RV Solar Calculator - Fiberglass RV


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Old 01-18-2020, 10:08 AM   #1
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Name: Henry
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RV Solar Calculator

I am considering having Solar Power installed in my Big Foot 25RQ. I am trying to determine how many amp hours we draw per day. Using the Go Power website calculator, a big draw seems to be the furnace fan. We use propane to run the furnace when boondocking. How many hours would the furnace fan typically run per 24 hour day, while camping in 20 to 30 weather, with the thermostat sit @ 60? Using the furnace, rather than a Mr. Heater Buddy, gets warm air to the holding tanks.

We also use the refrigerator on propane. How many duty cycles would be expected? The Go Power website only has 16 as the max input for their calculations. Would the refrigerator not be on for 24 hours each day?

Using 10 hours for the furnace we have a amp hour daily draw of 367 amp hours. Does this seems reasonable?

Also we have a built in 2500LP generator, which sucks down propane like you wouldn't believe, hence the interest in solar power.
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Old 01-18-2020, 10:27 AM   #2
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Without knowing the amp draw of your furnace, it is difficult to come up with an amp hour draw, but I suspect your 367 amp hour per day is high. If it draws 5 amps (high for a small RV furnace) at 10 hours, that would be 50 amp hours. The refrigerator on propane draws less than 1/4 amp off & on unless you have a version that uses a heating strip to defrost.

Typical small trailers with LED lighting usually run around 30 - 50 amp hours per day with overnight furnace use. If your daytime temperatures are in the 20F - 30F range, it will be higher, but I doubt it would go over 60 - 70 amp hours unless you have an inverter and use it to power high wattage appliances.

The only real way to determine usage is actual measurement. If you do lots of dry camping, installing a battery monitor will be a useful addition, and will tell you what you actually use.
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Old 01-18-2020, 10:43 AM   #3
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Jon, sorry I misstated. I meant to say the estimated TOTAL amp draw, that is, for all things using elec in the trailer, is 367 amp hours per day. I will try to find out what the amp draw is for the furnace fan, maybe in will be in the manual. It will take a a few minutes to get to the trailer and ferret out the info.
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Old 01-18-2020, 10:51 AM   #4
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If you’re not pressed to install a system right away, you could install a good battery monitor to measure your actual usage.

- Bogart Engineering
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Old 01-18-2020, 11:28 AM   #5
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Thanks John. I would like to have it all installed prior to this next camping season.

I sent the following email to the CEO of Big Foot RV:

What is the amp draw of the furnace in my trailer. I am getting ready to have solar panels installed. The furnace fan seems to be the largest user of electric.
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Old 01-18-2020, 12:42 PM   #6
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Originally Posted by Rzrbrn View Post
.... I meant to say the estimated TOTAL amp draw, that is, for all things using elec in the trailer, is 367 amp hours per day. ..
What the heck are you running in the trailer?

IDK what your furnace is but I have a good guess what it would draw, and even with that on the high end of my estimate and adding everything that a typical camper would be using while boondocking, I would guess that your estimate is at least four times too high. On the other hand if you are running a microwave over, hair dryer, etc. with a big inverter, then it might be low. But in that case I would suggest some adjustments to get down to 50-80 amp-hours a day AT MOST.

Once you have your power budget figured out, then its time to figure out how much power you need to generate and store, based on time of year, tree cover, an (almost) worst case scenario regarding weather and cloud cover, etc.
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Old 01-18-2020, 01:31 PM   #7
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Quote:
Originally Posted by Rzrbrn View Post
Jon, sorry I misstated. I meant to say the estimated TOTAL amp draw, that is, for all things using elec in the trailer, is 367 amp hours per day. I will try to find out what the amp draw is for the furnace fan, maybe in will be in the manual. It will take a a few minutes to get to the trailer and ferret out the info.
I believe your estimate is still too high. If you are really using 367 amp hours per day, you would need at least 1000 watts of solar, twice that during the short winter days with low angle sun, and at least 700 amp hours of batteries, or 6 225 amp hour 6V deep cycle batteries, more if you want to cover a couple cloudy days. While there are large class A RVs that use systems this large, I don't think you would need one in a fiberglass trailer.

As an example, I am currently dry camping at the Long Term Visitor Area in Quartzsite, have had 50F - 60F days & nights in the 30's. I have an inverter & often use my microwave, a toaster, and make a 5 cup pot of drip coffee each morning. I typically use 40 - 50 amp hours per day, and replace it with 420 watts of solar into a pair of 100 amp hour lithium batteries. The heaviest usage was after 3 cloudy days when I was down 76 amp hours.

Again, I'd check my calculations for usage.
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Old 01-18-2020, 02:27 PM   #8
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What I am reading is very good news. I can only say that I used the Go Power calculator and thought I was being conservative. Trying to attached the Go Power Calculation as a pdf.
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File Type: pdf Go Power! Recommended Products_18_01_2020_14253.pdf (181.8 KB, 19 views)
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Old 01-18-2020, 02:34 PM   #9
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I tried to estimate usage in the Go Power calculations. I don't really see anything excessive except for the Furnace Fan. I am not at all sure if the 80 amp hours means the fan runs continuously for 10 hours or that it cycles on and off for 80 amp hours: Big Foot RV has not responded back to my question. I suppose the fan time can be cut back. We use the Microwave to heat up frozen breakfast, reheat lunch and maybe warm something up for dinner: I don't much like cooking outside in the snow or in cold rain.
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Old 01-18-2020, 03:10 PM   #10
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Not sure if my picture will upload correctly since I am on my phone.. But according to my VictronConnect, my furnace uses between 7 and 8 amps when in use. At rest (No lights, no tv/radio, no fridge on, etc) I am using a constant .2 Amps.

My suggestion is to install accurate power monitoring like the Victron shunt. Then you will know exactly what your power needs are. Of course this is just battery powered devices. It won't measure AC (shore power or generator) current. Also if you are planning on using solar to power the air conditioning or microwave you need to factor in using an inverter.
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Old 01-18-2020, 03:12 PM   #11
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Thanks for the info Jon.
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Old 01-18-2020, 03:38 PM   #12
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Here is the charging done by my Victron Solar Controller for the first part of the month. It is in Watt Hours, so to get it to amp hours you need to divide by 13.6 (the float voltage of my batteries).

The white is bulk, the narrower darker band is absorption, and the light blue float.
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Old 01-18-2020, 05:39 PM   #13
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JonV, my interpretation of the graph is that on Jan 4 you used about 93ah, but most days you used about 75ah. Is this correct?

JonR, Big Foot RV said that the furnace uses about 8 amps when in use, exactly what you are showing. Since the fan is not on constantly, I suppose that means when the fan is not on it uses almost nothing. So one question is how long does the fan actually run over one 24 hour period, which would then be the actual amps used during that period.

If the above actually makes sense, don't be fooled into thinking I actually know what I am talking about.
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Old 01-18-2020, 06:19 PM   #14
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Originally Posted by Rzrbrn View Post
.... So one question is how long does the fan actually run over one 24 hour period, which would then be the actual amps used during that period....
Eight amps sounds a little high I think the NT-16 in my Scamp draws under three amps when running. But 8 amps is a safe estimate and might be right.

The duty cycle is hard to prefect. It depends on the thermostat setting, outdoor temperature, wind, and even the number of people (or dogs) in the camper, as well as a few other things.

While I understand your desire to get an estimate on power use based on the experience of others or using an online calculator, IMHO that is not the best way to approach it. Some people (Glen) claim to sleep in very cold weather and only run the furnace for a short time in the morning, using maybe as little as 1 amp hour a day for the furance. Others such as myself prefer to live in a camper no colder than 65 F no matter the conditions outside. In that case either, or both, the electrical power and amount of propane become precious resources.

And when you start talking about using microwave ovens, coffee makers, etc. there is even greater variation. Perhaps I should repeat that if you are trying camp off the electrical grid, even with solar, then propane is the best thing to use for cooking, heating and the fridge. IMHO making adjustments in the use of energy sources is superior to trying to replicate a home environment where you have endless electric power available.

So we come back to the suggestion to use a current tracking battery monitor. That is to say one with a shunt. The first reference given to you was for the Trimetric. That is what I have and I like it but there is much to be said for the Victron Energy monitors (like Jon has). If I had to buy one again it might well be the Victron. IMHO a current tracking monitor is the best way to figure out how much power you use for various things under varying conditions and to know how effective your charging system is. Get one now, do some living in the camper, and you will have a good idea about the solar system requirements for next season.
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Old 01-18-2020, 06:25 PM   #15
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Thanks Gordon.
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Old 01-18-2020, 06:50 PM   #16
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Quote:
Originally Posted by Rzrbrn View Post
JonV, my interpretation of the graph is that on Jan 4 you used about 93ah, but most days you used about 75ah. Is this correct?

JonR, Big Foot RV said that the furnace uses about 8 amps when in use, exactly what you are showing. Since the fan is not on constantly, I suppose that means when the fan is not on it uses almost nothing. So one question is how long does the fan actually run over one 24 hour period, which would then be the actual amps used during that period.

If the above actually makes sense, don't be fooled into thinking I actually know what I am talking about.
There is some inconsistency between the Victron solar controller readout in watt hours & the battery monitor reading in amp hours. According to the battery monitor, the most amp hours I've ever used (since installation last summer) was a discharge of 72 amp hours. Since the monitor subtracts amps out of the battery & adds amps going in, the solar panels area likely producing more, ie during the day some of the energy from the solar panels is going directly to loads, and would not be counted by the monitor.

Since I often save charging & using high current devices until after the batteries are full, the solar statistics are more accurate as long as I'm dry camping. Again, what comes from the solar panels & does not go into the batteries is not registered by the battery monitor.

I'm guessing using the float voltage of 13.6 to determine amp hours from watt hours. During the charging cycle, it typically goes from a starting voltage in the morning as low as 12.58 to as high as 13.24. The finishing voltage is almost always 14.43V as long as there is enough sun to get the controller to the absorption stage. So, the watt hours is accurate; the amp hours is a educated guess.

I will admit that since I switched to lithium batteries, I have not done anything to attempt to keep usage down, and am probably using 20 - 30 amp hours more than I did when I depended on a pair of 6V 232 amp hour wet batteries. I love lithium!
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Old 01-18-2020, 07:28 PM   #17
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According to the specifications shown on their website a 25 ft Bigfoot comes equipped with 30,000 btu furnace
From what I could find on the internet the current draw for the 30,000 btu furnace is 8.0 to 9.2 amps
My Casita had a 16,000 btu furnace , my Escape has a 12,000 btu furnace
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Old 01-18-2020, 07:32 PM   #18
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Originally Posted by steve dunham View Post
According to the specifications shown on their website a 25 ft Bigfoot comes equipped with 30,000 btu furnace
From what I could find on the internet the current draw for the 30,000 btu furnace is 8.0 to 9.2 amps
My Casita had a 16,000 btu furnace , my Escape has a 12,000 btu furnace
I believe the Bigfoot furnace is ducted, which requires a more powerful fan than the direct output furnaces most smaller fiberglass RVs use, so the 8-9 amps does not surprise me.
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Old 01-18-2020, 08:16 PM   #19
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There is some inconsistency between the Victron solar controller readout in watt hours & the battery monitor reading in amp hours. ...
This is an important point and one that I ignored in my post.

A battery monitor that reads via a shunt on the battery will be quite accurate regarding current into and out of the battery, and therefore good for judging the state of charge of the battery when you compare the net charge / discharge against the capacity of the battery.

It can also be most helpful in measuring the load on the battery from the stuff in a camper but only when the battery is not being charged. You can use this information to figure out the daily power use.

However if you use only a shunt at the battery then it will not be so helpful in knowing the total power consumed in the camper when the battery has BOTH a load on it and also a charger running. That is because the load (camper) is powered by both the battery and the charger (solar).

In this case a second shunt and monitor can be used that measures only the current going to the load (camper) independent of the shunt that measures the current charging or discharging the battery. Some people do use two shunts in this manner.

But one shunt measuring current into and out of the battery can still tell you how much power the various things in the camper use (when the battery is not charging of course) and when the battery is charging can also tell you at any given moment if the load is more or less than the charging (solar) power being supplied and by how much. As an example, lets assume that your furnace draws three (3) amps when running, and it alone is running (no lights, etc). Also assume that you have solar panels in use. If the battery monitor shows a positive two (2) amps going into the battery, than you know that the solar is providing two (2) amps for charging and also the three (3) amps for the furnace, a total of five (5) amps from the solar panels (typical for a 100 watt panel with some clouds). If the monitor showed a negative one (-1) amp, and again you know the furnace draws three (3) amps and is the only thing running, then the one (-1) amp recorded by the monitor is coming from the battery and the solar panel is making up the other two (amps) that the furnace needs (when fan is on).

Quote:
Originally Posted by Jon Vermilye View Post
..
I will admit that since I switched to lithium batteries, I have not done anything to attempt to keep usage down, and am probably using 20 - 30 amp hours more than I did when I depended on a pair of 6V 232 amp hour wet batteries. I love lithium!
Indeed.. if the OP insists on running his 120 VAC stuff, microwave, etc... then the posts about your lithium set up is required reading.
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Old 01-18-2020, 10:50 PM   #20
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Charts and website calculators are well and fine but sometimes there is little point to them when you have other restrictions driving your solar generation ability. Just get out there on the roof with a tape measure and see what you can fit on the roof. Use that as your starting point for sizing your solar system. There is a limit to how many and what size of solar panels you can fit up there.

So then you work backwards figuring out what if anything you need to quit doing to make it work so you can run your heater and fridge.

People forget that you can start with the total sum of the potential capacity and then subtract instead of adding. If you have a surplus of energy great, because then you can still make out OK on those cloudy days when there is not a lot of solar gain.


Then the next thing you want to look at is matching the optimum battery storage size for the amount of solar panels you have up top. You have size and weight limits on how big of a battery bank you can have in a fiberglass trailer.
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