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Unplugged: How much power am I using?

| Updated Oct 9, 2013

Blaaaaaaaaaaah!  We woke up to the shriek of the propane detector at 5 am in a parking lot outside a completely full campground on Natchez Trace.  We had arrived exhausted after changing a flat tire – in the rain – very late the night before and found no available campsites.  It was an unpleasant way of learning that the propane detector in our new-to-us Roadtrek is also an excellent low battery alarm.  We knew our single house battery of unknown vintage might be weak. But until we spent a night unplugged and needing the furnace, we hadn't experienced a problem.

If you spend all your time in campgrounds plugged into shore power, you needn't concern yourself much with the house battery or batteries.  Just remember not to leave your 3-way fridge running on battery when you spend 3 hours in a museum (switch to propane).  But if you spend much time boondocking (unplugged) you should know how much power you're using and how much you have left.  One internet site on adding solar to your RV said the first step should be to install a battery monitor and learn how much power you use.  Only then you can design a good solar system for your needs.

Trimetric showing battery at 100%
Trimetric 2025RV shows the battery at 100%

We replaced our old battery and we also bought a battery monitor – a Trimetric 2025RV – from We agree that it has been one of the most useful upgrades we have made to our Roadtrek.  The box arrived with a wiring harness long enough for a Prevost, a rather intimidating 500 amp shunt, and the display to be mounted on the wall.  We looked at it and scratched our heads.  Maybe the local RV dealer would install it?  They were less than enthusiastic, so we carefully read the instructions and did it ourselves.  It was actually quite easy.

Installing the shunt requires mounting it in a  location near the battery. Buy a short piece of heavy battery cable, available at auto parts stores, to run from the negative terminal on the battery to the shunt, with the original negative cable attached to the other terminal on the shunt.   We ran the wiring harness for the monitor behind the wall panel into the pantry cabinet.  We mounted the display on the pantry wall above the TV.

Shunt Installed
Shunt on left outside of battery compartment, holes into compartment not yet sealed

With the Trimetric battery monitor installed, we quickly learned and made notes of how much power each item in our Roadtrek used.  In addition to the voltage of up to 2 batteries, the Trimetric can display the number of amps being used –or watts if you prefer  Don't deplete a wet cell battery below 50%. The Trimetric has a nice display of % full.  Our single deep cycle battery is 80 amp hours.  Those with two 6 volt batteries may have as much as 220 amp hours.  So we had 40 amp hours to use before recharging, either by driving, plugging into shore power,  running generator, or by solar.    Here is what we discovered:  Your Roadtrek may be different, so make your own chart.

Power Use ChartFor example, using the table, if we run the Fantastic Fan on 2 overnight – say 10 hours – it will use 1.5 amps times 10 hours or 15 amp hours.  If we watch a DVD for 2 hours, that would use 1.5 amps x 2 hours or 3 amp hours.  Meanwhile the sensors work all the time, plus the Snyder kit on our fridge runs, so we have a constant draw of 0.4 amp hours. If we are stationary (and unplugged) for 12 hours that would add 0.4 x 12 for 4.8 amp hours.  So we will be using 15 + 3 + 4.8 = 22.8 amp hours — well within our 40 amp hours available.  Our biggest power hog is our laptop computers.  We use 12 volt adapters, but they can still use 3-5 amps.  We are careful with our computer use when boondocking.  We learned that we rarely exceeded 30 amp hours a day in power – which is probably on the low side for most Roadtrekers.

With a battery monitor you can keep an eye on your power use and prevent damage to your battery or batteries by not allowing them to discharge too far.  It will give you peace of mind when you wonder if you have enough power to run a CPAP machine or a furnace through the night.  It'll provide you with the information you need about your power needs when you consider upgrades to your batteries or charging system,  or adding a solar system.

Trimetric Diagram


Mike Wendland

Published on 2013-10-09

Mike Wendland is a multiple Emmy-award-winning Journalist, Podcaster, YouTuber, and Blogger, who has traveled with his wife, Jennifer, all over North America in an RV, sharing adventures and reviewing RV, Camping, Outdoor, Travel and Tech Gear for the past 12 years. They are leading industry experts in RV living and have written 18 travel books.

10 Responses to “Unplugged: How much power am I using?”

May 22, 2016at8:26 am, Dave Martin said:

I just completed the installation of a Trimetric 2025RV in my 91 190P and while it seems like a great addition, I encountered an issue that might save someone the hours of frustration I had. I ordered mine with the 100 amp shunt because I didn’t plan on running any large loads like an inverter and also thought it might be slightly smaller and easier to find a spot for it close to the battery in the tight compartment. Another factor was that the 100 amp shunt provides amp readings down to 0.01 (which didn’t seem critical to me.)
Once everything was connected I went to check it out and was getting some very odd readings. Turning on the FantasticFan resulted in a 24 amp increase in current draw. Turning off the propane leak detector resulted in a drop of 2.5 amps. Plugging in to shore power showed an initial charging rate of 280 amps. It looked like the decimal point must be off by a factor of 10.
I spoke to the guy at BestConverter and he didn’t have any explanation. Although I was 98% sure I had wired it right the first time, I took it apart and confirmed that the shunt was wired correctly. I kept reading the manuals looking for something and finally found it.
When using the 100 Amp shunt, the monitor must be set to operate on L3 rather than the default setting and the P11 setting must be changed to indicate that it is attached to a low power shunt. While I’m very happy to have the Trimetric, it would have saved me lots of trouble to have just gotten the 500 amp shunt.

December 02, 2013at4:04 pm, Jack in Jax, N3FYP said:

This is an excellent ‘first step’ in gaining an understanding – and control – of your DC (battery) usage…but the key is that ‘% used’ measurement which Lynn and Roger mention. Much of one’s DC consumption varies with seasonal and personal variations while boondocking: e.g. frig use when it’s hot vs. cold, entertainment use on a rainy day, or laptop use when some route planning is in progress. So not only would you want to avoid dipping into the house bank’s capacity beyond 50%, but you also will want to have total house bank capacity substantially beyond one’s ‘average daily use’…for just those occasions.

October 09, 2013at1:40 pm, Neil said:

Good, sensible, useful nformation. Thanks!

October 09, 2013at10:31 am, Ron Dempster said:

My RT 190 has a battery monitor installed by the previous owner from a Electric fork lift truck which seems very accurate and probably simple to install

October 09, 2013at9:20 am, Lyn said:

I don’t know electrical systems, but I can follow directions really good. I tried reading the Trimetric how to install manual I downloaded, but I need much simpler directions – places I called for install sounded even more clueless than I am.

So, all I need to do is install the shunt (as you described) and somehow? connect the harness (ignoring the behind the wall thing)? Any help with connecting the harness would be most appreciated.

October 09, 2013at10:10 am, Roger & Lynn Brucker said:

The Trimetric instructions are intimidating because they cover so many different applications (home, boats, RVs etc). Installing the wiring harness is not hard. There is a one page wiring diagram in the instructions that as all the necessary info. We labelled the wires at both ends and connected as shown on the sheet. About a year later (when installing a backup camera) we ran another wire up to the engine compartment to be able to monitor the voltage on the engine battery (but this is not necessary). We will take a look at the instructions and see if there is a way to make them easier to follow. We found the programming more confusing than the wiring harness – like figuring out to set and use a new multi-function watch.

October 10, 2013at8:20 am, Roger & Lynn Brucker said:

We put some more detailed instructions at
We have a diagram under construction but it is not complete yet. But hopefully this will answer your questions.

October 13, 2013at6:57 pm, Lyn said:

Thank you so very much – really clear instructions!

October 09, 2013at7:39 am, Campskunk said:

a battery monitor is one thing i don’t have, and wish i did. without one, even knowing the voltage doesn’t really tell you much. any load going out or charging current going in will move the voltage off the battery’s true state of charge, so without a battery monitor you are just guessing. nice article.

October 09, 2013at7:28 am, shari groendyk said:

This is great information, Lynn and Roger. The monitor would help demystify the whole energy use issue. Re the fridge, we had that very thing happen to us in Hershey in our 2001 Pop. Toured the RV show while our fridge sucked up energy in dc mode for 3 hours in the parking lot. Woke up to the screaming gas detector at 3:00 a.m. in Walmart’s parking lot. We knew better, but we just weren’t thinking …. The monitor would definitely make us more mindful of usage.

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