Have you ever wanted a portable solar power system for your RV, boat or cabin, but thought the subject was a proverbial can of worms, and the people who wrote about it or discussed it spoke in a strange techno-geek that made your eyes glaze over? Making matters worse, you may have even taken some baby steps by looking at some systems at Amazon but became discouraged by the component prices.
It wasn’t that things were stratospherically expensive, it was the whole idea of throwing several hundred bucks at something you felt you knew so little about. The whole idea was a bit unsettling.
My goal with this article is to impart some of what I have learned about portable solar power systems, and how easily and yes, economically, you can incorporate a small, useful solar system into your camper or vehicle, to make your weekend trip or campsite experience the best it can be. After reading this, it is my hope that you will view a portable solar power system as a straightforward and worthwhile investment, and have the confidence to pursue a system of your own.
Build Your Own or Buy Your Own System?
One of the more exciting things for me about the solar power industry has been the ascension of the portable solar generators that are proliferating in the market. These are basically a power plant in a box, with a handle, that are from about 20 to 50 or so pounds, depending on the battery type, and are designed with portability as the first priority.
I have reviewed a couple of examples of these on my site, such as the Xantrex XPower Powerpack and the Renogy Portable Solar Generator. These small systems are an excellent way to dip your toe into solar power because they are ready to use systems – the difficult decisions have all been made.
The only piece missing from them is the solar panel. So, instead of reading about inverters, combiners, controllers wire gauge, etc, and catching analysis-paralysis, you can read up about just the PV module (solar panel) that would work best with your generator, and the available space on your rig in which to haul it around.
If you are completely new to this industry, have an immediate need for a portable solar power system, and your power needs are light to moderate, such as keeping a cellphone and laptop charged, a couple of lights and a fan and maintaining a small fridge or 12VDC cooler, this may be the only type of system you ever need.
That’s Great – But I Want More Power!
I can certainly relate to this! The quest for more power is a familiar one. To me, the whole point of getting into portable solar power systems in the first place was to find a way to power a small air conditioner without relying on the power company. This is necessary where I live in Florida because of our frequent and often violent summer storms.
Once the storm passes and the cleanup begins, the demand for power is at its peak but the supply is often at zero. It is frustrating, to put it mildly, to be in an area with light damage but still no power for sometimes days. This is usually because the grid in which you live has been switched off for power line worker safety.
Even more frustrating is when your neighbor (you know who you are, sir!) happens to be connected to a different grid and his lights are on, while you sweat in the dark for a week!
Quick, true story: last summer (2018), a friend living in a very small subdivision of about 30 houses, lost power for an entire month! The county had just completed a road widening project next to the subdivision and as part of that, also raised the road a couple of feet. When the storm came through, the change in topography caused flooding to their backup generator area and they thus failed. The road work had created a water dam that was preventing the natural runoff that had always occurred. Here’s the best part: the county refused to allow them to pump the water away from the generators so repairs could be made! As runoff, they would not allow indiscriminate pumping!
So, the county widens the road and turns a portion of the adjacent private property into a wet retention pond, then refuses to allow its owners to remedy the problem! The “polluted runoff” was somehow acceptable when the county directed it towards the subdivision, but it became Chernobyl-level waste when the residents tried to direct it back where it had always flowed before.
I would bet that an off-grid solar power salesman or woman could have cleaned up in that subdivision right after that storm.
The independence afforded by generating your own electrical power can be life-changing. However, I want my portable solar power system to be able to handle my climate control, specifically, my Florida summer cooling (air conditioning) needs. Anything less in my mind is just not adequate. What do I have to put together to accomplish this?
Where Are We Going With This?!
Here it is; you want a portable system that is powerful enough to operate a small, portable air conditioner on a stifling hot and humid Florida summer week. I use ‘week’ as my goal because anybody can abuse and potentially destroy a battery bank over the course of a day or even a weekend running a large appliance. Running the same stuff for an entire week requires proper planning and execution to make it possible.
The best way to size or determine how much power you need to generate is to know how much your electrical equipment requires to be operated and work up from there.
I am planning to use a 5000 BTU air conditioner. You want the smallest possible unit, mine uses just under 500watts of power at 110VAC to operate its compressor. This equates to 4.5 amp hours of power. (watts divided by volts).
My inverter is 90% efficient, so its internal circuitry consumes 10% of the incoming DC power to transform it into the AC or alternating current required by the air conditioner. This case I would need about a half amp for every hour my compressor runs, and less than a tenth of that when just the fan is running.
Since I am cooling just a personal sleeping space (the smaller the better), I am counting on my compressor to run briefly and infrequently once the area is cooled. I estimate that 10% of the time, I’m drawing roughly 5 amps total, and 90 % of the time, just a half amp. The idea is to use no more than 5.50 Ah hours of power and 55 Ah over 10 hours.
I have seen power specs online that show the need for literally thousands of amp hours of battery capacity per night to operate a small air conditioner. This may be true if you are cooling an entire motor home or cabin, but if you scale things down and cool just the place you sleep, I think you will discover that it isn’t as difficult as you have been led to believe.
Battery Power is Everything
At the heart of every portable solar power system (and most stationary systems as well), is its energy storage. In fact, only a grid-tied inverter system, which shifts surplus energy over to the grid uses offsite energy storage, Every other system stores electrical energy locally in some sort of battery or battery bank.
Every component link in the solar power production chain is important, however your battery setup is arguably the most important usability factor.
There are interesting battery choices these days that are well beyond the traditional flooded led acid types that have been with us for a very long time.
You will run into terms such as valve-regulated led acid, AGM sealed led acid, GEL-AGM, lithium cobalt oxide, lithium ferro phosphate, lithium iron phosphate, dry cell, wet cell, non-spillable… etc. It quickly becomes a lot of information.
The best advice I could give about batteries is to buy a sealed AGM battery if you’re on a budget and a lithium iron phosphate battery if you can afford its higher upfront cost. When you analyze batteries from a lifetime cost basis, where initial cost, number of charge cycles, size & weight, maintenance time etc are all factored in, the expensive batteries become the obvious best choice. It may be counter-intuitive to you, or it may not be, but a $1700 lithium battery and its very long service life and vastly superior discharge ability is less expensive to own, over its entire expected life, than the 4 to 6 sealed or flooded deep cycle batteries it effectively replaces in your rig.
Plus, the expensive battery used a fraction of the amount of space, did not out-gas, and was a fraction of the weight of the lead-based batteries.
How Much Power Are We Talking?
The most powerful Renogy battery in their lineup is a 170 Amp hour lithium, pictured above. It also has a 2C discharge designation, which is a measure of speed at which it can fully discharge, at maximum output, or in this case 2 hours.
Lets compare that to a decent quality, sealed AGM Vmax 200 Ah deep cycle battery,
First, the Renogy lithium:
- 2176 watt hours of energy
- 170 Ah @ 2 hour rate to 10.5 Volts
- 2000+ discharge cycles (7000 at 50% Depth of Discharge ‘DOD’)
- 46.3 pounds
- 2660 watt hours of energy
- 200 Ah @ 1 hour rate
- 600 discharge cycles (not likely possible if your DOD consistently approaches 50%)
- 112 pounds
There are many additional ways to compare batteries such as recharge rates, energy density and other factors. Lithium technology is superior to lead batteries in every way except the cost of acquisition. Were the upfront cash outlay the same for both types, this discussion would be unnecessary.
How Much is Enough?
Using my 5000 BTU portable air conditioner as my guide, will these batteries provide enough stored power to properly it for a 10 hour period? Let’s see….
The AC compressor uses about 500 watts of power, but only about 50 watts when just the fan is running – which means that 90% of the time its using 50 watts, 10% of the time its using 500 watts.
So, this means approximately 1 hour at 500 watts and 9 hours at 50 watts per night.
Renogy battery: 2176 watts divided by 500 watts = 4.35 hours, 2176 divided by 50 watts = 43.52 hours.
Since you want to stay above 50% discharge with the lithium battery, (not necessary for just for the sake of comparison) you cut these numbers in half. Therefore, you could expect a single 170 Ah Renogy lithium battery to supply over 2 hours of compressor run time, and also double the required fan only time required by the 5000 BTU portable air conditioner.
Note here that it is considered practical to use considerably more than 50% of the energy stored in a lithium based battery. This would be abusive and damaging to a led battery. For comparison purposes, I decided to use the same 50% discharge rule.
VMax battery: 2660 watts divided by 500 watts = 5.32, 2660 divided by 50 watts = 53.2 hours.
Limiting the discharge to 50% gives us a real-world estimate of 2.66 hours compressor time and 26 or so hours of fan only time.
Just based on the math, the Vmax battery with its higher amp hour rating would also be able to handle the job of running the little air conditioner.
Keep in mind that if the wheels could come right off this scenario if any part of the equation is wrong. Using an older or less efficient AC unit, trying to cool a larger space, turning down the thermostat too far etc, any of which could make the compressor switch on more frequently, would negatively impact the real world result. Clearly, experimentation is key.
Important to note here is that the lead battery will wear out far faster with this kind of load than will the Lithium battery. Deep cycle lead plates can handle the occasional deep discharge, but their design and chemistry are better served by light discharging and recharging. The plates simply break down far earlier in their life cycle with repeated demands for deep current draw.
This is why you end up replacing a led battery at 3 or 4 times the rate you do a lithium battery, for otherwise the same system.
You can always purchase additional led batteries to mitigate the current demand by splitting the work between the batteries and, therefore, extending their service lives. The problem here becomes the extra space requirement and weight that a second, 100 plus pound battery requires.
For a cabin or house, sure, grab 2 or more of the Vmax batteries and monitor carefully their DOD and you should get a few years of reliable power from them. You haven’t saved money over lithium if you do this, but you do have redundancy, and you do also have more continuous as well as surge output.
There are always trade offs to be made when it comes to power storage, like anything else. But since we are focused on portable solar power systems, we want as few batteries as is possible for the job.
Recharging Lithium Batteries is Faster Too!
This is a ‘biggie’ for any portable solar power setup. It’s one thing to pack enough battery power for a night or weekend camping trip. It’s quite another to plan for and execute an extended, powered, boon dock excursion.
To successfully pull off the latter, and end the trip in just as good a shape as you started, you are going to have to address battery charging.
We worry a lot about how much power we’re drawing from storage, now we have to be concerned with how much and how fast are we replacing that power.
This is where things like PV modules,aka, solar panels and their charge controllers come into play. But the battery type is critical here, and there is nothing better (or as good) as Lithium-based batteries when it comes to recharge rate.
The main, simple, explanation for this is that, unlike lead batteries, lithium can be quick-charged right back up to 100% capacity without the need for an ‘absorption’ phase that eats up time with the lead acid type. Lead batteries must spend hours after about 80% recharge is achieved in their absorption phase. Rapid-recharging lead batteries is simply unheard-of.
Another bonus with lithium is their vastly superior ‘working’ capacity. A 170 Amp hour lithium battery will output virtually all of its power without ill effect. Even if you regularly and repeated discharge a lithium battery to 80 or 90% and beyond up to its rated power output, you can still expect to get from 2000 to 6000 charge/discharge cycles from it and experience DOD rates above 50%. Their output power curve is also essentially flat. Whereas the deeper you go when discharging a lead battery, the faster they ‘peter-out’. Their electrical pressure, if you will, starts dropping rapidly.
Lead batteries are impaired and often damaged at a DOD of just 50%, and 500 to 800 charge/recharge cycles are the norm.
If you can afford the upfront cost outlay of lithium iron phosphate batteries, you will be paid back in longevity, power output, weight and space savings, and overall reduced cost over any type of sealed lead battery.
Last but not least, partially charging lithium batteries does nothing to impair their output or longevity. They perform just as well, albeit not as long as with a full recharge, on the days when you lack the time or the sun to fully charge them.
Lead batteries should always be fully recharged, and you significantly degrade their performance and lifespan by repeatedly ‘charge-starving’ them.
Lithium batteries are really the best thing to ever happen to enhance the reliability and usability of a solar power system, portable or otherwise.
Controllers, Inverters, Isolaters and Other Light Reading….
OK, so we have a good starting estimate for the amount of power we are planning to draw, and the power storage requirement this give us. We need to address the power input side to keep our batteries working and healthy, hopefully, night after night. Let’s start with a properly sized solar charge controller.
The two main charge controller technologies are Maximum Power Point Tracking (MPPT) and Pulse Width Modulation (PWM). Because of the advantages of the MPPT controller in terms of efficiency and scalability compared with PVM, we know this is the better choice.
To choose the correctly sized charge controller, we again rely on Ohm’s law: Amps x Volts = Watts (this case the wattage of my solar panel or panel array).
My 100 watt, 12 volt PV module in full sun outputs 8.3 amps. Way too little to keep my AC going. So, I plan on acquiring more panels and producing 600 watts at 12 volts for a total of 50 amps of direct current, (actual will be closer to 40 amps).
So I want to look for an MPPT solar charge controller capable of managing 60 amps or greater. Most good MPPT controllers can handle a bit more than their indicated rating, but I want a scalable system – one that I can add solar panels to without danger of sending too much current to my rig.
How Long Is This Going To Take?
The answer my friend, is blowing …… never mind! The question now becomes, will my 600 watt panel array and 60 amp MPPT charge controller refill my 170 Ah lithium or 200 Ah led acid battery on an average day? To answer that we must again make a few assumptions and guesstimations, as follows:
I will count on 5 hours per day of sun, and will be replacing 100 amp hours of power used since the previous day. (this might be OK as long as my overnight draw is 50 – 55 amps).
My 600 watt, 12 volt solar array, because of weather, resistance issues, and inefficiencies should produce roughly 400 watts, which is 33.3 Amps.
I would expect that on sunny days, with 5 full sun hours, I would generate about 166 amps at 12 volts from my PV array. There will of course be days with little or no sun, but where I live, those days are quite rare.
This does point to the obvious problem of stretches of time of poor weather and insufficient sun to provide the 2 to 3 minimum hours required to keep my battery minimally charged. During these stretches of time we still have choices such as augmenting our power generation with wind power or gas generator (spit), or, keeping things as simple as possible, using the alternator of the vehicle that brought you to place where you are contemplating these issues!
For this to work, we need a device called a Battery Isolator
Basically, this is an electronic switch that connects your solar system battery to the vehicle’s battery, so the vehicle’s alternator is able to provide charging current to both batteries.
Keep in mind that in most cases, you are asking your car or truck to charge two different types of batteries, and you may be putting a heavier load on the vehicle’s charging system than it is prepared to handle.
Still, the right isolator can be another tool or option to keep your batteries healthy and producing sufficient power to keep your rig, and you, going when you otherwise would not be able to. I would rather have an isolator on not need it, than otherwise have no way to charge my camper battery during a week of rainly weather. (which you just know will happen the very first week you get your new system on the road for a shakedown).
For your lead-acid powered camper, check out Renogy’s 200 amp isolator:
Don’t Forget the Inverter
Quality really counts here. This is because the more efficient inverters usually generate less heat and heat tends to kill electronics. If you go cheap on the inverter, you end up going cheap often. End result is you pay more and are continuously aggravated by low reliability and usability.
Most people advise to just get the largest inverter you can avoid. I think this is kind of silly advice. If you can afford only a high quality 75 watt inverter, you’re out of luck because your 500 watt air conditioner will not run. This is an exaggeration but is designed to make a point; buy the best quality inverter that is large enough to handle what you will demand from it.
I like and trust Renogy products and they have a lineup of quality, pure sine wave inverters that will work in my setup. For about $150, their 700 watt unit would work, but $200 more gets you 6 times more capacity! Their 3000 watt, pure sine wave unit allows room to grow and can easily handle the 500 watts my air conditioner will draw.
For more information on inverters, check out my article here: How to Choose the Best DC to AC Power Inverter.
I Feel Dizzy and slightly Queasy …
And well you should! Discussing portable solar power systems from (literally) top to bottom is the stuff of apprenticeships and college curricula, yet you’ve tackled it in one sitting!
We now have at least a rough idea of the incoming and outgoing power requirements associated with our portable air conditioning system. We have an idea of the PV module array we will need, and the supporting hardware; charge controller, inverter and possibly isolator required for extended trips, of longer than a weekend, away from the electric power grid.
The devil is, of course, in the details of this or any system. We still have to run cables, connect switches, mount panels – if we go that route, monitors, batteries and of course, (and possibly most important), create an isolated space that will be climate controlled as a result of this effort.
I hope that you are encouraged by the possibilities raised by this article and use it to form the basis for the beginning of your own quest to provide a completely air-conditioned, restful sleeping area, in silent fashion, for extended periods of time even in the most remote location you ever thought of reaching.
Let me know how it works out for you.