990’s are still worth their weight in gold.

Some day I’ll tell you how I got my “Skunk Works” patch…or no, I can’t.  But I can tell you that the outfit I was with really knew their poop when came to DC power systems.

Ad of the Week

Out of Pro Sound News – the ad.  Not only does their ad feature the headline “Post Apocalypse Wireless Mics…shown being worn by over-sized cockroaches, but their tag line rocks, too:

Made in the USA by a Bunch of Fanatics.”

Keep them in mind…that’s the kind of kick-ass ‘tude that makes it in the world, any more.

Ya’ll come back Monday when we will return to the light-hearted frivolities. Including reader comments on this series and some field reports from actual DC power users on their learnings along the way.

Meantime, write when you break-even,



Coping: With Solar Power — 19 Comments

  1. George,Although we have trod different paths to get electrical redundancy, since you have the expertise, what are your thoughts about EMP/HEMP/ Carrington event protections? I am spun up on the “frequency ” of the Carrington cycles but the EMP/HEMP possibility from enemies is of significant concern.
    Many thanks, Graham

  2. better option for batteries. Rolls Battery Bank 12VDC 428 Ah 2 Rolls 6v, 428 Ah.

  3. Pardon my ignorance…why do you recommend a a 1000W/2000W inverter for a 200W (2x100W) set of panels.

  4. Can you recommend a marine/RV deep cycle battery(ies) to complete the Sol-Ure kit for those of us planning on linking over to Amazon and buying the whole set-up.

    • Just the local interstate 6 v golf cart batteries are fine. If you buy them from afar, you get screwed on shipping. Also, find some old cores for trade in or work two or three dealers around so they don’t screw you with a “core charge”

  5. i really wouldnt publish that because it would give people ideas that can hurt someone

    • another reason not to hotwire your solar or generator to the house is when linemen come through your block they might think there is power on your block unless they can actually hear the generator where as if you are a registered back feeder they will know on their onboard computers

    • We agree to disagree on this point: the kind of people who commit mean malicious acts generally don’t have the money/brains to read a column like this one.

  6. i say never never never also unless you are me i have an old house that has 2 old pull out master fuse holders with handle rings on them , i made up a short 2ft jumper extension cord with both end males and took 2 plastic wire ties and connected the jumper cord to both handle rings ,so when power is out the fuses and jumper cord go to the other room as one piece to go from inverter to female wall socket

  7. Hi George-A question on the backfeed problem. If you have a genset with an induction generator and the utility goes down, there is no reactive current and the induction generator cannot produce power. If you have a synchronous generator connected to the grid without an automatic disconnect, when the grid goes down, the synchronous genset will be trying to supply power to an infinite load which will stop it dead in its tracks and catastrophically destroy the genset. Is this correct or did I get electrical engineering 101 wrong?


    • Hi Mike (cold up there this time of year!)
      You’re right…which is why automatic transfer switches are so necessary…. the best way is still a disconnected genset that auto starts the gen set – see Xantrex – I think they made one…

  8. Reading you on solar power is making me think about the option. I wonder, what is the capacity of your source (KW)? Do you use batteries to store it? What is capacity of them?
    If any other pertinent info you may find help me in this endeavor, please, let me know.
    Thank you much, Rafaela

  9. George –

    You are correct about the dangers of backfeeding a generator into the grid. But you do not need a transfer switch to make it safe for the lineman doing repairs. All you need to do is switch the main breaker to the house to the off position and it serves as a transfer switch. A transfer switch is nice as it is generally near the generator connection to the house but you can accomplish the same thing by disconnecting the main breaker in the basement.

    • The main breaker does NOT serve as a transfer switch. It serves as a line disconnect, and one that you must carefully check each time you energize the house wiring from the generator or other local source. It’s a terribly dangerous idea for the following reasons:

      1. Each time YOU must be absolutely sure that the line disconnect is OFF before energizing any of the wiring from your local source.

      2. Each time YOU must be absolutely sure that local power is disconnected from the main panel BEFORE the main breaker is energized.

      3. YOU must be absolutely sure that nobody else will flip a breaker, including the main under any other circumstance. How will you do this – by padlocking the panel?

      4. YOU must be certain that YOU will NEVER make a mistake, since that could cost a lineman his life. Don’t forget, YOUR power will uplink through the mains transformers on the pole and feed 7000 volts as far as they reach. Potentially, the voltage could be boosted much higher. If you are lucky, your breakers will be blown or your system fried before anyone gets killed.

      5. YOU must be certain that nobody else in your household will EVER make a mistake – EVER. I wouldn’t even trust myself to do that every time under stressful circumstances.

      6. You need to be certain that all of the thousands/millions of other people “just like you” won’t fall prey to any of the above mistakes.

      7. Using a transfer contactor, you can be certain that your house will “fail over” to your backup system automatically. Using a proper transfer switch makes sure that none of the above mistakes(and liability) will happen on your watch.

  10. Sent you a diesel related e-mail. Home Depot lower back brace helps me. Great e-power article. We are set up like a B-707/747 electrical system redundancy but are going to add solar per your great tips. Many thanks, Graham

  11. George-I am 71 female and electrically,technically challenged. I live at 1650 feet in the Catskill hills-wind blows a lot but it is beautiful. Right before Y2K I bought a 5000 watt generator and had my well pump electric line fixed as a male plug so it could plug into the grid if the electricity was on or into the 220 line from the generator if electricity(grid) was down. The water goes into a gravity feed LP hot water burner so I can take showers. The LP fireplace works without electricity. I also have a 110 line to a power bar so I can run the refrigerator and a few lights. I built a post and beam house that is super insulated on a sand box with 10″ pipes circulating heat from the 22 foot ceiling. New York State and the Federal government had some really good financial incentives for a 6K solar system, so I had that installed but without batteries because I didn’t think I could handle their maintenance. My 3 big dogs(2 German Shepherds and a Golden Retriever) are my watch dogs. I love reading your web site to know what you and others are doing.