How Efficient is our Solar Setup? 2 Year Report

Two years ago, we installed a solar array on the roof of our truck camper. The ten-panel array looks impressive. But how efficient is it, really? Today, we are reviewing the amps, volts, and watts yielded to determine how efficient our solar setup really is.

Solar Power for RV Living

Powering an RV can be very simple if limiting. There are plenty of RV parks with full hookups that provide the conveniences of grid living for a mobile lifestyle. But for people like us, who prefer to live off-grid, solar power is more than an auxiliary backup to keep the camper in maintenance mode. It is our primary power source.

When we gutted our vintage 1970 Avion C11 truck camper we removed all the propane tanks and related appliances. Instead, our new camper is fully electric with an 8640-watt DIY LiFePO4 battery bank, a roof covered in solar panels, and some backup power from our alternator. Where many RVs fall back on propane to run their power-hungry heaters, AC units, refrigerators, and microwaves, we depend on our solar power. So every amp counts.

Installing Solar Panels

Like so many of our projects, our solar array is far from the ordinary RV arrangement. The curved roof of our Avion truck camper poses an odd challenge: how many solar panels can we reasonably install on our camper? It is a long and involved debate that we meticulously documented in our post How Many Solar Panels Should I Buy For My Truck Camper? But in the end, we agree that we will install ten: eight flexible panels hugging the curved edges of the roof and two semi-rigid panels centered on the top.

A Hypothesis

With our solar panels selected and installed, we can estimate our anticipated power for our solar setup. Solar panels come with ratings for their potential output. Each of the ten panels installed is rated for a max output of 5.93 Amps at 18.56 volts for a total of 110watts. The panels are wired in serial sets of two and then connected in the junction box in parallel, to create a 2S5P system.

Altogether, our solar setup should add up to a potential output of 29.65 amps at 37.12 volts for a total of 1100W. Victron, the manufacturer of our solar charger estimates that an 1100 W array will yield an average of 4,000 Wh per day.

But all of this is conjecture. A hypothesis, if you will. Solar output varies by the time of year and other environmental factors. But that kind of average should have us sitting pretty. When we installed the panels, we were confident that, if anything, we were overdoing it. After two years, however, the data tells a different story.

The Different Story: Untangling Solar Output Data

Our Victron solar charger not only regulates the incoming voltage and amperage from our solar array, but it also logs how much is coming in and how it is managed. When the batteries are low, the solar charger pushes in the amps in bulk mode. As the batteries reach full capacity, the solar charger pulls on the throttle for a more controlled absorption mode. Once the batteries are full, the solar charger cuts off any more flow and sits in float until power is needed again. And throughout it all, the solar charger logs the voltage, amperage, wattage, and time in bulk, absorption, and float. Well, at least for some of the time.

A Note on the Data and Conditions

While we have had the solar panels in operation since October 2020, it appears that the logs beyond the last 30 days were purged. As a result, we only have data for our solar setup from July 7th to August 6th. For reasons we’ll get into later, we don’t have relevant data beyond August 6th.

This purge is unfortunate for many reasons. We are missing out on the ability to compare average seasonal or location-based solar yield. But at the very least, the data we do have represents some of the longest days of 2022, starting shortly after the summer solstice. Even on August 6th, the most recent date recorded, the sun rose at 5:57 AM and set at 7:48 PM, making for nearly 14 hours of daylight.

On the other hand, this data block also coincides with a build period for our camper. As a result, the camper—and, thus, the solar array—is off the truck and parked in a static location. While there is nothing directly above the camper, there are tall trees nearby that shade the roof in the early morning and evening. Thankfully these represent periods with the least solar to collect. So, the data we do have represents the peak sun and hours.

30 Days of Solar Power

With these long summer days, we hope to see the solar panels maximize their potential output. The array can generate a maximum of 1100 watts. But, referring to our data mapped out in Table 1 below, we only record a peak output of 720 watts.

Our daily yield is similarly disappointing. We estimated that the solar array should generate 4,000 watts per day. Even on our best day, however, we have a total yield of 2,130 watts, almost half what we anticipated. Even more disheartening, across the full 30 days, we average only 1466 watts per day.

To be fair, despite the long days we are experiencing, we are in a humid climate. And while the weather cannot account for all of the missing watts, it certainly can account for the variation from day to day as we layer in weather data in Table 2 below.

Daily solar yield and max power output for from July 7 to August 6th, 2022

Date	Yield (Wh)	Max. PV power (W)	Max. PV voltage (V)	Time in bulk (m)	Time in absorption (m)	Time in float (m)
7/7/22	1400	642	37.81	802	0	0
7/8/22	1860	720	39.19	525	0	0
7/9/22	1220	645	39.96	247	0	0
7/10/22	150	81	38.87	233	0	0
7/11/22	2040	697	38.23	841	0	0
7/12/22	1590	425	37.85	839	0	0
7/13/22	1360	649	41.16	514	312	0
7/14/22	1260	699	41.44	511	152	170
7/15/22	1270	537	41.26	571	134	135
7/16/22	1250	603	40.94	679	126	17
7/17/22	1060	613	38.15	797	0	0
7/18/22	1790	695	37.76	832	0	0
7/19/22	1540	647	40.59	553	160	98
7/20/22	1160	665	38.71	436	0	0
7/21/22	220	106	38.68	247	0	0
7/22/22	2060	576	38.22	832	0	0
7/23/22	2130	595	36.74	818	0	0
7/24/22	1750	720	40.27	820	0	0
7/25/22	1320	468	37.57	781	0	0
7/26/22	1510	643	37.18	808	0	0
7/27/22	1900	621	37.12	817	0	0
7/28/22	1590	650	37.03	800	0	0
7/29/22	1230	741	37.76	758	0	0
7/30/22	1520	674	37.88	821	0	0
7/31/22	1290	441	37.38	822	0	0
8/1/22	1750	618	37.79	809	0	0
8/2/22	2130	602	37.42	808	0	0
8/3/22	1990	629	39.48	718	101	0
8/4/22	1450	467	39.41	813	0	0
8/5/22	1460	494	40.51	659	136	0
8/6/22	1200	705	38.93	448	0	0

Environmental Variables On Solar Yield

There are a lot of variables that can affect the solar draw, but it is particularly interesting to see what difference weather can play in solar yield. The difference that light fluffy clouds can make while passing in front of the sun is dramatic. And over the last 30 days, there have been a lot of clouds. Even on our best recorded producing day, August 2nd, we experience scattered clouds. And while these clouds do mute the results, passing clouds also make room for the direct sun at other points.

Such is the case on July 8th, a day of scattered clouds and yet our highest wattage. On that day, the solar array manages to peak at 720 watts. That’s 380 watts less than it is capable of. But there could be another environmental variable at play as well: the heat.

Solar panels are rated to operate within a given temperature range. Our panels have a maximum operating threshold of 185°F (85°C). That may seem like a high threshold. But these black panels exposed to direct sunlight on a hot summer day can reach temperatures far higher than the ambient temperatures. And as the panels overheat, their output drops. So, along with the clouds, the summer heat is another mitigating factor.

Solar Setup Daily Yield & Weather Conditions

Date	Yield(Wh)	Max. PV power(W)	Average Temperature	Sky
7/7/22	1400	642	81.8°F (°C)	Mostly Cloudy
7/8/22	1860	720	83.6°F (°C)	Scattered Clouds
7/9/22	1220	645	78.4°F (°C)	Broken Clouds
7/10/22	150	81	79.2°F (°C)	Partly Sunny
7/11/22	2040	697	81.5°F (°C)	Passing Clouds
7/12/22	1590	425	82.9°F (°C)	Broken Clouds
7/13/22	1360	649	80.8°F (°C)	Broken Clouds
7/14/22	1260	699	81.0°F (°C)	Scattered Clouds
7/15/22	1270	537	82.8°F (°C)	Broken Clouds
7/16/22	1250	603	84.3°F (°C)	Broken Clouds
7/17/22	1060	613	83.3°F (°C)	Partly Sunny
7/18/22	1790	695	77.2°F (°C)	Partly Sunny
7/19/22	1540	647	81.1°F (°C)	Broken Clouds
7/20/22	1160	665	88.2°F (°C)	Scattered Clouds
7/21/22	220	106	85.8°F (°C)	Scattered Clouds
7/22/22	2060	576	85.4°F (°C)	Passing Clouds
7/23/22	2130	595	87.1°F (°C)	Scattered Clouds
7/24/22	1750	720	85.7°F (°C)	Broken Clouds
7/25/22	1320	468	82.5°F (°C)	Broken Clouds
7/26/22	1510	643	80.4°F (°C)	Partly Sunny
7/27/22	1900	621	85.8°F (°C)	Broken Clouds
7/28/22	1590	650	82.0°F (°C)	Broken Clouds
7/29/22	1230	741	77.9°F (°C)	Broken Clouds
7/30/22	1520	674	78.7°F (°C)	Broken Clouds
7/31/22	1290	441	79.0°F (°C)	Broken Clouds
8/1/22	1750	618	80.2°F (°C)	Broken Clouds
8/2/22	2130	602	84.3°F (°C)	Scattered Clouds
8/3/22	1990	629	83.3°F (°C)	Scattered Clouds
8/4/22	1450	467	82.0°F (°C)	Broken Clouds
8/5/22	1460	494	80.4°F (°C)	Broken Clouds
8/6/22	1200	705	81.5°F (°C)

Hourly Output for our Solar Setup

While the total daily yield and max power vary dramatically by day, some hourly patterns might shed light on our perceived solar shortage. Over our 30-day record, we can see a notable period that represents some of the highest total yield and power on the weekend of 7/23/22 and 7/24/22. So, we are breaking down this period to highlight the efficiency.

Breaking Out A Week

Over the 23rd and 24th, we start to see a pattern in the short-lived peaks. To confirm our suspicions, we extend the test into the week to compare the output between the dates July 23 and 29. Overlaying the hourly output, it’s pretty clear that we consistently see a minor peak in the mid-morning followed by a drop and a more pronounced peak around noon and 1 pm. The fault is environmental but the consistent form of this graph across the week indicates that our problem is not the weather. The trees that the camper is parked next to that shade the panels in the morning and afternoon.

That’s one mystery solved…

Hourly solar power output from July 23 to 29, 2022

Conclusions

Our results are far from what we anticipated when we first installed the panels. While environmental conditions play a part in the disappointing solar output of our array, it is far from the full explanation. Something is wrong. It could be wiring. It could be the arrangement. It could be the panels themselves. But whatever the problem is, to continue in our current arrangement is impractical. We should have the capacity to generate twice as much power as we currently are. Power that we are sorely missing to run our air conditioning and other necessities to make it through our current heat wave. We have to find what is wrong and fix it.

What’s Next for Our Solar Setup?

Before we can fix our solar array, we need to identify what is causing our poor output. And that calls for one of our favorite activities: running experiments. We have started a list of questions to answer:

• Are any of the individual panels underperforming and limiting overall yield?
• Is there a problem with how we wire our panels?
• How dramatically does heat affect the output of solar panels? And how effective are cooling methods?
• Does the curve of our camper roof affect our solar output?

I suppose there is another big question that we’ve left hanging throughout this article: why is there no more data after August 6th? It’s nothing to worry about. Given the less than stellar results from our last 30-days of solar power collection. we preparing to run some experiments to see what we can do to improve our output. To this end, we are disassembling our solar and power setup to test a few configurations to improve our yield. So keep an eye out for upcoming posts about our findings.