By Mike Sokol – RVelectricity and the No~Shock~Zone
Brought to you by CarGenerator.com
Integrating solar panels into your boondocking plan
While you could just move to a cabin in the woods and live totally without electric power like humanity did 100 years ago, few of us want to give up our current lifestyles. So if you want to camp without shore power from a campground pedestal or run a generator all day, there are some special technologies needed if we want to maintain our electrically charged lifestyle while off the grid. Enter the solar panel – which is the topic of Part 4 of this series.
Now, this is just a primer on solar panel theory, not an installation series. But it’s important that you know just what’s possible with a given number of panels. But as they say in the car business: “Your mileage may vary.”
What about watts and watt-hours?
Okay, even if you’ve been following some of my other articles about the differences between power and energy, now’s a good time for a quick review. A watt is simply a measure of power. There are lots of ways to produce a watt of power, but in the electrical world it’s simply 1 ampere of current flow with 1 volt of potential. Since 1 amp x 1 volt = 1 watt, even I can do that math in my head. If we have 1 ampere of current flow at 12 volts, then we can see that 1 x 12 = 12 watts. Up that to 10 amperes of current at 12 volts and now we have 10 amps x 12 volts = 120 watts.
But what about time?
Good question. You’ll notice there’s no time in our simple watt equation above, so if we want to know how much energy is expended we need to include just how long this wattage is occurring. Back to our example: If we have something producing 120 watts of power for 1 hour, then that’s 120 watt-hrs. Up to to 10 hours and now we multiply 120 watts x 10 hrs = 1,200 watt-hrs. Pretty simple, right?
What does this have to do with solar panels?
Well, the reason we need to understand this is to calculate just how much energy we can get from a 100-watt solar panel. While you might think it’s as simple as multiplying 100 watts times 10 hours in the day (1,000 watt-hrs), it’s not that simple. That’s because the 100-watt rating for a solar panel is only on a bright day with the sun directly overhead. And that only happens for a short period of time each day. Throw in the angle of the sun for the season, plus cloudy skies some of the time, and you’re lucky to get 300 watt-hrs of energy per day from each 100-watt solar panel. Maybe a little more energy on hot/sunny days; and certainly less on cloudy overcast days. But 300 watt-hrs is a good place to start.
Okay, let’s see how many solar panels you might need to fully recharge a 100-amp-hr lithium battery which can be safely discharged down to 0% of capacity. We’re going to ignore flooded cell and AGM batteries for this exercise because you don’t want to discharge them below 50% of capacity.
Since it’s a nominal 12-volt system we just multiply 12 volts x 100 amp-hrs and get 1,200 watt-hrs of stored energy. Let’s run the numbers for a Lithium battery. That suggests that we could pull 100 watts of power from this battery for 12 hours, or 1,200 watts of power for 1 hour. Of course, these are just rough numbers because your actual numbers will vary due to voltage drops and losses during high-amperage situations. But the basic theory is on target.
So how many solar panels does that take?
Now that we have everything in watt-hrs it’s pretty simple to divide 1,200 watt-hrs of battery capacity by 300 watt-hrs of energy per 100-watt solar panel and see that you’ll need at least four 100-watt solar panels to recharge a single 100-amp-hr Lithium battery. Just do this: 1,200 watt-hrs / 300 watt-hrs = 4 panels.
If you want to fully recharge a pair of lithium batteries with a total energy storage of 2,400 watt-hrs, then you’ll need perhaps 8 solar panels of 100 watts each. Now, it really doesn’t matter if you wire these in series or parallel or series/parallel for these calculations. If you have a proper PWM (Pulse Width Modulated) solar charge controller, then watts-out is going to become watts-in, because energy is energy.
How much energy do I need to boondock?
Well, that the $64,000 question, isn’t it? Now that we know it will take around 400 watts of solar panels to recharge a single 100-watt-hr Lithium battery, we just need to know how many batteries are needed to “run what we brung.” As you may remember from my Vitrifrigo/Danfoss/Briter experiment in the spring, a 12-volt DC compressor refrigerator would run for around 36 hrs. on a single 100-amp-hr Lithium battery.
So in 24 hours it would use about 70% of the energy capacity of that battery, leaving 30% in the tank. That suggests that if the only thing you wanted to run was a single 12-volt DC refrigerator, you could likely get by with 300 watts of solar panels (3 panels of 100 watts each). However, you should go with at least 400 watts of solar panels (4 panels of 100 watts each) if you want to run anything else at all (like a CPAP machine and some lights).
Can I run my air conditioner from solar panels?
Well, in my Hybrid Inverter experiment last week I found that a single 100-watt-hr Lithium battery would run an air conditioner for maybe 1 hr. (depending on compressor duty cycle due to ambient temperature). With that we can calculate that it would take 2 batteries to run it for 2 hours in the evening to cool down your RV bedroom, and that would require maybe 800 watts (8 solar panels of 100 watts each), just for this 2 hours of coolness.
Stone cold crazy…
If you want to go crazy and be able to run your air conditioner all night long (8 hours), it would take around 32 solar panels of 100 watts each to produce 300 watt-hrs x 32 panels = 9,600 watt-hrs of energy. Yikes! That’s a lotta solar panels!
While nobody will likely be building an RV with 3,200 watts of solar panels on the roof anytime soon, if we scale our expectations back a bit we can combine a few technologies to get additional power without having to build a solar farm.
No matter what you do, in order to run any 120-volt appliance in your RV while boondocking you’ll need an inverter. And to get maximum flexibility it should be a Hybrid inverter that will allow you to intermix the DC power from batteries with the AC power from a smaller power source.
One solution I’ve seen is using a 3,000-watt hybrid inverter that can run your RV air conditioner for maybe an hour or two on battery power alone. But a small 1,000-watt inverter generator could be used to supplement maybe 400 to 600 watts of rooftop solar panels.
CarGenerator™ as solar panel backup
Another possible solution for non-air conditioning boondocking is to bring along a CarGenerator™ inverter which will allow you to do emergency recharging of your RV batteries when the “sun don’t shine.” Of course, if you already have a 2,000-watt inverter generator that’s a great solution as well. But if you only need supplemental recharging power occasionally, don’t want to store, maintain and lug around a generator along with its gasoline container, then CarGenerator could be a great backup solution.
If you think that CarGenerator might be a possible backup power solution for your solar/boondocking adventures, please read this extra write-up from Jonathan Schloo, the owner of CarGenerator, and sponsor of this boondocking series.
Take it away, Jonathan…
1000 WATTS OF POWER ANYTIME BUT WITHOUT SOLAR
How about having 1000 watts of solar power available day or night, rain or shine, but without the installation expense, weight, or hassle or worrying if the sun is shining.
Our tow vehicle provides the equivalent power of ONE THOUSAND, 1000 watts of solar panels on demand at the push of a button anytime day or night, rain or shine. I can use that power to recharge my trailer batteries, or power up devices in my Airstream trailer, make coffee, recharge my laptop, watch TV and more. It costs me 1/4 gallon of diesel fuel per hour.
Wanted to share a very simple way you can measure and calculate just how much power you can safely pull from your vehicle at idle, for recharging your trailer batteries or powering up your Airstream shore power cord using an inverter, or even powering your house during a blackout.
Manufacturers spend big engineering dollars to make sure the vehicle can fully support all accessories from the alternator. My approach is to simply switch off all those accessories and pull that power out and convert it to usable AC power for your trailer or home backup power. The measurement method below determines, conservatively, exactly how much power.
If you’re a little bit handy and you can find where your alternator wire is, here’s what I did.
Buy a DC Clamp meter capable of 200 amps DC or more. Search Amazon for: Meterk Digital Clamp Multimeter 4000 for example.
1. Place the jaws around the alternator cable and let the vehicle run for 5 or 10 minutes and everything settles down, battery is fully recharged from the starting motor, etc.
2. Check amps readout. In my case with a GL350 Diesel SUV, it shows 25 amps for basic engine overhead, with no accessories switched on.
3. Switch on all the accessories your vehicle has, including cabin fan on highest AC setting, window defroster, all heated seats, full running headlights, radio, etc.
4. Check amps readout. In my case, it shows 110 amps. Subtracting these numbers shows that all my accessories total around 85 amps. So if I switch them all off, I can very safely pull out those 85 amps x 13.5v equals 1147 watts.
5. The alternator doesn’t care if it’s powering a cabin fan and headlights or powering a DC inverter. The pure sine wave inverter I use is 90% efficient so I can safely pull out 1,032 watts of power when the vehicle is parked and not using accessories.
This is roughly equivalent to having ONE THOUSAND watts of solar panels on your Airstream roof in perfect sunlight. But it works anytime day or night rain or shine.
There you go…. a safe, easy, simple way to determine how much power, at minimum, you can pull from your vehicle. This is a fairly conservative approach. In fact you could pull more power by high-idling, which many trucks support, or you could install dual 220-amp alternators so they can pull easily 200+ amps at idle.
Read more or purchase a CarGenerator™ HERE
For more info on the SL-2000 Hybrid Inverter click HERE
For more info on the SL-3000 Hybrid Inverter click HERE
Brought to you by CarGenerator.com
[Editor: You can find the Meterk Digital Clamp Multimeter 4000 which Jonathon mentions above on Amazon.]
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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