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dumb question...

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Posted by merlin-7 on January 6, 2007, 10:33 pm
 
This is a planning question so I have no real numbers here, so lets take a
few items for granted..

 Lets say you have 5 hours of full sun per day and want 1 day of reserve
power.

(inverted) You want to run a full size fridge, a 1500 watt microwave for 30
minutes a day. A washing machine for 30 minutes a day.(washing machine would
only be used in daytime while panels are in full sun) plus a TV (no real
numbers here but lets say 100 watts, mostly when panels are in full sun for
4 hours a day)

(dc load 12 volts) ham gear...5 amps at night for 3 hours average ...
                             lights ....cfls 100 watts 4 hours....average...
                             other.....100 watts 5 hours......average...

 What I think I need here is a bit of an unbalanced system. that is, more
panels vrs. battery storage.

 Looking at the above. How many watts of solar panels and AH of battery bank
would you start with?
 Rember, this home is on grid, I am just looking for a starting point for
off grid.

 heating, cooking etc. will be done with gas.

 I do not expect you all to do all the math but some of you will have a good
idea anyway...

 Thanks
 Joe ki4ilb












Posted by Eric Sears on January 7, 2007, 9:48 am
 
wrote:


I can't say how realistic your guessed numbers are - but the fridge is
the bigger load - so it might make a lot of difference what you choose
(or already have!).

(I would have said that in many places 3 - 4 hours was a better
average over a year, for a fixed panel - but YMMV). Certainly 5 hours
is possible for summer time.

In very round terms, the power usage EXCEPT the fridge are around
2kwh+. The fridge (of the size you describe) might very well add
another 2kwh (or at least 1.5kwh).

Unless you have some way of determining which appliances operate
"when", you might need a pretty big inverter (ie if the microwave were
running and the fridge switched on, the peak load could be excessive -
which would call also for a larger battery).

Assuming you limit your max load, and only need one day of reserve,
probably about 1kw of panels might do it (very rough guess).

As far as battery bank is concerned, you highest load is likely to be
created by the microwave (except for a brief switch-on surge for the
fridge). A 10kwh battery might be sufficient - but running a 1.5kw
load on that for any length of time will probably drop its voltage a
bit! I think most would opt for something like 15kwh battery.
You should be able to work out the amp-hr - it will depend on the
voltage of your system.

Having grid power as a secondary source (when solar insolation is
low), is a definite plus (its much what I do here with a lot less
solar at present). My "less than 450 watts" of panels provide a max of
about 1.8kwh on a good day in summer. An MPPT would add to that.

Eric Sears  ZL2BMI.

Posted by Ron Rosenfeld on January 7, 2007, 3:56 pm
 

In order to come up with reasonable numbers, you need to provide accurate
values for your electrical usage.  You have not provided any numbers for
the fridge or washing machine.

I will assume 1.6kWh/day and 300W while running for the washing machine.

Since you are on grid, I will assume that the 5ESH is an average for the
year, and that this is how you want to size.

If you were off-grid, you'd want to size for the worst month.

But you state that you are "looking for a starting point for off=grid", so
I'm not sure what you want to do here.  But we'll use the 5 hours/day, and
also assume you will be using high-quality batteries that you can discharge
80% DOD.

Because of your 12VDC load, I also assumed a 12V system.  But a 24V system
might be more appropriate with a DC-DC voltage converter, depending on your
eventual total size.

Your wire sizing will be less, and your inverter choices wider.

That being the case:

DC Load:    1080 watt hours per day
AC Load:    2700 watt hours per day
Inverter losses      15%
Net AC req    3176 watt hours per day

Array Sizing Procedure
        
DC load in watt-hours/day    1080Wh
AC load expressed as DC        3176.47    Wh
Total load            4256.47    Wh

System nominal DC voltage             12 volts
Total load in ampere-hours            354.71    Ah
System losses/safety factor multiplier        1.2    
Adjusted total daily load             425.65    Ah

daily average insolation             5 ESH

Required array current                85.13    amperes

Current at maximum power (Imp) of selected module    7.1
  (These are BPSolar 3125 12V/125W modules)
Number of parallel modules required            11.99
Line 12 rounded up to next whole number            12
Number of series modules required            1.0
Total modules in array (Line 13 X Line 14)        12


Battery Sizing

Selected days of reserve    1                    
                        
Adjustment for cycling capacity        0.8                    
Adjusted total daily load        425.65    ampere-hours

Recommended nominal battery capacity    532    Ah at    12    volts or
6.38    kWh
    

--ron

Posted by BobG on January 7, 2007, 4:50 pm
 Great numbers Ron. I changed the title to something more interesting
for readers scanning titles.


Posted by merlin-7 on January 7, 2007, 5:37 pm
 


discharge

 I do not have real numbers at this time, It is for my home when I move in 6
months or so...

However, you did confirm what I had expected...I will need to go to A 24
volt battery bank. Not so much as a current loss or wire size thing but the
problems and cost of charge controllers.
 If I understand the numbers correctly, 1 mx-60 charge controller can handle
the system you outlined above at 24 volts but not at 12 volts. correct?

 Now I will need to find a good 24 to 12 volt converter in the 30  amp range
for my ham gear.
 I am also thinking about having a small array (400 watts or so) to run my
ham gear and lights.( I have that now) and building the new system from
scratch for the inverter/ac....

Or, I could, get a small 24 to 12 volt charge controller to go between the
large 24 volt battery bank and a small 12 volt battery bank (just something
to handle the load spike when I transmit)
 Is this realistically possible?

 I also have a mixed set of solar panels at this time that works ok for 12
volts but would be a problem adding them to a 48 volt string without loseing
some ouput.

 Thanks joe KI4ILB





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