Help someone new with a system design?
EarlyRiser
Registered Users Posts: 1
I live in an apartment that has a south facing outdoor patio.
My idea is to put a couple of Golf cart batteries, maybe a 100 watt solar panel to power my TV, cable box, desktop computer, monitor, printer, Lamps, modems, routers, speakers and basically power my home office only.
Are two batteries enough for this assuming I get reasonable amount of light? Is a 100 watt solar panel enough?
How much of an inverter would I need and which charge controller?
Thanks!
My idea is to put a couple of Golf cart batteries, maybe a 100 watt solar panel to power my TV, cable box, desktop computer, monitor, printer, Lamps, modems, routers, speakers and basically power my home office only.
Are two batteries enough for this assuming I get reasonable amount of light? Is a 100 watt solar panel enough?
How much of an inverter would I need and which charge controller?
Thanks!
Comments

Re: Help someone new with a system design?
What are your goals?
You won't save money! but you might have some a small back system. It's doubtful you will have enough energy to run all the things you want to run for very long. You can measure your loads with a KillAWatt meter and find out how much energy they take to run.
A 100 watt solar panels is only likely to give you .250 kwhs of energy in a typical sunny day, your desktop computer likely uses that much energy in an hour. It might run a laptop for 510 hours.
The charging rate for 2 golf cart batteries would normally require 200 watts of array(panels) as a minimum.Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite Epanel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
 Assorted other systems, pieces and to many panels in the closet to not do more projects. 
Re: Help someone new with a system design?
As Photowhit saysStart with measuring your loads. Here is a link to what a KillaWatt meter is (you can get them and other brands at many hardware stores and onlint too):
http://www.solarelectric.com/kiacpomome.html
Once you know your loadsThen you can start planning a system. ButIn any case, usually your best bet is to start with extreme conservation. A laptop computer may use 1/10th the power of a desk top system. LED lighting, looking for low power networking/printers, turning stuff off when not used (setup computer for "sleep mode", etc.).
Most of us underestimate our loads and overestimate how much power a solar system is capable of providing. Just to give you some ideas... Two 6 volt @ 220 AH batteries for a 12 volt @ 220 AH battery bank.
Say you use 25% of the battery per day of "no sun" (13 days of storage and 50% maximum discharge for long lifeOr 2 days and 50% discharge for this example): 12 volts * 220 AH * 0.85 AC inverter efficiency * 1/2 days storage * 0.50 max discharge = 561 Watt*Hours at 120 VAC per day
 561 Watt*Hours per day * 1/5 hours of use per day = 112 Watt average discharge rate
 561 Watt*Hours per day * 1/0.52 end to end system eff * 1/4 hours of sun per day = 270 Watt array nominal
 14.5 volts charging * 220 AH battery * 1/0.77 panel+controller derating * 0.05 rate of charge = 207 Watt array minimum
 14.5 volts charging * 220 AH battery * 1/0.77 panel+controller derating * 0.10 rate of charge = 414 Watt array nominal
 14.5 volts charging * 220 AH battery * 1/0.77 panel+controller derating * 0.13 rate of charge = 539 Watt array "cost effective" maximum
If, however, you wish to run the system daily for 9+ months of the year, I would really suggest a minimum rate of charge of 10% or more... If you run the office during the day and can use the excess energy (after battery bank is charged) you could get from a 10% array: 414 Watt Array * 0.52 system eff * 4 hours of sun average minimum = 861 Watt*Hours per day minimum for ~9 months of the year
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Help someone new with a system design?
"If you want to run the system ~5 hours per day, then that would be:
561 Watt*Hours per day * 1/5 hours of use per day = 112 Watt average discharge rate."
If you were calculating watt average discharge rate for say a refrigerator that cycles for 20 minutes each hour (for a total of 8 hours in a 24 hour period) would the calculation be the same? In other words why would the calculation not be 561 Watt Hours X 8 hours or 4,488 watt average discharge in a 24 hour period?
Thanks,
Jim 
Re: Help someone new with a system design?
That is why a KillaWatt type meter is so nice... It is a kWH (kiloWatt*Hour) energy use totalizerBasically like the odometer on your car measures the total amount of miles driven between readings.
A refrigerators that uses:
120 Watts * 24 hours per day * 20 minutes * 1/60 minute cycle = 960 Watt*Hours = 0.96 kWH per day (a very efficient full size refrigerator in cool weather)
To run the above would take:
960 Watt*Hours * 1/0.52 off grid system eff * 1/4 hours of sun (~9 months minimum a year) = 462 Watt array minimum (full sun, tilted to latitude)
As you can see, a refrigerator is usually one of the largest loads for a small, off grid home.In other words why would the calculation not be 561 Watt Hours X 8 hours or 4,488 watt average discharge in a 24 hour period?
Note that Watt*Hours is a total amount of energy used (like Miles driven).
Watts is a rate, like Miles per hour:
50 Miles per Hour * 10 Hours = 500 Miles driven
500 Watts * 5 hours usage = 2,500 Watt*Hours used
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Help someone new with a system design?
Or:
120 Watts * 8 hours (1/3 of a day) = 960 Watt hours.
Such a calculation does not include startup demand, any nonrunning power required (as for controls that might be always on), or the occasional defrost cycle.
That is why measuring is better than estimating. 
Re: Help someone new with a system design?
Thanks Bill. I just ordered a Killawatt meter so maybe I can figure this out.
I understand the 120 watts * 24 hours per day * 20 minutes, but how did you come up with the 1/60 minute cycle?
Also any written material you can refer me to would be appreciated. I have seen watt hours calculated several times in articles (for refrigerators) online where they simply take rated wattage and multiply it times hours per day used to get watt hours used per day.
Jim 
Re: Help someone new with a system design?
From your description:say a refrigerator that cycles for 20 minutes each hour
Or 20 minutes out of 60 minutes... 20/60 = 1/3 = ~0.33 = 33% duty cycle.
Depending on room temperature and internal cooling loads (a refrigerator making ice uses more power than one not making ice, etc.).
Don't get too wrapped up in getting things down to 1% accuracy. It is easy to have 10% variation in sunlight (weather, pollution, etc.) and 10% differences between how two different people design and run their systems.
However, using the Watts rating on the label is usually going to give you highly inaccurate power usage estimates. An energy star rated refrigerator will easily use 600 Watts for the defroster heating element (on/off during a 12 hour defrost cycle)And use around 120 Watts for upwards of 50% of the time, or more as the weather turns hot.
The KillaWatt meter adds all the usage together and gives you how many hours you have been measuring. Say you got 3.2 kWH for approximatly 70 hours of run time on the meter:
3.2 kWH used * 1/70 hours measurement time * 24 hours per day = 1.1 kWH per 24 hour day
Pretty much like figuring MPH, estimating how many gallons the trip will take, etc...
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Help someone new with a system design?
And in the summer, depending on the room temperature, it can use considerable more. My EE model uses about 1kwh per day in the cooler months but in the dead of summer is closer to 1.5kwh per day.OffGrid in Central Florida since 2005, FullTime since June 2014  12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total)  Custom built singleaxis ground mounts  Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnetDC, Mate3, Hub10, FW500 AC/DC  24 x Trojan L16REB Batteries 1110ah @ 48v  Honda EU7000is Generator and a pile of "other" Generators  HomeMade PVC solar hot water collector  Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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