A short history of the sun

Several people wrote to me yesterday to say “thanks” for yesterday’s blog post on solar.   It’s amazing to me how much information there is on the Internet about RV solar power, and yet how little of it is actually useful or even accurate. So I’m going to write a little more about it today.


Wednesday was a less challenging day for solar power than I had expected. By afternoon the skies cleared up and we had good power generation for a few hours.  You really get the bulk of power between 10 a.m. and 2 p.m., no matter what time of year or latitude, if you have fixed panels that always face directly upward like we do.  (People with tilting panels have a big advantage, because they can capture light at a more direct angle during the morning and late afternoon. I’d like to install those on the Airstream but so far I haven’t found a practical and cost-effective solution.)

The batteries started the day down 34.7 amp-hours.  I used the laptop for eight hours, and Eleanor used hers for about an hour, plus we recharged phones and camera batteries.  Even with this relatively heavy load, the batteries ended up at -15.4 amp-hours (a net gain of 19.3 amp-hours).  When you figure in the power we used while the sun was shining, we probably generated about 40-50 amp-hours during the day.  Not bad for a half-cloudy day.

I give these statistics as guidelines of how things might work for you, but it’s important to keep that the bottom line of solar use is that every situation is different. The key variables are: sun angles (time of year, latitude, time of day), cloudiness, panel generating capacity, and storage capacity.  A lot of the websites go on and on about wiring losses and other electrical engineering details, but in real life a single leaf on your panel can have a much larger effect on power generation.  Don’t get hung up on whether your wires are big enough if you haven’t first tried cleaning the glass.

Because there are so many variables, it’s impossible to answer the question I get all the time:  “Is my system big enough?”   Big enough for what?  From trip to trip, I never know how much power we are going to generate in advance (I’d be a great weatherman if I could).  The best description I ever heard was that “solar makes your batteries bigger.”   Think about it that way and don’t worry about having unlimited power — even with a generator, it’s an illusion.

I’m just happy that we can camp for long periods without power connections, at least in the summer.  We’ve been here at Horseneck Beach since Saturday.  Just for comparison, if we had the original factory batteries and no solar panels we would have run out of power on Monday.

Now, since I mentioned generators, I feel obliged to explain why people who have generators often are seriously deluded about what’s really happening when they use it to “re-charge” the batteries. It really doesn’t work, at least not with the standard gear that comes with most trailers.

The reason is based on the fact that batteries will only accept re-charge at a certain rate.  As they get more charged, they resist, and so the rate of charge declines.  It doesn’t matter how big your generator is; you could plug that battery into a nuclear power station and it still won’t charge any faster.   A “smarter” charger will do better than the really dumb 2-stage chargers that seem to be installed in most trailers, but only to a point.

For example, your batteries might accept a charging rate of 15 amps (DC) when they are really heavily discharged, and 5 amps when they are 25% discharged, and 1 amp when they are 10% discharged.  If you’ve got an 80 amp-hour battery bank, getting from 90% to 100% charge could take eight hours or more.  That nice quiet 2000-watt generator you use will produce a whopping 150 DC amps at its normal maximum output rate, which is obviously way more than the batteries will accept at any given time.  The rest of the power is wasted, unless you are running the microwave or some other AC appliance while the generator is running.

The other problem is that the factory installed “battery monitors” are almost always cheap-o versions that guess at the batteries’ state of charge by measuring voltage.  This is incredibly inaccurate, especially those lousy units that show the battery condition using Red, Yellow, and Green LEDs.  Would you drive a car with a gas tank gauge that just showed red, yellow, and green?  Even worse, these units will show Yellow when there’s a heavy power demand even if the batteries are full, and they will show Green when the batteries are actually quite discharged but have recently been charged just a little.  Imagine that the car’s gauge went to Yellow every time you pressed the accelerator.

Try it sometime.  Use your batteries for a day or two, until they show Yellow constantly.  Then plug in for 30 minutes, unplug, and watch as (miraculously!) the monitor reports Green or “100%”.  Don’t believe it.  That’s what is sometimes called a “surface charge.” It’s a symptom of the battery monitor being fooled because it measures voltage.  The voltage pops up for a short time after charging, but it won’t last.  To get an accurate view of battery charge using voltage, you need to let the batteries “rest” (no drain, no charge) for at least an hour.  That almost never happens in a camping situation.

So here’s the scenario I see all too often:  After a day of camping, the owners decide it’s time to charge up the batteries.  They fire up the generator, plug in, and let it run for an hour or two.  The voltage-based battery monitor says all is well, so they turn off the generator and go to bed secure in the knowledge that they are “all charged up!” Except they really aren’t.

In two hours, the best that generator is can do is pump in maybe 10 amp-hours, if the batteries were moderately discharged to start.  Rather than being “100%” the reality is that if they started at 70%, they might now be at 85%.  So the next morning, the campers wake up and use a little power for the water pump, and by 10 a.m. they are amazed to see that they are back in the Yellow zone.  What happened?

So they plug in the generator again, and this time they run it for three hours, getting up to 88% charge.  At this point the batteries are really resisting further charge, so only about 1 or 2 amps of the 150 amps that generator can produce is actually getting into the batteries.   The next day, same problem — the battery monitor says they are still stuck in the Yellow zone.

Solar has a huge advantage here.  A steady all-day charge will get your batteries up to 100%. It’s like the turtle and the hare.  With batteries, slow and steady wins the race. If you have both a generator and solar panels, use the generator only when the batteries are heavily discharged (for an hour or so in the morning, for example) to get the bulk charge done quickly, and then let solar finish the job over the course of the day.

If you only have solar, keep in mind that during the morning and mid-day, moderately or heavily discharged batteries will probably accept every amp the panels can generate.  Then the charging rate naturally slows down.  In our case, by mid-afternoon the batteries are usually in the 90-100% range, and the charging rate has slowed to perhaps 1 amp.  If the panels are still generating 5 amps, we have surplus power, and so that’s the time of day we plug in all of our rechargeable accessories like phones, cameras, Kindle, laptops, etc.  This strategy takes maximum advantage of the power being generated.

Another good time to use a generator is when power demand is high.  It’s much easier to avoid using battery power (by being plugged into the generator) than to try to recharge battery power later.  So if you have small batteries, use the generator in the evening when you are making dinner, and any power consumed will be supplied by the generator.


  1. says

    In my years of searching, that’s probably the best description of solar charging I’ve come across. It’s also a great endorsement for solar panels!

    Have you come to a conclusion whether roof-mount solar panels are better or worse than separate modules? (Roof, of course being vastly more convenient, but separate being able to be repositioned for maximum direct sunlight impingement?)

    Are you planning on going to the Penn Woods Autumn Leaf rally at the end of the month? Barring circumstances, we’ll be there!

  2. says

    Thanks, Craig. Generally speaking I would always opt for a repositionable/tiltable panel if possible.

    Sorry to say we won’t be at the Penn Wood rally. At the end of the month, we need to be in Virginia and then heading south.

  3. dave.gt says

    Thanks for the solar panel discussion. Even I can follow that with only one question at the moment. How do know what each fixture/computer/camera battery will pull from the batteries? Do you have a chart that you put together?

  4. says

    Dave, just about any device with an AC plug has a rating plate that tells how much power it pulls. But we have a highly accurate amp-hour meter in our trailer (the Tri-Metric 2020 that I’ve referred to before). We can see the exact amount of power that each device uses when we turn it on.

  5. Brett says

    If you don’t have the Tri-Metric and you are trying to do the math you need to factor the 12 volt to 120 volt conversion. 1 AMP draw AC is a 12 AMP draw DC, so do your math carefully!

    The calculation is AC AMPS*12 Volts=DC AMPS Then factor run time (hours) to get the total AMP hour draw.

    So a 1AMP AC draw will use 12AMPS of DC power in 1 hour.

    Class dismissed!

  6. says

    Careful doing calculations with AC amps. The above calculation is kinda-sorta correct, assuming the AC amperage is given in RMS. 1 amp RMS at 120 volts would actually be 10 amps at 12V DC (not 12).

    If you’re running an AC device off an inverter connected to a 12V battery, the 12A answer might be pretty close, however, because the inverter wastes maybe 10-20% of the battery energy in the process of conversion.

    If you’re trying to take that back one step further, (to how much solar capacity you need to replace that battery energy) you have more losses in the system – the battery charging efficiency and the solar controller efficiency both affect the result.

    However, there are lots of other issues using the label on an AC appliance. The biggest of these is that the AC appliance probably gives the instantaneous peak maximum that the device could ever draw under any circumstances, rather than the actual average you’ll see in normal operation.

    As Rich says above, the only very good way to see how much battery load an AC appliance will create is to plug it in, turn it on, and watch with a good battery monitor to see how much current is actually being pulled from your battery. Doing the math from the specifications on the label – even if you’re an engineer – won’t get you very close.

    As an example, we have a “1000 Watt” inverter in our trailer. We have an espresso machine that says “1600 Watts” on the label. Seems like it could never work, right?


    Running the machine on our inverter and watching the current on our battery monitor, we see the DC current cycling from 20 Amps to 120 Amps in about 5 second intervals.

    That 120 Amps is only about 1450 Watts (depending on the exact battery voltage) and it only happens for about a half second out of every 5 seconds. The average power is much lower – I’m guessing about 850 Watts (which would work out to about 70 Amps DC). Also, looking more closely at the specifications of the inverter, it says that 1000 Watts is the maximum continuous output, but that peaks of up to 1600 Watts can be tolerated for a few seconds, and smaller peaks for a few minutes…

    The results? Every day we have two nice cappuccinos while dry-camping on solar – even though doing the math with the specifications says we should be drinking stove-cooked drip coffee.

    NOTE: Our inverter has great circuit protection with resettable breakers. That makes me comfortable with the “try it and see” method. Your inverter might not be so well protected, so be very cautious plugging big loads into your inverter if you’re not sure.