May 23, 2012

Electrical Calculations and Design

Throughout this whole trailer build, we always wanted to build this thing "off the grid."  In other words, we wanted to have some of the conveniences of the world (lights/plugs/chargers/etc.) but didn't want to have to plug in anywhere.  So with some research we decided that solar would be the way to go.

The design of this system is actually quite easy.  Utilizing solar panels you "trickle charge" your battery as it is depleted and can do this with a solar charge controller.  What the controller does is it charges the battery and then cuts off the solar power when the battery is fully charged.  Think of it as a computer for your battery - it's "smart!"

Before purchasing anything, I first had to determine exactly what we would be powering so we would know what wattage of solar array we would need.  This meant we had to research and decide on all of the fixtures we wanted for the trailer so we could input their wattages into our calculations.  GAH!  After we gathered all the info I used the following method to calculate our potential electric consumption:

This is consumption over a period of 1 weeks time (typical method of calculating solar capacity)

To determine watt hours = (Watts x hours per day use) / 5 hrs [5 is the typical peak charging time available during a normal sunny day.]

Devices and Data 
12VDC Light (cabin)   10Watts     0.83Amps     6hrs/day use     = 12Watt hours
12VDC Light (cabin)   10Watts     0.83Amps     6hrs/day use     = 12Watt hours
12VDC Galley Light    10Watts     0.83Amps     6hrs/day use     = 12Watt hours
12VDC Fan (cabin)     20Watts     0.5Amps      6hrs/day use      = 24Watt hours
Laptop (AC voltage)    180Watts   1.5Amps      5hrs/day use      = 180Watt hours

Total = 240Watt hours / Day
Total = 1680Watt hours / Week

If we work through and determine what wattage of solar array we need we utilize the same technique, we finally decided on two 20 watt solar panels which we got from Northern Arizona Wind & Sun.

40 watts x 7 days per week x 5 hours per day of charging time = 1400 watt hours/week

Even though these numbers don’t match (1400 watt hours vs. 1680 watt hours needed) we decided that it would suffice, not only based on the cost of the panels but also on the sizes.  We only had a certain amount of space on the roof for the panels and any panels we found with a higher wattage output were too large.  This will just mean we have to be a little more careful with how much electricity we use and "rough it" a bit more.

Whew!  I know that was a lot of math (and a lot of words) but it gives you an idea of how to calculate out the size of a potential solar array on any of your future projects!  And because posts without pictures are boring, here is a photo of me in the midst of wiring which I will go into on the next post!


  1. I'm curious, how may batteries will you have? Im trying to figure out my own system and am trying to figure out how many batteries I will need

    1. @jamison get the amps of everything plugged into your battery bank. Multiply each amp rating by how many hours you will be using it. Add everything together and you have your amp hour need. Batteries range in size and capacity.

      lets use this post as an example
      Devices and Data
      12VDC Light (cabin) 10Watts 0.83Amps 6hrs/day use = 4.98 amp hours
      12VDC Light (cabin) 10Watts 0.83Amps 6hrs/day use = 4.98 amp hours
      12VDC Galley Light 10Watts 0.83Amps 6hrs/day use = 4.98 amp hours
      12VDC Fan (cabin) 20Watts 0.5Amps 6hrs/day use = 3 amp hours
      Laptop (AC voltage) 180Watts 1.5Amps 5hrs/day use = 7.5 amp hours* (10.5 @40%)

      * these amp hours need to be exaggerated by up to 40% to make up for loss via inverter.

      Daily amp hours= 28.44 Amp Hours
      Weekly amp hours= 199.08 Amp Hours

      For a daily recharge [solar panels] you would only need 1 very low capacity deep cell battery. For a weekly recharge [via generator, boondocking, charging off car power while driving etc...] you would need 200 amp hours worth of battery capacity. You can get a single 200 Amp hour deep cycle lead acid battery, two 100Ah, etc...but this is the MINIMUM capacity you can have with those useages, if you use your laptop too long and you run out of charge, you get to spend the rest of the night in the dark [or just get out and run your genny :D ]

    2. Great thanks, I think I need to start planning it all out on paper

    3. Thanks for the comments! I was planning on getting a deep cycle marine battery, but was still worrying about the amp hours that I'd need. Much appreciated help! I wired a battery meter into the interior cabinetry so I'll know exactly how much voltage I have left, but overall I was going to wing it and try out different things. If I can't get onto my laptop during the day so be it! I'd rather be roughing it anyways... sometimes its good to get away from it all...

  2. If you haven't bought your solar panels yet I can help you out. My name is Nathan Wrzesinski and I am the founder of Project Upcycle.
    [ ]

    If you would like I can design and build you a panel that can more than fit your electrical needs. All I need are a few pictures of your roof and your dimensions. I can also probably do it cheaper than a big box panel.

    LOVE the teardrop by the way, it looks GREAT! I saw your page on and now I'm subscribed :)

    If you are interested in reducing your electrical usage you can get 60w adapters instead of 90w for your laptops. If you REALLY want to cut back on power look into getting netbooks for traveling. I am running an eMachines 1919 netbook with a 30w power adapter.

    1. I actually ended up purchasing two units a few months ago when I was working on ordering components for this thing. I'll explain the units I bought in another post soon, but thanks for the offer! It took some research but I actually found two that will fit exactly on either side of the fan on the roof perfectly.

      I'll definitely be looking into a travel charger for my laptop, Thanks for the great idea! saving electricity is going to be a challenge but I think we're up to it. After all we've been roughing it in tents up until this point with no electricity!

      Thanks for the comment!

  3. Enjoying your blog, I too found you through Tiny House Blog and subscribed. :-)

    Which way are you running the grain on your oukume? Your outer ply grain for this application should run perpendicular to the curve so that you are bending the plywood in it's weaker and less stiff direction. You can adjust your spar spacing to suit, shorter spacing along the curve and a wider spacing perpendicular, and you will maintain your strength without really gaining any weight. Great advantage if an anisotropic panel. :-) Of course, probably a little late in the project to be useful advice if you haven't already done it that way, unfortunately, but good to keep in mind for later components.

    On battery bank sizing above, don't forget that the capacity you actually plan to use needs to be tripled for lead acid batteries or doubled for agm. The rated capacity it's the maximum usable charge, but if you actually draw down to that regularly, your battery life will be much shorter before replacement is necessary.

    Looking forward to seeing the project progress! I've had something very similar in mind for a while, but unfortunately have too many projects on my own plate right now. :-\


    1. We actually only used the Okoume for the side walls, the ceiling and roof are birch veneer plywood - much less expensive and easier to work with! I don't think we mentioned directionality in our post about the ceiling install, but you are right... perpendicular is the best way to bend the wood.

      That's good advice about the battery too, we still have to select one! Thanks!