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RV Electrical: Solar Power

Photographer and cyclist and Mac expert and software engineer Lloyd Chambers is available for consulting on general Sprinter considerations at his usual consulting rates via phone, or in person in the Palo Alto, CA area. Save yourself hours and mistakes by discussing issues up-front. More about Lloyd....

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What is the objective reality about solar power for Sprinter vans and similar?

This discussion is not intended to apply fully to very large RVs which might be capable of half a dozen large high quality solar panels and without materially affecting wind drag and where parking in the sun is all but a given due to size. It speaks only to smaller RVs, like the Mercedes Sprinter.

Some people add solar to a Sprinter to be 'green'. Some people add it for charging. Both reasons are almost always rationalizations which cannot hold up to the facts of how Sprinter vans are used.

The only scenarios in which solar might make sense are when there are regular multi-day periods of parked operation where the engine is not started and the van sits in ample sunlight all day long and daily power needs are no more than 500 watts (e.g., 100 watts for 5 hours). If there is no auxiliary battery, then there is almost no value, since the starter battery has very modest capacity, and idling for 10 minutes will recharge it. But even for these scenariors, the money put into solar just does not add upp versus even a relatively small 100 amp high quality lithium battery charged by the alternator.

What follows is a series of assertions that are based upon analysis involving cost, complexity, reliability, battery life, park-vs-drive, seasonalities, miles driven, power draw per day, inferior functionality by displacement of dollars into solar vs better choices, alternator charging power, free charging while driving, and so on. Most solar installs are rationalizations which step away from all this context—if that makes someone feel good, that’s fine, but it should be acknowledged as a fantasy that makes one feel good, and nothing else.

  • Solar imposes a scarcity mentality upon its users. It is not a fast-recharge power source. so one frets over how much power is being used and how long it will take to recharge the batteries (days for a nice-size battery). Can that 2nd bag of popcorn can be microwaved? Can I use my computer another hour? No joy.
  • Solar is good for topping off and trickle charging and modest daily power needs. But if that’s all you need, one good lithium battery and the stock alternator already solve the issue and will yield far superior versatility day or night any time of year.
  • Solar is not as primary power source. It is a supplementary power source. It it is a large expense that adds complexity and increases risks of failure and battery problems. And it only supplements a primary source in a meaningful way when conditions are optimal, which means parking in the sun instead of the shade and drastically lower value in winter months, precisely when more power is needed (heating, lighting, at least for me in the mountains).
  • Solar panels are usually mounted in a way to significantly increase drag (on a Sprinter or van), thus reducing fuel economy day and night for every mile driven. They also create a wind noise nuisance.
  • Roof mounted solar panels with associated rack to protect the panels (mandatory) cause a loss of fuel economy via wind drag. This is generally not less than one mile per gallon, which is *huge*. Those losses in fuel economy apply for every mile driven, the losses increasing exponentially at higher freeway speeds, all times of day and night, every time of year. Yet driving is precisely when the alternator has plenty of capacity to charge far more rapidly than solar, so there is no value to solar when driving. Reduced fuel economy, and no power needed or stored from solar while driving = guaranteed net loss. This is as anti-'green' as it gets: 18 or 19 mpg instead of 20 mpg is a massive carbon burn compared to the power gained by solar. When manufacturing costs of solar are included, Sprinter solar users are as anti-'green' as could be designed in.
  • Solar cannot charge a battery past 100%, so if the battery is near full, solar accomplishes nothing. What it can do is keep a battery topped off, but if a good lithium battery is chosen and proper discharge and charge protocol is followed, this “topping up” idea is pointless and even a negative for most users, since lithium batteries should be cycled regularly, not left to sit at 100%.
  • The argument for solar vs alternator charging is absurd. The solar panels and associated rack to protect them kills fuel mileage and driving range by up to 2 mpg—a huge net loss. The primary alternator can charge a minimum of 2X to up to 20X faster, day or night 24 X 7 X 365. A 2nd alternator can charge up to 40X faster. Driving 30 minutes charging via the alternator is equivalent to solar for an entire day! Or just idling for one hour easily beats the best solar rig. Since my photography involves driving at least every few days, there is zero value to solar for charging, since the driving will deliver far more power. Plus, I have to idle for heat in cold temperature, which charges at the same time (and when there is *zero* solar potential).
  • Buying a van so as to scrimp and save on electricity is an absurdity: shoulder a pack and ride a bike if the carbon-free thing is your thing. Otherwise, go enjoy yourself by installing a proper battery(ies) charged by the alternator(s).
  • There is not enough roof area on a Sprinter van for a good daily charge unless one uses little power. Solar displaces useful and fun stuff on the roof, too. Solar means cutting holes in the roof, running more wires, adding charging and power conversion hardware (which wastes power), etc.
  • Solar only charges when the sun shines... and my idea of a good camping spot involves shade, which is the best air and only air conditioner I need in many places. While the solar panels do shade pat of the roof of the van most of it is not shaded and the side of the van is what soaks up the heat most of the day. No matter what, install excellent insulation for heat or cold.
  • If you’re out having fun all day, solar can provide a good charge for modest usage—all good—but what if I want my 300 watt computer system to work on for 3-4 hours in the evening along with some microwaved popcorn? Solar falls farther and farther behind, so recharging requires a more robust charging source anyway. Put the money wasted on solar into a good battery, or two.
  • Sitting in an RV park? There is shore power.
  • Sitting in the middle of nowhere for a week? Get a larger fuel tank and idle the engine for one hour every 3 days into a good battery. Park in the shade and enjoy yourself instead of roasting.
  • The money spent on solar is far better spent on a good high capacity battery in a smaller and lighter form factor (lithium battery).
  • Get a 2nd alternator if you need air conditioning. Plan on idling because even two of the largest and best solar panels put out very little power versus what AC consumes.
  • Solar power decreases substantially at my peak travel time (September through November). And in the hot summer, I want to park in the shade so when I come back to my van it’s not 110°F inside.
  • Late in the season, the sun peeks weakly into canyons only a few hours per day, and often there are clouds that greatly reduce solar output. Or snow or rain. So other power sources must be relied upon anyway. Furthermore, the engine must be idled for heat, which at the same time provides massive charging power versus zero for solar.
  • Solar can damage battery lifecycle longevity, see the note below.

Bottom line: relying on solar, you spend all your time avoiding electricity use instead of doing what you want to do. And it might actually be a net loss when fuel economy is reduced from wind drag.

In many if not most cases, solar is a feel-good irrational non-solution for anyone looking for a reliable power supply; too slow to charge, useless at night and of diminishing value in the non-summer months. That said, solar is good for those who leave their van parked for long periods. But it is very hard to justify on any other basis when a 2nd alternator is available.

ROI analysis — Zamp 200 watt portable solar panel kit

The Zamp 200 watt portable solar panel kit is a standalone collapsible solar panel. I am using it for an ROI (return on investment) analysis because it can generate much more electricity by being placed optimally to the sun versus rooftop panels, the connection costs are low, plus the vehicle can be parked in the shade (!), so air conditioning is far less of a power draw, if any.

Charging via the alternator is 2X to 20X faster, as per my testing. As per the discussion above, solar is mainly a feel-good waste of money unless one parks for days at a time and cannot idle the engine and power needs are low. There is just no rational justification for it with a Mercedes Sprinter diesel and one decent battery.

Assumptions: 120 watts charging power 6 hours a day for 180 days a year, half that for the other half of the year. This is probably optimistic.

120 watts * 6 hours = .72 kW per day = 130 kW
60 watts * 6 hours = .36 kW per day = 65 kW

= 195 kW

If we use current high California rates of $0.36 / kWh, the value of that power is 195 *.36 = $70 of value per year—and that’s using the high end of the rate range.

At the on-sale price of $1078.65 and 9.25% California sales tax, the cost is $1178, which means that in power terms, it will take 16 years to just break even. And unless it is a high-grade SunPower panel, its output will degrade substantially by year 10, so I’m going to call it 20 years to break even, and that’s being kind. Way before then we’ll have 48V vehicles and diesel will be 10% to 20% more efficient.

Note that 195 kW is equivalent to recharging (from 10% to 100%) my 400-amp Lithionics lithium battery 43 times = once every 8 days (it is rated for about 2300 such cycles!) In other words, it takes 8 days to recharge it, which is 100% worthless to me at least. That same 195 kW could be charged in about 180 hours of idling when the battery is low. At 0.8 gallons of diesel per hour @ $3.49 per gallon, that’s $144. Plus I can charge even while idling even while using a kilowatt of power.

None of the above takes into account the wasted human time and effort of dragging along and setting up the panel, cleaning it, hardware to attach/stow it, risk of theft or damage, panel failure, proper connection terminals on the outside of the van—so I’ll add $2000 for wasted human time storing, cleaning, setting up and dealing with the solar panel plus materials and labor for a proper install (terminals on/in the van).

In a nutshell: solar panels on a Sprinter make zero financial sense and probably no sense at all unless there is no option to idle the engine. They only make sense as a convenience for travel, just as does an high-capacity and high quality lithium battery. But primarily solar is a feel-good rationalization that cannot stand up to reason.

Solar can damage battery lifecycle longevity

The Lifeline/Concord lead acid battery technical document states:

For repetitive deep cycling applications (deeper then 50% depth of discharge), chargers should have an output current of at least 0.2C (20 amps for a 100 Ah battery). If an output current is below this value, the cycle life of the battery may be negatively affected.

Translation: solar is a problematic for large batteries. The Lifeline/Concorde GPL-16T is a 400 amp battery, and I was considering a pair of them in series (for 12V). Assuming "C" of 400 amps, the amperage required is 0.2 X 400 = 80 amps. That’s about 1000 watts—virtually impossible with solar on a sprinter van under the very best conditions on the equator. Hence, guaranteed battery damage over time if one charges with solar after discharging the the batteries beyond 50% , which I would want to do routinely.

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