Energy Budget

[Spaceman Spiff]

Finally! I will be starting a camper build that I will be spending 6 - 8 months living out of it in the southwest, starting in September.

I have developed an energy budget that I would like critiqued (hope this works, I attached an excel worksheet as a text file). Please tell me where you think I am wrong. Don't worry about hurting my feelings; I worked in product developement for 30 years and the blood sometimes ran ankle deep when we were 'discussing' technical abstractions. -- Spiff

---

Sorry, columns didn't line up! I'm frustrated. Will try again later. -- Spiff

 

[SternWake]

I think your fridge consumption estimate might be a little light, unless you add a bunch of extra insulation. The actual consumption can vary a lot depending on how much warm stuff is placed within and the ambient temps as well as the number of door openings.

But I think 2 100 watt panels should be enough to cover it. I only have ~200 watts, and am in the SW, and use way too much laptop and still have enough juice, most of the time. In Summer I have a good surplus.

 

[Spaceman Spiff]

Thanks SternWake for wading through my messed up .txt file :blush:
I tried for over an hour, yesterday, to put a formatted table into this editor without success. So, I tried attaching it as a excel .txt file. Comes up OK on my computer, but when it opens as an attachment, the formatting is off! #@%^&*( I have gotten so used to WISIWIG that I have forgotten how to manipulate a dumb editor.

I wanted to put the whole budget in for completeness, but there are only two items that I am unsure of:

1. Amp-hours per day from solar.
I will have 2 x 100W solar panels mounted flat on my camper roof. Using data from a solar energy handbook, I calculate those two panels generating ~50 amp-hours per day, in Quartzite, AZ., in January. Any of you that have experience in the SW in winter; does this sound OK? I have NO experience with solar, so I am flying blind.

2. Amp-hours per day used by refrigeration.
Thanks Willy, highdesertranger, and Bob for answering this in another thread. Truckfrig TF65, +90° ambient: 2.5A x 35% duty cycle x 24hrs = 21 amp-hrs per day. Sound reasonable?

This is now my basic budget (don't bother trying to read the .txt file):

200 Watts solar____________________________50 amp-hours/day
225 amp-hrs Crown 6V golf cart batteries
TF65 refrigerator__________________________ -21 amp-hrs
Fantastic fan on medium for 10 hrs___________ -11 amp-hrs
Laptop charging____________________________ -6 amp-hrs
Cell phone charging_________________________ -3 amp-hrs
CO & LP detectors_________________________ -2.4 amp-hrs
Misc small loads (under 1A)_________________ -10 amp-hrs
(LED lights, charge controller,BMS,refrig fans)
BALANCE__________________________________ -3.4 amp-hrs

This is my problem: I am riding on the harry edge with no margin. If I have a cloudy day or park in the shade, I won't replenish my batteries. So, unless I am off by ~ 5 amp-hrs, I am going to have to cut down on my laptop and cell phone usage. I am deliberately going with worst case numbers, because.....Murphy.
I am still debating wiring to the truck alternator, but I do not want to have to rely on running the truck or hauling a generator and charger with me.
-- Spiff

 

[compassrose]

http://handybobsolar.wordpress.com/

 

[Willy]

I'd think you'd be getting more than 50Ah a day. Right now, having loaned out my 64 Unisolar panel to a buddy, I'm running off a single 120W panel. This is powering my Engel cooler, 17W super skookum solar fan, laptop, led lighting, cellphone charging.. and I'm fully charged with power to spare before noon. For winter I'll scoop that panel and be running 184W.. and be fully charged by around 1pm. This is also up in BC, Canada, which doesn't get the kinda light you get down south in AZ.
The only other thing I have is a battery isolator that runs to my 'dedicated' fridge battery. For the most part, it doesn't do much, since my drives are only for a few minutes, but it's there just in case. The fridge battery is just a starter battery (which is PLENTY), and I've got 2 group 27 120Ah 12v deep cycle batteries for everything else. The charge controller is a cheapish dual battery PWM unit and works fine. This past winter, I had 184W of solar up on the roof, but for about abt 10 yrs before that I'd been getting by with about 150W (buncha small amorphous panels and my Unisolar). ..Willy.

 

[Spaceman Spiff]

For you techno geeks out there: I found this solar calculator.

http://www.batterystuff.com/kb/tools/solar-calculator.html

Not sure where there 'fudge' factors are from (what is a 'worst-weather multiplier' :huh: ), and I usually don't like to use 'calculators' that work behind the scenes, but it might give some of you a giggle.

I also found some modeling software from the National Renewable Energy Lab, but have just started looking at it. Probably don't have time to do more than skim the surface. Got a camper to build.

http://www.nrel.gov/rredc/models_tools.html

-- Spiff

 

[akrvbob]

I hate solar calculators--I don't use them and don't recommend them.

Your usage is very similar to mine and I spend every January in Quartzsite with 190 watts on the roof and it's enough. If it's cloudy and you get low, you just cut back until the sun comes out again. If it's not enough, you buy more solar. I know that sounds flippant but you can add a 3rd 100 watt panel for not much money or a complete system for $180

That brings me to what I DO recommend to everybody, buy ALL the solar you can afford and can get on the roof. No one ever says "I have too much solar!"

It's cold in the desert in the winter. You won't need the fan at all and the fridge won't come on much.
Bob

 

[Spaceman Spiff]

Calculators are tools, and like any other tools, you have to understand how to use them safely and correctly.
Experience like what Bob, highdesertranger, Willy and others have shared tells me that 200 watts of solar should be plenty. At this point it is mostly academic; my analysis doesn't agree, and the engineer in me rebels at that.
But sometimes you have to go with experience and figure out the why it works (or doesn't) later; that is where I am at. I have ~70 days until I start my next big adventure, so I have to start building. I can afford 200 watts of solar panels and my roof will handle the weight, so that is what I am going with.

On a related note - Does anyone have experience with the Blue Sky SB2000E or SB2512i_HV charge controllers?
-- Spiff

 

[SternWake]

I have a SB2512i that I bought in 2007. I also have the IPN pro remote battery monitor which allows one to customize voltage setpoints.

I doubt 100 watt panels will need the higher voltage capability of the sb2512i HV, unless you are choosing to wire them in series.

My only regrets with my BlueSky purchase is that I did not pay the extra 35$ for the iX model. It has a battery temperature sensor and allows many other features that my sb2512i does not.

YOu're not going to have issues with just 200 watts, not in the SW, even with the panels flat mounted and non tiltable.

But tilting can do a lot in winter months, especially if it is cool outside.

I would recommend adding some extra insulation to the TF-65. I was considering going with that unit as it would have fit in my space, but then I would not have been able to add extra insulation, and ultimately I wanted the made in italy Vitrifrigo vs the made in china Truckfridge, even though they are both very similar

 

[Spaceman Spiff]

Thanks SternWake,

I doubt 100 watt panels will need the higher voltage capability of the sb2512i HV, unless you are choosing to wire them in series.

My geeky side says to have the capablility to wire in series, but because I will size the wire from panels to controller for minimal voltage drop, there is probably no advantage to doing it.

My only regrets with my BlueSky purchase is that I did not pay the extra 35$ for the iX model. It has a battery temperature sensor and allows many other features that my sb2512i does not.

Good to know. My reading of the BlueSky sight led me to believe that all their MPPT charge controllers accepted their temp sensor; probably assumed something I should not have. I definitely want battery temp sensing.

But tilting can do a lot in winter months, especially if it is cool outside.

Thinking of doing something like this:
http://www.wanderthewest.com/forum/topic/6711-dual-solar-panels-in-series-tilt-able/

I would recommend adding some extra insulation to the TF-65. I was considering going with that unit as it would have fit in my space, but then I would not have been able to add extra insulation, and ultimately I wanted the made in italy Vitrifrigo vs the made in china Truckfridge, even though they are both very similar

I have made accommodation in my design for 2" of insulation around the sides, top and bottom of the frig. Won't know until I get it if it is possible to insluate door and back. I will be adding forced air circulation to the heat exchanger chamber.

If you don't mind my asking, what were the reasons for choosing Vitrifrigo over Truckfridge? There is ~$200 difference in price. But I am developing a definite bias against buying 'made in China'.
-- Spiff

 

[SternWake]

You will be able to add insulation to the back lower portion, but not much around the cooling unit itself. I squeezed in one layer of reflectix between box and compressor on the side, but not the bottom. i did not want to limit airflow from the condenser fan around the compressor or compressor controller.

I added a little extra insulation behind the removable door panel, but I don;t believe it is effective as in very humid weather, the stainless door panel I used will sweat.

I went with the VF over the TF for the Italy factor, and also because the VF came with a 120MM fan covering the whole of the condenser and attached with screws, rather than a 92mm fan covering only a portion of the condenser and held onto it with Zip ties. Also the VF has a recharging port, ther TF just Crimps the lines. Also, the TF places the expansion valve in a position that would have inhibited me from putting the Noctua Fan on the other side of the condenser, which makes it quieter and fits my whole no recycled airflow mentality better. Had I got a TF, I could not have done the Mods I am so pleased with, and the 150$ difference now 2 years later, is forgotten. I hate wishing I just spent the extra, like on my 2512i instead of the 2512ix.

I rarely think back a wish I saved x amount previously, Often I look back and regret not paying the extra for what I really wanted, and should have purchased.

 

[Mac J]

https://tools.carmanah.com/src.web/GoPowerCalculator.htm?state=RvDiv

Here's another calculator. I know RV Bob doesn't care much for calculators but I have no other resource. (Calculators are BTW always biased to the ideal situation and can only be used as a guide) but what else have we to measure by?

 

[Spaceman Spiff]

I agree SternWake. Learning when to buy cheap and when to pay for quality is a difficult lesson that some never learn. It's always good to be reminded.

Thanks for the link, Mac J. Take a look at it tonight.

(Calculators are BTW always biased to the ideal situation and can only be used as a guide) but what else have we to measure by?

Or biased to whatever they are selling.
One of the things that used to bug me is the number of highly educated people that take the output of computers without thinking. 'If the computer says the sky is pink your eyes must be wrong!' One has to have a basic understanding of what the answer is or calculators can get you into a lot of trouble. -- Spiff

 

[Mac J]

Sure but that becomes a 'catch 22' because people don't always know what they don't know. Let's call it a working chart instead then.

---

Somewhere along the line of education one has to trust their references.

 

[akrvbob]

Spaceman Spiff wrote:
At this point it is mostly academic; my analysis doesn't agree, and the engineer in me rebels at that.
==============

Exactly my problem with calculators. The results almost never match the real world. The calculators require so many assumptions and guesses they are nearly worthless.

But let me tell you a system that works 100% of the time with zero failures:

Buy the largest system you can afford and fit on the roof.
If you find that isn't enough, conserve on power until you can afford more or be ready to buy new batteries every year or two.
Bob

 

[Mac J]

So...toss out all the reference books, calculators, charts, graphs, computers and go by gut instincts because the written data may be flawed?
Wouldn't that mean that we should toss out your written advise too?

I don't know Bob. 'buy all you can' seems a little fast and loose to me.

---

I am glad it works for you.

 

[Willy]

Pssst.. the cake is a lie! ..Willy.

 

[Mac J]

Pssst.. the cake is a lie! ..Willy.

I'm not sure what this means Willy.

 

[Willy]

From a game. big cake Has a somewhat ambiguous meaning too. ..Willy.

 

[SternWake]

Bob's advice is sound, as the roof space is the limiting factor on most vehicles. Solar panels are relatively cheap these days, so the person who 'calculates' they need precisely 214 watts, will likely find that they only have room for 250 watts anyway. Is somebody really going to say no I don;t want 250 watts because a calculator told me I'd only need 214 and I can save 14.95 buy getting these two 115 watt panels?

More solar than you need just means your batteries are happier and will last longer, and you do not need to worry about it and you have a bigger buffer for bad weather.
Less solar than you need will just stress you out, and cost more in the long run as you replace batteries more often and freak out about your electrical consumption and your batteries "which no longer hold a charge".

The initial price difference, between too much and too little, is not significant. Not these days.

So take the advice of those who have done this lifestyle for years, and Stuff as much solar on your roof as possible , or you can be a doubter, and take your time to arrive at the same place after much stress, hair pulling and premature battery replacements.

Obviously those with larger RV's can stuff more wattage on their roofs than a van dweller, but in general these types of RV's are going to be larger consumers of electricity, with many devices which require a generator anyway.