Coping: The Prepper’s Guide to Electricity

If you’ve been a reader for more than a month, or so, you may have figured out that electricity and I get along pretty well.

Not in terms of Franklin’s kite flying (right) or doing some of the math of James Clerk Maxwell, whose work was ultimately perverted by Oliver Heaviside to mask routes to advanced technologies like anti-gravity…

Regardless, we need to make reasoned judgments about things electrical.  I won’t go into the details, except to say first class commercial FCC ticket age 16, extra class ham forever, on four patents on battery state of charge instrumentation (as a team member) and building my own grid-interactive solar power system which co-powers this content sort of qualifies me as an expert.  Our mixed solar and wind system on on cruising sailboat we lived on for 10+ years also counts as hard-learned lessons….

In recent week’s I’ve  had advertising inquiries from outfits that wanted me to sell ‘em advertising for solar power producta.  That got me to thinking:  Time to haul out the BS detector and teach you how to read it.

Basics of Solar Power Part 1

1.  When you are buying solar power you are buying full sunlight watts.  Power divided by price should be your thinking here.

Right now, you can go to Harbor Freight and buy a 45-watt portable solar power kit for $189.  Let’s to the math, shall we?   $189 divided by 45 watts means you would be paying $4.20 per watt.

Now, there are other systems out there that will charge as much as $250 for a 20-watt system, but they will include a couple of laptop batteries.  Still, that works out to how much?   $12.50 per watt.

Real solar outfits, like will sell you solar for less than  a buck a watt if you know what you’re doing.  Hmmm…how hard a choice is that?

Now, in all fairness, when you see these “portable systems” they can’t be exactly compared with a “real” solar power system because people like me figure that we won’t be doing a lot of hiking about with laptops if the SHTF.

Why?  Because if the SHTF there won’t be an internet and with no internet and no printed manuals on food storage and preps, what would you need a solar laptop charger for?

Sorry to ask embarrassing questions like this, but think about it:  If you are going camping and really feel the need to write out in the woods, get a little computer with no hard drive that will give 8-hours+ of use, cost less than the solar rig (have a terrible keyboard admittedly) and call it good…

Remember, the reason prepper doesn’t just buy stuff that says “Solar” on it because it’s the “preppy thing to do.”  Hell no!  You buy this stuff because it pencils out in dollars and cents over time and until the end of the world arrives, you can still cost-justify it.  Oh, helps if it works and can actually charge a battery from dead to chock-a-block full in less than 6 hours.

2.  I have seem some of the damnedest solar panel holders for portable panels imaginable.

Me?  I’m too damn dumb to write those checks.  I’ll just find a rock and prop the panel up on that.  Or lean it up against a tree, the side of a car, or whatever.

But what about wind blowing it over?” 

When was the last time your car blew over?  Seriously?  (*Florida residents in the path of past hurricanes need not send wise-acre remarks, thank you.)

3.  Most people are woefully ignorant of battery basics for long life.

Battery facts:  Charge them too slowly and they won’t get fully charged.  Only plan on 6-hours a day of full sunlight for planning purposes unless you are going equatorial in your adventuring.

Why?  Yes the sun comes up earlier and goes down later than 6-hours worth.  Again, tracking arrays are a pretty story, but they cost money and I have yet to see one that pencils out over a 20+ year product life.

Do not discharge a lead-acid (car) type battery more than 60% of its rated capacity.  You can go deeper with lithium and NiCad, but…batteries only cycle so many times.

Battery charging must be done in not more than 4-hours, and ideally 3 1/2 hours, because the last 2 1/2 hours is for finishing charging if you’re going to be done in six hours.

Let’s use a good battery (Group 27, 100 Amp-hour deep cycle) and figure how long it will take to charge and how much panel:

The battery capacity is 100 Amp-hours and we will use 60% of that.  So we need 60-amp-hours per day of charging.  60 amps for one hour will not do it.

That’s because a discharged battery will accept charge at very high rates (like 60 amps) but only until the battery is about 85% full.  Then, the “finishing charge” (which we call “acceptance voltage”) will take about 2 1/2 hours to 3 1/2 hours (in cold climates) regardless of what you think should happen.

In multiple-stage charging there are three parts to what goes on.

a.  Bulk charging.  The battery will accept all the current you can shovel into it as long as the battery voltage does not exceed “gassing voltage”  *(bubbling off Brown’s gas which is highly explosive).

b.  Acceptance voltage:  Call it 13.8 to 14.5 volts for a 12V battery, but see manufacturer’s spec sheets for your battery.  The colder it is, the higher the acceptance voltage (finishing) voltage can be.  I’ve worked on issues involving cold-weather cycling problems where the acceptance voltage should have been 15.2 V but that was below freezing.  Using 14.1 for 2.5 hours at room temp may be a good guestimate, or 14V for 3 hours.

c.  Float voltage:  13.2V for a typical car battery.

Car batteries die in winter because they never get fully charged (12.8 may be a good general alternator setting in Phoenix, but not in Milwaukee in winter.

Phoenix batteries blow up in summer because alternators go 13.8 volts in many cases and that’s above the gassing voltage and say goodbye when the Brown’s gas goes poof!  In the engine compartment…it’s ugly.

So let’s do the calculation:  We need 60-amp-hours to recharge this old car battery. 

We need to get 50-amps of charging done in 4 hours to get about 85% of the energy back into the battery.  (It’s a bit more, actually around 52 amp-hours, but it’s early).

So the source (whatever it is) needs to put out 50-amp-hours in four hours. 

Amp-hours divided by time equals current per hour:  50 divided by 4 hours is 12.5 amps for four hours of bulk charge.

Now how many watts is this?

Remember PIE?  Power (in watts) equals current in amps [abbreviated i] times volts [abbreviated e].

So if we need 12.5 Amps times 12 volts, means we need a 150-watt panel.  And we need it for 4 hours.

If you think you can actually use a ham radio for hours on end and recharge it with a 45-watt panel, lotsa luck on that.  You can, but the recovery time won’t be six hours.

We can discuss fine points all day.  Yes, a smaller panel might work, but there is still the matter of the charge controller.  But when people build solar lash-ups they really need to have all the pieces.

4.  The most meaningful non-panel choice is in how you regulate the power coming off a panel.

If you are in now hurry (small panel) you can buy a panel that is 1.5% of battery capacity (like a 2 watt panel) and leave it hooked up to a car battery for life and it will still work for months (or years) if you service the battery.  This leads to a long discussion of battery CEF (charge efficiency factor), self discharge rates, tables and math that will hurt your brain.

Second choice is a PWM controller.  Pulse width modulated like the old Trace C-40s (See the Schneider C-40) we used to use.  Or in marine applications the Cruising Equipment Company (old alma mater) InCharge regulators for high output marine alternators. 

If you think your car or truck is really a survival platform with a stock battery and conventional alternator, hate to be rude here, but grow up!  If you want high performance electrical call my old friends at Balmar and get a real high output alternator.  (

Just remember that a “high output alternator” without advanced regulation on it is really a joke. 

Might work in a truck where the engine runs 8-hours, but not if you’re building a real life survival system which I designed for countless sailboats where power management is key…oh the stories… back to point:

For solar source, the simple Schneider C40 is what you need at minimum.

Here’s how PWM works:  Picture the power coming off your solar panel as a loaf of bread.

When the battery voltage is low, the whole “loaf of bread” is shoved into the battery.

When the voltage gets up to acceptance voltage (14.1 or whatever is set) then the break begins to get sliced.

As the current (at 14.1 volts) continues to drop (or the voltage tries to go higher) then “air” begins to appear between slices.  Until, with full battery, you are doing mostly air and now much power.

At the end of the timed cycle (time at acceptance voltage) the timer drops the PWM to float voltage (13.2 or whatever you’ve set) and now you have a full battery.

5. MPT controllers are a little different.

They play by the same charging rules, but they are more expensive because they incorporate a small buck-boost inverter and a microprocessor to extract as much power as possible from your solar panels.

Every solar panel has a “sweet spot” where it puts out the most power.  Some combination of voltage and current.  What happens if the max power point is at 16 volts for a panel?  Well, dumping that directly (or via PWM controller) into a battery will load the panel too heavily and its output (watts) will be lowered.

So in a Maximum Power Point Tracking controller, the charger remembers what voltage on the panel delivers max power, and then always delivers the right load for the panel to see and then it shoves whatever the battery needs out the other using PWM or other techniques on that side.

The result is that an MPPT controller can give 15-20% more output under certain conditions (like winter with low sun angles).

Tomorrow, I’ll go shopping and give you a real solar package idea which would support basic home ops including a radio and so forth.

But I wanted to get you thinking about not under-sizing the panels which is the worst thing you can do…but remember you can’t just plug in that much power without a controller if you don’t want to blow up batteries.

Feel free to send in questions…I will try to answer them here, on on the discussion group side.

For Peoplenomics subscribers, look up the word “robust” (as in home power) in the Master Index and you’ll find details about our “real” system:  3.5 kw of panels, stacked sine wave inverters, dual MPPT controllers and more…pay particular attention to wire size, too.

Controllers are not 100% efficient, so our 150 watt panel might need to be upsized to a 165 watt panel even if the charge controller claims 90% efficiency.  Or a 180-watt panel if they happen to be cheaper.


Write when you break-even


16 thoughts on “Coping: The Prepper’s Guide to Electricity”

  1. Well George, looks like you have the foundation built for your next ebook. Thanks for especially for the tickler that standard auto or truck alternators are not the best route to take.

    Any good links or thoughts appliances/equipment which can be run off the output of a 12V alternator?

  2. On the other hand, Greece would not have its woes if they had simply taxed the hell out of some of their shipping magnates but again, I repeat myself, it’s all drearily similar to present US directions.

    No, George, this is simply more socialism. Tax the rich becomes proof of concept for tax the peon. The problem is government taxing at all. Historically, there is a 3x multiplier for creating jobs, One full-time job with benefits creates three additional jobs in the economy. The more taxation, the lower this multiplier becomes until it becomes zero as we are seeing in United States today. To those unaware of this concept this effect shows up as a reduction in the velocity of money. Government jobs and government entitlements actually have a negative effect on an economy. The founding fathers had it correct when they allowed taxation by the federal government only in the form of excise taxes, taxes that can be avoided, and import tariffs. Direct taxation by the federal government was specifically prohibited by the Constitution. Today there are two direct taxes in United States, the IRS income tax and Obamacare. In a certain industry which shall remain unnamed this equates to double penetration for American taxpayers.

  3. George:

    Enjoyed your article on PV systems. (1/22/2015)

    I browse through often and marvel at the PV prices.

    How does one know if the PVs being offered are any good or will last?
    I bought a single 200 Watt PV from sunelec a few years ago and it did not produce rated output under the best of conditions.

    Are there good, better, best brands of PVs?
    Which ones should I buy when I start building my 10KW system?

    Gary C

  4. Nobody replying. You must of fried there brains. Kept me awake after lunch and that’s hard to do. How about working something up with car alternators. We could put them up in the air and attach air blades to them to charge the batteries. This can be done cheep, buy the alternators at the junk yard . I plan on trying this when I retire from my day job. But maybe someone out there has tried this. Please write in. Zenny

  5. “Right now, you can go to Harbor Freight and buy a 45-watt portable solar power kit for $189. Let’s to the math, shall we? $189 divided by 45 watts means you would be paying $4.20 per watt.” IF my math is correct – that is the first 45 watts – after that – and the power kit is paid for (well – yes there’s more math to it and it will really take more watts to make this work) but once the kit is paid for – the only cost to more watts is what – sunshine?

  6. Dear George, Good Article. Funny(Write when you break-even). I am rolling on the floor. I have 4-120 watt panels and 6 – 6 volt 405 Amp-Hour batteries So 1215 Amp Hr @12 volt, with a Trace C-40.I use multiple 12 volt 2000watt inverters with smaller ones to run lights and. Six(6) hours charging is about right in winter, but 8 hours is what I get in summer. It will take me about 10 years to break even. But it sure is nice having a backup for my lights and other appliances. Thanks for the info.

  7. Would a Flywheel electrical storage system be better or worse?
    I know they are expensive, but a flywheel should last quite a bit longer.

  8. Wow; great! Looking to make a home-size purchase (off-grid, seasonal dwelling) by spring. This is just what I need. Am anxiously awaiting tomorrow’s article, hopefully detailing specifics for a very cold climate situation. Thanks.

  9. Great info as I learn how to set up off-grid system. Q1) flooded batteries (&AGM?)give off hydrogen when recharging…does this mean you cannot have them inside house? Q2)Are there practical limits to cable length from a)panels to controller & batteries, or b)batteries to DC output device?

  10. When/where do I find out how to set up a home to be totally solar like how many panels for the average 1200 square foot home extremely well built environmentally ?

  11. We hv a 12kw northern lights fueled by 750 gal of diesel sitting in the water behind the house in the form of a boat we have used it twice in power outage and also have a system set up with our hybrid car and inverter keeps the lights on and depedant on how advanced you want to get can run other stuff I can’t find the link I used but it’s out there. We sell xantec and have had only one issue with warranty .

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