Case Study - Sizing a system for a pop-up camper trailer

Author's Notes:

I am a professional renewable energy (RE) author and avid flyfisherman, and I've lived 12 miles off the power grid for the last 15 years, powering my home entirely from solar and wind power. I receive a ton of email each week with questions about the basics of designing RE power systems, ranging from tiny systems for a camper trailer to large home power systems. This article is my attempt to explain the basics of it for a complete beginner. Hopefully I've eliminated most of the jargon, and at least explained all the jargon that's used.

High mountain lake sunset
Yep, that's a ring from a rising trout in the lower righthand corner! We caught and released over 200 trout on that 2-day camping trip. It was the shakedown cruise for our new old pop-up camper and its new solar power system. The system worked great!

Vintage pop up camping trailer

Our vintage 1968 Starcraft pop-up camping trailer, the "Sally Ann."

Unfortunately for his employers at, DanF is a trout bum. That means that at certain times of the year, he is 'unavailable' for work because of a particularly nice insect hatch on our home waters here in Northern Colorado, or indeed anytime all his hours spent staring at a computer screen start turning his brain into mush and making his eyes go googly.

DanF's employers, who are probably now reading this new web page, are particularly advised to note how often this problem occurs. He apologizes sincerely to them for trying to convince them last fall that our company web server cluster facility was located on the shores of Hohnholz Lakes on the Laramie river for better security, and that weekly trips up there were required for regular servicing. In reality, our servers are located along the North Platte river in the stretch between Seminoe Resevoir and Pathfinder Resevoir in Wyoming, often called the "Miracle Mile."

According to the best medical research available, the only sure cure for these fatigue and 'googly eye' afflictions is a few days of fishing. Since DanF's Dad has a vintage 1968 Starcraft pop-up camping trailer available for use, DanF figured that installing a simple solar power system in the "Sally Ann" and writing a web page about it would be a great way to catch the late summer damselfly hatch near Walden, Colorado and the October caddis hatch in Lowell, Idaho and still pretend that he was actually working the whole time!

System design considerations

(and how we addressed them)

How much power do we really need?

Not a whole lot for us--we go camping so we can flyfish, and get away from all the modern contraptions and their noise. We need lights at night, a portable radio / CD boombox, and battery chargers for our 2-way radios, digital cameras, and GPS units. Bright lights are essential if we are tying trout flies in the evenings. If we bring our vintage Grumman canoe, we do use some serious power with the electric trolling motor--more on that later.

There have been other people at certain campgrounds we've stayed at recently who seem to need satellite television dishes, big screen TVs, electric Mister Coffee machines, electric refrigerators, and all kinds of other stuff that we are trying to get away from. They put all this stuff right in their campers! We are boggled by this attitude towards wasteful power use, and the noise, fuel cost, and maintenance factors of running an infernal combustion engine at the edge of the wilderness. Perhaps we are Luddites, but we do like our peace and quiet. That's why the 'Sally Ann' is powered only by (silent) solar power, with the option of a (again, silent) shore power connection.

12 volt DC versus 120 volt AC power systems

A few years ago, just about everyone that powered a camper (or small cabin, for that matter) from solar used a 12 volt DC (12 VDC) system--that's a direct feed to your cigarette-lighter-plug gadgets off of a 12 V battery or bank of batteries. Problem is, 12 volt power has lots of losses in the transmission, and to reduce the losses things need to be wired with very thick, expensive wire that's hard to work with. That's why most 12 VDC lights and appliances are very small...a typical car cigarette lighter plug and outlet can't handle much power before becoming hot from electrical resistance and melting your dashboard, or (hopefully) blowing the fuse first.

In the last 5 years, 'power inverters' have dropped dramatically in price and become widely available--the internet, truck stops, the local NAPA auto parts shop, everyone sells these gadgets now for cheap. Most are around $150 or less. They convert 12 volt DC power into 120 volt AC power, also known as 'house current.' That's what's available at every electrical outlet in your house in town! If you are not in the USA, inverters are available worldwide for every different electrical scheme, you just have to buy the right one for your country. With an inverter, instead of buying all new light bulbs, boom boxes, toasters, radio/GPS/camera battery chargers and such for your 12-volt camper, you can use the stuff you already have, or buy new ones from K-Mart for cheap. A typical (and highly efficient) compact fluorescent (CF) light bulb for 120 volt AC house current will cost you only a couple bucks at Home Depot or the local supermarket. A 12 volt DC version will cost you at least $10 per light bulb, be available only online or at a specialty RV shop, and probably won't be built by a major US or European manufacturer from whom you can obtain a refund if it doesn't work right.

We decided to go with 120 volt AC inverter power for the Sally Ann. A quick trip to K-Mart, and all our lighting and appliance needs were met at low cost. Buying all those gadgets again for 12 volts DC would have cost far more than the cost of the inverter! We ended up with a 1500 watt Whistler inverter, much more than we need, but it was available cheaply used. If we were to actually RUN a 1500 watt load from it, our little marine deep cycle battery would be dead in less than an hour. To pick the correct size of inverter, total up the power draw in watts for all the lights, appliances, and other gadgets that you might be running all at the same time, and pick an inverter with a 'continuous' power output of a couple hundred watts above that. 'Surge' power output that's advertised is not a useful spec--if you ever reach this number in the power you are using, your inverter is too small. Also, be sure to follow the inverter manufacturer's recommendations for wire size from your battery to the inverter--this wire needs to be both thick and flexible.

inverter installed in camper

1500 Watt Whistler inverter installed under counter, to keep it safe from water drips, bourbon and beer botches, chili spills, and tipsy flyfishermen.

Power versus Work versus Energy

Folks often get confused about the correct units in which to measure power consumption. Watts measure power being used or gained at any given instant. But Energy is what's most important to you, not power-- Energy is power measured over time. It doesn't matter that the big TV in your camper uses 200 watts. The real issue is how many hours per day you have the thing on! So, the proper units to use are watt-hours or amp-hours (A/H), not watts or amps. Ten amp-hours of use means that you ran your boombox that draws one amp for 10 hours--or that you ran your big TV that draws 10 amps for one hour. You get the idea.

Appliances and lights are always rated in watts, and they are always marked on the back for how many watts they draw at full power. Batteries are rated in amp-hours of capacity. The only math you need to do for designing a simple RE system is converting watts to amps. Then you multiply by time to get amp-hours. Watts = amps x volts, and amps = watts / volts. So, a 60 watt light bulb powered from your inverter will use 1/2 amp at 120 volts AC (60 watts / 120 volts = 0.5 amps). But your battery is 12 volts! So to power this bulb, you need to do the math for what's really coming out of your 12 volt battery-- 60 watts / 12 volts = 5 amps.

Choosing the battery

The Trojan CB-27 battery we installed in the Sally Ann stores 100 amp-hours of power. BUT you never, ever want to take any battery down below 50% of its capacity--most batteries can only do this for you a couple dozen times, then they are ruined. For long battery life (2 years or more) don't take your battery down by more than 30% of capacity. If you can afford to replace your battery every year or two, you can be more abusive.

The 30% discharge guideline says we can frequently draw down the battery on the Sally Ann by 30 amp-hours, with occasional drops to 50 A/H (half capacity). So, for decent battery life we could run that big 60 watt light bulb for at most 10 hours to take our battery down to half capacity (100 amp-hours / 5 amps = 10 hours.) If we ran our cute little 1000 watt electric heater from inverter power, the math would be: 1000 watts / 12 volts = 83 amps-- a bit over half an hour to the 'battery critical' condition of 50% discharge. Cold Cranking Amp (CCA) ratings on batteries are useless for measuring storage capacity. CCA is for car engine starting batteries, not RE systems. The number you need to look at is amp-hours (A/H). You might have to ask your battery dealer for this information--it may not be printed on the battery.

You can combine multiple batteries for more capacity, if you have the space. Standard 'marine deep cycle' batteries are a good choice for a tiny system, they withstand deeper discharge much better than car engine starting batteries. For a larger camper or a cabin, you might consider golf cart batteries (T-105s), which have to be installed in pairs because they are 6 volt, not 12 volt. Home or cabin power systems often use 6 volt L-16 batteries, which have lots of capacity and are extremely durable--but they are large, heavy and expensive, and usually much more than can be fit into a camper. So-called 'gel cell' batteries have the advantage that they can be tipped over without losing electrolyte, but in general they are much more expensive and fragile than the run-of-the-mill 'flooded lead acid' battery we used in the Sally Ann.

Trojan CB-27 battery in the battery box, with lid removed. Inverter is mounted above it.

Trojan CB-27 battery in the battery box, with lid removed. Inverter is mounted above it. 100 amp-hours capacity. Note how thick the inverter wires are -- this is essential. The battery box is screwed to the floor so that nothing can move.

Lighting choices

A 60 watt incandescent light bulb wastes more power as heat than it turns into light. Think of any incandescent bulb as a space heater that produces light as a byproduct! Fluorescents (FL) and compact fluorescents (CFs) are FAR more efficient, and they should be the only bulbs you even consider for an RE power system. LEDs are also an option--they are not very efficient (despite many advertising claims), but they do emit most of their light in one direction, eliminating the need for reflectors. Our lights are a pair of 7 watt compact fluorescent (CF) bulbs, plus a CF fly tying lamp that uses 13 watts, for a total of 27 watts with all lights on. That's 2.25 amps at 12 volts, so we could then run all these bulbs at the same time for 13 hours without damaging the battery (30 amp-hours / 2.25 amps = 13.3 hours.)

The solar panel

We didn't want any glass in our solar panel because of long drives on bone-jarring dirt roads, so that ruled out all the normal models intended to be mounted on houses. My remote house in the mountains runs on such solar panels, but we feared breakage during our driving. We also did not want to permanantly mount the panel to the roof of the trailer--we're lucky the old girl doesn't already have a leaky roof, and putting holes in it sounded like a bad idea. Plus, a flat mounted solar panel doesn't do much good unless you are near the equator--for maximum power generation, solar panels should be facing directly at the sun.

There are some really cool flexible solar panels available--you can roll them up for storage, and even mount them on the upper deck of boat and walk on them--but they are much more expensive than standard models. For solar panels, it all boils down to dollars per watt. Standard, 'single-crystalline' or 'poly-crystalline' panels mounted under glass for homes run about $5 per watt right now. 'Amorphous' panels are up around $6-8 per watt, and flexible amorphous panels can reach $10-12 per watt! Amorphous panels have the disadvantage of making much less power per square foot than single or poly crystal, but have the advantages of not being so brittle and fragile, can be built into flexible panels, and are affected less by partial shading.

We couldn't see the logic in paying more for one flexible, roll-up solar panel than what the pop-up camper cost us in the first place! So, we compromised--our solar panel, made by Innergy and mostly used in remote river gauging stations and other telemetry applications, uses single crystal cells embedded in fiberglass instead of under fragile real glass. It gives 30 watts of power in full sun, and is only 19" x 36". It came in at $5.97 per watt, much better than roll-up models, and it makes more power for its size than amorphous designs. We built a simple aluminum frame for it that we can prop up near the camper after we arrive at a remote campsite. Shading is a big factor in solar panel output, even one branch shading part of a solar panel reduces output to nearly nothing. With the 25 foot extension cord, we can put the camper in a shady spot and set the panel in the sun.

our glass-less, fiberglass embedded solar panel with homebuilt aluminum frame

Our glass-less, fiberglass embedded solar panel with homebuilt aluminum frame. The tree shading shown in the picture is BAD--the panel is hardly putting out any power because of the shading.

Solar panel wire sizing

Remember that we discussed earlier how difficult 12 volt DC electricity is to move around compared to house current? Proper wire thickness is essential in connecting solar panels or else much of the power coming in will be lost as heat (just like in a toaster, only the solar panel wire won't even feel warm to the touch.) Wire losses of around 5% of your panel's output are acceptable. Rather then go into all the math, the easiest way to do it is Google up 'wire size chart' online, or get a printed one from any large renewable energy dealer's catalog. Simply look up your panel's output in amps at 12 volts, and read the maximum distance you can go on certain gauge of wire. Extension cords are great for this, but be sure to check on the package what the actual wire size inside the cord is. Generally, 10 or 12 gauge is the largest available at the hardware store...get the best and thickest you can afford. The more power your panel puts out, the thicker the wire you need and the shorter the distance you can go. For us, a nice 25 foot, 10 gauge extension cord cost about $25 and has very little loss at only 2.5 amps from our solar panel--we could add a couple more solar panels later and still be able to use this cord with under 5% loss.

Solar panel controllers

Large home-sized solar power systems always have controllers to keep the battery bank from overcharging. In our tiny system, we did not include a controller. The reason is that standard 'flooded lead-acid' batteries are not damaged at all by 'overcharging' -- defined as continuing to pump electricity into the battery after it's full. All that happens is that the battery electrolyte level drops more rapidly as the battery bubbles. If the electrolyte level drops below the top of the plates, the battery is ruined. It's important in any RE power system, with a controller or without, to check the battery electrolyte level monthly and refill it with distilled water when it gets low. In our case -- we'll never have that battery bubbling on a fishing trip, we use too much power every night. When the trailer is parked at home waiting for the next trip, we plug in the solar panel for a day or two each week to keep the battery topped off. If your battery is bubbling, it's no problem -- but don't smoke around it, and be sure the hydrogen is properly vented to the outside. In a camping trailer power system, the only time the battery might be bubbling is when the trailer is in storage for the winter with no power usage. If you for some reason chose a gel cell battery, a controller is REQUIRED, and will cost you $50-100.

Back to input vs output

We just did all that math about how much you can draw down a battery. Now look at our input from the solar panel (30 watts, 2.5 amps) versus the output from our lights (27 watts, 2.25 amps.) Here in Northern Colorado, a good clear day gives an average of about 6-9 full sun hours on a panel, it varies greatly by season. Multiply that by the amps output of the panel to figure how many amp-hours are coming in. 30 watts = 2.25 amps x 6 hours = 13.5 amp-hours (winter), and for 9 hours of solar exposure (summer) it's 20.25 amp-hours. That's not too bad--we could run our 27 watts of lights for 6-9 hours at night after a sunny day. What if it's overcast, or rainy? Solar panels put out nearly nothing unless they are in full sun. After a few cloudy days, we'd be dipping deep into the batteries capacity to run our lights for 6 hours a night. That's where shore power can be handy!

Renewable energy versus shore power

In a house in town, you'd call it 'grid power' instead of 'shore power.' It's your basic electrical outlet into which you can plug your camper electrical system. Most campsites that provide shore power also provide shore water pressure. This is absolutely worth the small amount of money it costs per night! Besides having potable water under pressure hooked right into the trailer, a simple 10 amp battery charger from the auto parts store can be running at any time we are connected to shore power -- that 10 amps coming in is like having 4 of our 30 watt solar panels running at once. Even so, the cute little 1000 watt electric heater we carry along to warm up the camper while making early-morning coffee is simply NOT feasible to run off of a renewable energy system, no matter how many solar panels and batteries were installed. We run it off of shore power only. It's the same in town--electric ranges, water heaters, space heaters and such use far more power than even a large renewable energy system could provide.

Is a transfer switch needed?

All a transfer switch does is switch your electrical outlets from shore power to inverter power and back again. Some are automatic, some are manual. All are expensive. To save money, we simply wired up a pair of outlets in the Sally Ann--one is for shore power (if it's available) and one is for inverter power. Simple, cheap, and can cause no confusion if properly labelled. The only thing an inexperienced operator would have to remember is to NOT plug the 1000 watt electric heater into the inverter output--shore power only for that power hog of a (very handy) gadget!

I already have a generator, will it help?

You bet! In a remote campsite where there's no shore power, a generator could be really handy. And the battery system means that you might not have to run your generator late at night to watch TV--you could instead boost your battery with the generator during the day, and keep a silent campground at night by running your TV off of the inverter. In our case we'd simply plug the generator into our shore power input, and both run loads directly from it, and use the remaining generator power to charge our battery with the standard NAPA battery charger we haul along with us. Get the biggest battery charger your generator can handle and that you can afford. Generators give you the most watt-hours per gallon of gas when you run them at 50% of their rated load or more, and run time will be much less to fill your battery when using a big charger.

How about water pressure?

Once again, we kept it simple. There was already a standard camper-style water faucet over the sink in our camper. If there's shore water pressure available, we push down on the handle to get water. If we are using water from our self-contained water cistern (a new, clean, food-grade, white 5-gallon bucket) we pump manually from it via the same handle. An electric water pump would have been an option, but adds more electricity use and possible reliability problems to the water system. It would be irritating to be without water just because the battery was low, with no sun or shore power to charge it back up.

What about the trolling motor?

On some lake fishing trips, we bring our vintage Grumman canoe and small electric trolling motor along. It's easy to drain that battery way down with just a few hours of flyfishing, especially in windy conditions. So, we chose exactly the same battery for the trolling motor as we did for the trailer -- they fit in the same battery box and hook up the same way, so we can quickly exchange the trailer battery for the troller. Shore power with a battery charger or a direct connection to the solar panel are the best ways to charge up the trolling motor battery. Don't try to run your 120 vac battery charger from the inverter to charge the trolling battery! You'd be converting 12 vdc to 120 vac back to 12 vdc for charging, with losses every step of the way. It's much more efficient to charge the trolling battery from shore power or direct solar...that's the main reason we made it easy to swap batteries between the canoe and the camper.


You could spend lots and lots of money on a solar power system for a camper. In fact, I wish some of those folks with noisy generators on our last fishing trip had done so! The problem is, they would HAVE to spend a lot of cash to run all those power-hogging gadgets off of solar. The same problem crops up in designing a renewable energy system for a home or cabin--the more kilowatt-hours of power you use each day, the more batteries and solar panels you need. Conservation is the key--it's estimated that for every dollar you spend on energy conservation, you save $3-5 on the cost of the RE equipment needed to power your stuff. If we'd used incandescent light bulbs in the Sally Ann, our possible run time before the battery was empty would be about 6 times less! The same for TVs, computers and anything else you need to run in the backcountry. LCD screen TVs have very low energy draw compared to normal models, as do laptop computers. Use an old-fashioned stove-top toaster instead of an electric one. Install a propane fridge instead of an electric one. Pick which appliances you use on a trip, depending on whether you are hooked into shore power or not. The electric toaster oven is FINE on shore power, but not on solar! Microwave ovens are surprising --- they use lots of power (800-1500 watts) but are only on for a minute or two, so a small microwave is perfectly feasible on solar power.

The key to using renewable energy, and one great benefit, is that you MUST be aware of your power usage at all times! When you return home, you'll suddenly be aware of all the power you are wasting every day. You might even turn into a 'power ogre' like me, roaming the house finding TVs and lights that were left on and shutting them off!

Irritated rainbow trout

Irritated rainbow trout. I released him.