Living on Sunlight

tl;dr — Libbie’s rooftop solar generates all the power we need for extended boondocking.

And that makes us happy.

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But if you want to know way, way too much more …

We will spending most of our Haven’s Path time in remote areas boondocking: off-grid, without external hookups of any kind. That means we need to be self-sufficient for both water and power. For us, power management means solar management. 

Here’s what we’ve discovered.

First, Some Basics

Libbie, our Lance 1685 travel trailer, like most RVs, has two connected electrical systems: a 120-volt AC circuit like the kind in your house, and a 12V DC circuit like the kind in your car. The AC circuit is accessed by several standard household outlets. AC also powers the microwave/convection-oven, the air conditioner, the refrigerator (when connected to shore power), and anything we plug into the 120V outlets. Everything else runs on DC: the lights (all LED), the stereo, the TV, the fans, the water pump, several auxiliary systems monitors, and anything we plug into the 12V cigarette-lighter outlets – such as an inverter (more on that later).

The 120 AC circuits work when we’re plugged into shore power, such as at a campsite with an electrical hookup. They would also work if we had a portable generator running. From the beginning of our planning we’ve elected to skip the generator option, both to avoid the noise and gasoline and to live within more renewable guidelines.

The 12V DC circuits work whenever we have charge in our pair of lithium-ion batteries, which together hold 200 amp-hours. Those two rechargeable batteries remain at maximum charge when we’re plugged in, because Libbie uses shore power to keep them topped up. The batteries also get charged when we’re driving, with our tow vehicle acting as a generator.

When we’re not plugged in is when it gets interesting.

Like Photosynthesis, but with Silicon

Without shore power, the source of new energy we’ve chosen to rely on is sunlight. Atop Libbie are two 190-watt solar panels whose entire job is to charge the batteries.

It’s a race – can the solar panels keep up with the DC power drawn from the 12V devices we use?

Turns out, yes. Very much yes.

The Inverter Cheat

It turns out we can run some AC devices even when we’re boondocking. The magic trick is to use an inverter.

This tricksy device converts 12V power from the batteries to 120V power – but only at at low wattage, enough to power, say, a laptop charger or (thank Zeus!) our after-market Bose speaker. Not nearly enough for the AC or microwave, but enough to keep our power-drill and electric toothbrush batteries topped up. 

What We All Forgot from High School Physics

• Volts is like pressure, or charge. Sunlight on solar panels makes enough voltage to generate a current (in Amps), whose rate depends on the brightness and angle towards the sun.
• Amps is flow rate, or electric current, or power.
• Amp-Hours (Ah) is a unit of energy consumed or generated over time. For boondocking, the units that matter most are these Ah.
• Watts are work, the flow of electrons encountering resistance. Watts = Volts x Amps. So, for example, to convert watts to amps on a 12V DC system, divide watts by 12.

For context, we’ve read that typical RV users draw 75–150 amp hours per day. So, for example, that’s a current draw of 10 amps for 7.5 to 15 hours. We figure if we draw less than 10 amps per hour or charge more than that, we should be golden. It’s yes to both, by a large margin.

Our Kit

We have a pair of 190W solar panels, for a total of 380W. They operate at around 17V, per the manual. Therefore our peak ideal amperage, in direct, overhead (at the equator), maximum sunlight is roughly 380W / 17V = 22A. For a generally sunny-ish day at our latitude, our first guess is to round down to maybe 12A times maybe 8 hours of light to give approximately 96Ah of solar charge per day as a first guess. Most days would likely be less, we estimated, due to shade, clouds, dusty panels, rain, whatever.

Our two batteries hold 100Ah each, for 200Ah total, but the manufacturer recommends using only 75% of capacity for best battery life. That means we have 150Ah available without recharging.

So. The boondocking questions all simplify to:

1. How long can we go on our 150Ah capacity if there’s no significant solar recharging (like on a very cloudy winter day)?
2. Would our above estimate of 96Ah of solar charge meet our needs for an average day?
3. When do have adequate solar charge, how much can power can we draw that the solar cells can replenish in real time? That is, could solar generation outpace our peak usage?

Our Estimated Power Usage (for Science!)

Data geeks we are, we couldn’t help testing our total power draw, in order to make an estimate for DC power usage for a typical day boondocking. Many of these numbers are very rough, and all are guessing at what a typical day’s usage would be, and most are rounded to the nearest Amp-Hour, but, y’know, science.

DEVICE	CURRENT (Amps)	TIME (Hours)	ENERGY (Amp-Hours)
Interior Lamps (x5)	1	2	2
Jensen Stereo	3.3	4	13
12V TV	3.3	2	7
Bose Soundbar (via inverter)	4	2	8
Glenn’s MacBook (via inverter)	2.5	3	8
Rachel’s laptop (via inverter)	3.8	3	11
Water Pump	4	0.5	2
Variable-Speed Vent Fan	2	4	8
Bathroom Vent Fan	1.2	1	1
Handheld Vacuum	4	.3	1
iPhones (x2)	2	3	6
Miscellaneous Chargers (via inverter)	1	1	1
Power Jack (at Set-Up)	30	.1	3
Stabilizers and Slide-Outs (at Set-Up)	12	.2	2
Furnace Blower	4.6	1	5
Miscellaneous Systems and Detectors	.1	24	2
ESTIMATED DAILY TOTAL AMP-HOURS			75–80

All these guesses, added together, come in at maybe 75–80 Amp-Hours for a “typical” day, if there is such a thing.

So We Measured It

Just prior to our extreme temperature mayhem over Labor Day weekend, we ran a 6-day experiment without shore power: batteries and sunlight only.

Our solar controller tracks our actual usage, which ranged from 40–70Ah per day, depending on whether we watched a movie that night. And this was without being stingy for power usage at all. Which means that our estimates above of how much energy we might use were a little on the high side.

Along the way we also saw a max charging rate of 9 Amps, but we never drew more than that even with many 12V devices running. This resulted in our batteries stayed at 100% charge except briefly for one after-sunset movie.

To answer the questions from above:

1. We could go 3 days on battery power alone, without any solar charge. (This situation will not occur.)
2. Our solar panels produce maybe 80-100Ah per day, supplying about 1.5–2 times what we need for a typical day.
3. On a typical sunny day, we have never been able to use more energy than the two panels supply in real time.

In sum, Libbie’s rooftop solar panels (even dusty and in the partial shade you see in the picture) generate all the power we need for extended boondocking – so that’s just what we’ll do.