Is 150-watts of solar panels enough?

A quick primer of how many solar panels you need to boondock. Plus a link to my webcast on solar and batteries needed for 12-volt refrigerators...

Everyone,

During our live webcast on June 11, Tony Barthel (from Stressless Camping) and I had a lot of great questions about the amount of solar and battery storage needed for boondocking with a 12-volt refrigerator. And I also received a number of comments after the webcast about providing a simple overview of how to calculate the number of solar panels and batteries needed. Yes, I realize that all this terminology can be daunting, so here’s my basic primer on how these calculations work.

How to calculate the solar and battery requirements for boondocking

Hey Mike

One of the RVs I’m looking at has 150 watts of solar panels and a 100 amp-hr lithium battery. The salesman says that a 150-watt panel is plenty to keep my battery charged for a few days of dry camping. It also has a new 12-volt DC refrigerator, and I’ll need to use a CPAP machine at night. Is this guy correct, or is he blowing sales smoke up my skirt? —Alfonso

Dear Alfonso,

Ah, yes… This is a case of over-promising and under-delivering. Either he has no idea how solar works, or he’s just lying to make a sale. Let’s look at the numbers to see how this actually works.

How much battery storage does it have?

As I’ve noted many times here, a lithium battery can be discharged down to 0% State of Charge (SoC) thousands of times without damage. To calculate available energy we just need to multiply 12 volts times 100 amp-hrs to see that it equals 1,200 watt-hrs of energy.

Of course, if you had a 100 amp-hr flooded cell or AGM battery, they should only be discharged down to 50% State of Charge for best life, which would be 600 watt-hrs of energy.

How much energy will a 150-watt solar panel provide?

The general rule of thumb is that each 100 watts of solar panels will be able to provide between 300 and 400 watt-hrs of charging energy per day, depending on cloud cover, angle of the sun, and length of day. Since you have 150 watts of solar, that equals 1.5 times 100 watts of solar panels. So if you multiply 1.5 times the 300 Watt-hours per day per 100-watt panel, that equals around of 450 watt-hrs of charging per sunny day with 150 watss of solar panels. And that’s possibly as much as 1.5 times 400Wh per day which equals 600 watt-hrs per day in a sunny climate. But let’s use 400 watt-hrs per day with your 150-watt solar panel to account for real-world losses.

How fast will that charge the battery?

With everything turned off in your RV, that 150-watt panel will take at least 3 sunny days to completely recharge a 100 amp-hr lithium battery. That’s because 1,200 watt-hrs of battery divided by 400 watt-hrs per day of charging equals 3 days of time.(1,200Wh / 400Wh = 3 days)

How much energy will a 12-volt refrigerator use?

Of course, this depends on a lot of variables. But my experiments with 8- and 10-cubic-foot 12-volt refrigerators have shown that you’ll need around 900 watt-hrs per day to keep it running. So with a 1,200 watt-hr battery, it will drain the available power in a less than 2 days. Add a CPAP machine and a few lights and that 100 amp-hr battery could be dead by the first morning.

Can 150 watts of solar panels keep up?

No, because it can only replace 400 to 500 watt-hrs per day. So, that might extend the refrigerator by another half day of running. And that’s barely enough energy to replace the 500 watt-hrs a CPAP machine needs overnight!

So how much battery and solar do I really need?

If you want to dry camp (boondock) with a 12-volt DC refrigerator and CPAP machine for several days (let’s assume the CPAP machine will need 200 watt-hrs of energy per night), at a minimum you’ll need a second lithium battery, which is 200 amp-hrs or 2,400 watt-hrs of storage. But 400 amp-hrs of battery storage would be better.

And you’ll also want at least 600 watts of solar panels, which could provide 2,400 watt-hours of charge per day. This would allow the sun to completely recharge your 200 Amp-hrs of batteries in a day. And even if the sun doesn’t shine, you’ll still have at least 2 to 3 days of stored battery power.

Caveat emptor.…

Just like any other purchase, you’ll want to research the claims made by the salesperson. I’ll be doing more in-depth studies of solar panel and battery selection for boondocking, so stand by for more data. But in the meantime, I can tell you that 150 Watts of solar panel and 100 Amp-hrs of lithium battery storage will not be enough for your camping expectations.

If you want to watch a rerun of our live webcast on 12-volt refrigerator power requirements, click on the video link below.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Let’s play safe out there…. Mike