At A Loss

 I am new to the actual use of PV panels.  I have been reading about the
benefits of there use and I would like to incorporate them into my RV.  I
have calculated the total drain of my appliances and am now at a loss.  My
figures and vendor figures are completely different.  My calculations are
rather conservative, even with "fudge-factor" added in.  For example, I have
a DC Thermal Heater.  It draws 35 amps/hr.  It heats at a rate of 85 cfm.
My cabin space is only 170 cu. ft.  So realistically, after the initial
warming up period from the first heating cycle, the heater will only stay on
for a couple of minutes per cycle.  It typically turns on about 4 times per
hour.  So for every 15 minutes, the heater is on for 2 minutes.  That is
only 8 minutes of actual run time per hour.  My figures would show this
heater, under my application, to draw only 5 amps or less per hr.  Am I
thinking too far into this, or am I completely off in my math?

I would really appreciate anyone's opinion or advice at this time.  Thank
you.

 

Nonono. Incorrect units! Maybe it draws 35 amps (at 12 V, ie 35x12 = 420 watts
or 1433 Btu/h?) and the fan moves 85 cfm (with a 1433/85 = 17 F air temp rise?)


Sounds small, eg 170/8 = 21 SQUARE feet, if you have an 8' ceiling.
That's less than 5'x5'. Do you sleep standing up? :-)


Your math is OK, but... You might say the heater uses an average current
of 35x2/15 = 4.67 amps, for an average 4.67x12 = 56 watts of heat power.
Not much. Your body provides about 100 watts of heat power.

Nick

 

No you're not off base.  You have to figure actual energy consumption which
will obviously vary with cycle time.  


With regard to AC appliances, with which I'm familiar, on most the data
plate gives a maximum load which may rarely, if ever, be achieved.  Also,
for some appliances (pumps, refrigerators, and so forth) the startup surge
may be a good bit higher than the data plate numbers.

Arguably the best way to determine appliance consumption is to measure it.
There are meters available for AC; but I'm not certain about DC appliances.

You would basically size your panels to generate the daily amount of
electricity required; your other components to be able to handle the
maximum startup surge and running amount; and then decide about storage and
whether you want to provide for "enough" storage or run a backup generator
when needed.


-- ron  (off the grid in Downeast Maine)

 

  Lets say your heater takes 400 watts and runs 6 minutes per hour.
Over 10 hours it will run for 1 hour and thus take 400 watt hours
over a 10 hour period. A deep cycle 12v battery may be 400 watt hours
or more, but then you are out of electic in the morning. Maybe try two
deep cycle batteries minimum. Now you have to charge at least 400 watt
hours. If you have 5 hours of sun on top of your RV you would need 80 watts
worth of PV. Most RV panel are thin film and my experience is that a 40 watt
panel might actually deliver 20 watts.
  If they say you can get 5 watts per square foot with thin film your might
actually average 3 (rough estimates). That says you need more than 30
square feet of thin film RV panels. At maybe $40 a square foot that leaves
you with $1200 of panels. Pretty expensive for heating the inside of your
RV. I might swich to propane heat. Cheaper.

 

generator

While not disputing your advice, crystalline panels specifically for RVs are
available at about the same price, or a little more:
http://www.icpsolar.com/ssfamille.php3?id_rubriqueU
(Amazon.com product link shortened)