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.

The wind is back at Horseneck Beach campground.  The reservation website did warn that “sites can be breezy.”  People who come here regularly seem to be prepared for it, as I see a lot of kites and streamers attached to poles.  The big round rocks from the beach make handy weights to keep your belongings from blowing away, too.

So we broke out the kite that we’ve owned for many years and hardly ever flown.   Eleanor admits to being a hopeless kite pilot, and Emma has surprisingly little knowledge of kites, so there was some initial confusion, but eventually up it went, and it flew very well … until the inevitable crashes in the wild rose bushes. The kite survived to fly another day.

We didn’t do much else.  In the morning, we cleaned up the Airstream and relocated it to another campsite. (We’ve booked two more days here, but that meant we had to switch sites.)  I was tied to the computer the rest of the day, editing articles for the upcoming Winter 2010 and Spring 2011 issues.  Still, working near the beach on a beautiful sunny day gives the opportunity for very pleasant breaks, walking along the seashore or just enjoying the sun and salty breeze blowing through the trailer.

We had thought that in the evening we might hit one of the local restaurants for another seafood dinner, but of course being post-Labor Day, all of the tourism-related businesses are closing or reducing hours.  We settled for some interesting takeout from the local grocery and a movie at home.  It has gotten very quiet here, and that’s nice.

Solar report:  Since not much is happening, I’ve got an opportunity to talk a little more about solar.  Forgive me if this topic bores you, but I get a lot of inquiries from blog readers, so I know there is a need for real-world information.  Yesterday was full sun again, and with our 230 watt panel array, at this time of year (when the sun angle is still fairly high), we really can’t store all the power we generate.

We started Tuesday morning with a 36.0 amp-hour deficit, which is fairly normal as a result of using lights, water pump, and laptop the night before.  During Tuesday I was on the laptop for at least six hours, consuming about 9-10 amp-hours in total, and Eleanor used probably another 3-4 amp-hours charging her laptop.  Still our batteries were at 99% by 5 p.m., indicating that we generated more than 50 amp-hours during the day.

That’s all well and good while the sun shines, but the other half of a solar charging system is storage.  You’ve got to have battery capacity that is matched to your power needs and the capacity of your panels.  Long-time blog readers know that since January 2010 we have used a single Lifeline GPL-4D, which has a rated capacity of 210 amp-hours.  Today (Wednesday), we will see that half of the system put to the test.  The forecast is for gloomy skies and rain — in fact, it just started to rain as I am typing this.  Under the rain cloud, our power generation has been cut from 8-9 amps when sunny to a miniscule 0.8 -2.1 amps (depending on the thickness of cloud).  Essentially, at this moment we are generating only enough power to make up for the parasitic drain that is inherent in all modern RVs.  So, under those circumstances our power supply would be entirely reliant on the battery.

We started today with a deficit of 34.7 amp-hours. If we generate no power today, I would expect the battery to be drained by a further 20-30 amp-hours by 5 p.m., just as a result of my laptop use and normal parasitic drain. (Parasitic drain comes from the circuit boards and always-on electronic modules in the stereo, refrigerator, thermostat, propane leak detector, etc., and totals about 0.5 amps per hour.)  Tonight, we will use another 25-35 amp-hours, which means we could end up tomorrow morning with a deficit of 80-100 amp-hours.

Ideally you never want to discharge the battery by more than 50%, which means that at 110 amp-hours we need to either find some sunshine, or plug in to power.  If we are not cognizant of our power budget today, and the weather continues as it is, we’ll be up against that limit tomorrow.

Now, there are three practical responses to this.  (1) We could add more battery capacity.  A second Lifeline GPL-4D battery would double our capacity, but at the cost of about $500-600 and 135 pounds of added weight.   Also, given the rating of our solar panels, we would be hard pressed to recharge a deficit of 110 amp-hours under typical conditions.  Certainly we’d never be able to do it in a single day.  So our batteries and panels would not be well matched, although that’s not a big problem.

(2) We can plan ahead.  For example, we know that tomorrow we are pulling out of the campground and relocating.  There’s a very good chance that we will get at least partial sunshine while we are towing, which will yield power for the batteries.  (The tow vehicle does not add a significant amount of power to the batteries while towing, by the way — that’s sort of an urban myth.)  Or, if we know we are going somewhere that we can plug in, we don’t need to worry about reaching our power limit tonight.

(3) We can cut our power usage.  I could relocate to a coffee shop somewhere and use their power for my laptop.  I could take the afternoon off.  We could use only the LED lights in the trailer tonight.  We could use the campground showers to avoid using the power-hungry water pump in the trailer.  I find that a lot of people hate conserving because it makes them feel deprived, but we don’t mind so much.  If I’m “forced” to quit work early, I can live with that.

But I probably won’t get that excuse.  That’s because, in the real world, the weather changes.  It probably won’t stay cloudy all day.  Even as I’ve been typing this, the rain has stopped and some breaks have appeared in the skies.  Right now we are generating 6-8 amps, which is a very healthy rate for recharging the Lifeline. Despite fluctuations, we should generate at least enough power today to offset my laptop use.

It’s important to consider this, because people who don’t like solar power tend to invent worst-case scenarios to “prove” that it is impractical, and those scenarios often include cloudy skies.  That’s like proving generators don’t work when the gas tank is empty.  Especially in the northeast and northwest, it is quite possible to have extended cloudy periods that make solar impractical, but even in those cases it is useful.   The point is not to have unlimited power capacity.  For us, solar is a tool to enable us to camp peacefully while silent, maintenance-free power streams into our batteries automatically and extends our time in a great spot like this one.