One of the steepest learning curves when we first started using our camping trailer was learning how to manage our power. Camping used to be fairly simple. You had certain items that needed power, and when they ran out, you couldn’t use them. But with our camping trailer, suddenly we had large, expensive batteries that needed to be maintained. Unlike the lithium ion batteries in our devices, these AGM batteries could be damaged if they were drained too far. Additionally, core functions of the trailer relied on the battery (like the fridge, the control panel for the heat and water, and the fan for the furnace). When voltage got to low, we were warned by a loud, obnoxious beep from the LP detector. Our fridge might shut down. And we could shorten or kill our batteries.

We also tend to prefer getting out and away from crowds, which often means being away from power sources.

That brings us to solar. Solar is a great, sustainable, quiet power source. However, figuring out how much solar you need can be very tricky. Conditions can greatly impact your power. Hot days mean the fridge runs more. Cold days means the furnace runs more. And clouds, trees, or heat can impact the effectiveness of solar.

When you are thinking about developing a solar setup, there are really three considerations: how much power can you gather, how much power can you store, and how much power will you use.

Power Storage

Let’s start with the batteries. The chances are, you will be getting AGM Deep Cycle batteries for your trailer. They don’t require refilling like flooded batteries, and they cost less and perform better in the cold than Lithium Ion. You are also most likely to get 12-volt batteries. There are some advantages to having two 6 volt batteries in series, but if one goes down, you have no power (unlike two 12-volt batteries in parallel). We were power hogs, so two batteries was a must for us. We had two Group 27 AGM batteries, but with our Mantis, we are hoping to get something larger (group 31 or group 34).

Power Use

Net up, how much power will you use? We are power hungry. It was hard to estimate how much we’d use. First, I had to figure out what the main trailer power draws were. There are three main draws: the fridge, the combination furnace / hot water heater, and the A/C. Now the A/C runs on 120V, and draws too much power for any reasonably compact solar system, so we can count that out for the purposes of solar calculations.

Amps Volts Watts
Mantis Fridge 5.6 12 67.2
Truma Furnace 6.1 12 73.2
Truma Hot Water Only 0.9 12 10.8
Mantis A/C (6000 BTU) 4.9 120 520

Add to that the items we want to charge while out.

Watt Hours
iPad Pro 12.9″ 30.4
iPad Pro 9.7″ 41
(2) iPhone 6s Plus 20.9
Apple Watch 0.94

With those numbers as a starting point, the next step is to identify how long those items might run. The fridge is on 24/7, but the compressor (the main power draw) is not running all the time. The Mantis fridge draws 300 Wh per day at 25 Celcius / 77 Farenheit and 563 Wh per day at 32 Celcius / 90 Farenheit. Going backwards from there, that means 4.5 hours of run time on cooler days, and 8.4 hours of run time on warmer days. The Truma furnace does not come on frequently, so assuming you turn it off (or down) when not in the trailer, I assumed 4 hours of run time in the winter. In the summer, all I need is the hot water, and it’s not likely I’d run it for more than 2 hours a day. For devices, I assumed as full charge each day, which is generous, I doubt I need a full charge on everything. Lights and control panels are minimal draws, maybe 30 Wh per day (that’s just a guess).

So, given those numbers, here are the three main scenarios I mapped out for our power use. You can see in both the winter and in the hot summer, we run into more power usage, but the overall range is from 550 to 750 Wh per day.

Scenarios for power use for Winter (718 watt hours per day), Mild Season (447 watt hours per day), and the hot season (710 watt hours per day).


Solar Gathering

So, now that I know how much power I use, or at least have a guess to that usage, the next step is identifying how much solar I need. Now, there are two main choices when it comes to solar: portable panels and fixed panels.

Portable Panels Fixed Panels
Portable panels can be oriented to maximize sun exposure, but require more setup and fiddling to get setup right. They are great because you can park in the shade and put your panels in the sun. Fixed panels, on the other hand, are always connected, always drawing power, but their orientation and position is more limited.

In the Pacific Northwest, we are often in the trees or in the clouds, so in the end, portability is perhaps not quite as important, as it’s often hard to get the panels in the ideal position regardless. Even with portable panels, and direct sunlight, the sun moves, so unless you are moving the panels all day, you probable have limited direct sunlight. Now, in bright sunligjt, a 100W pane; draws 5 amps of 12 volt power for 60 watts. What it draws in less than ideal conditions is harder to determine. It will change depending on the conditions, and there is not a lot of consensus on the numbers. Most of the estimates I’ve found fall somewhere between 10 to 40% efficiency. There have been some studies in the UK (a very similar climate to the Pacific Northwest) that show 40% efficiency with the stratus clouds we generally get here, but to be conservative, I assumed 20% efficiency, to account for particular dark winter days.

So, using those numbers, I drew up a few estimates to help me gauge my ideal solar setup.


Chart outlining a variety of scenarios for solar power gain

Given those numbers, we decided that 300–320 watts (three 100 watt panels or two 160 watt panels) would be the ideal range for us. It also still leaves us room on our Mantis roof for one more 160 watt panel or up to 3 more 100 watt panels. I am leaning towards the fewer, larger panels, even though I could only fit three, therefore 480 watts of panels total on our roof.

Hopefully our solar journey is helpful to you. We have yet to have the system installed, but I’ll followup with an update after we do and get some use out of it.

2 thoughts on “Designing a Solar System

  1. Fascinating stuff! We’re planning for extended travels in a few years once retired. We were wondering about upgrading from 80 watts to 200, but see from your research that perhaps 400 watts might be more suitable. Thanks for doing all the leg work on this! Very helpful. Are you using 2 – 12 volt rv batteries?

    1. You’re welcome! Just be aware, these are still largely estimates, and it depends on your usage. I’m curious to see how our system pans out. We currently have a 40 watt portable panel, and it has not been enough.

      And yes, we are using two 12-volt deep cycle AGM batteries. Currently we have group 27s in our Cricket, but we are going to be getting group 31s in our Mantis. We’ve lasted anywhere from 2.5–4 days on the group 27s in the past, so we’re hoping tons of solar will help us last longer, especially on our 2 week and longer trips.

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