Hi Jeff

James

James,

As noted on this web site, we cannot provide specific design information on any solar project due to limited time available to answer each question on a free site, and with any design there could be critical information we do not have that could affect the advice given.  However, I always try to provide a direction for our readers when possible.

First, any off-grid system starts with identifying what electric loads are to be powered, and for how many hours per day.  Your list of this equipment has some serious problems which need to be addressed before even starting to complete a solar design.  For example, you list an electric stove which is almost never powered from a solar system due to high load, but it is OK to power a microwave oven since they only operate a few minutes.

You list running power tools each day, but make no mention of a well pump which we would expect to have on an off-grid rural installation.

You list a refrigerator as a constant 24 hour per day load, but refrigerators and freezers cycle on and off all day, so you need to find the daily kW-hour usage for your average room temperature to use and not an instantaneous  watt value.

You list a solar array output of almost 5 kWhs per day per kW installed which is highly doubtful.  This may be possible for peak summer sunny days, but this is not realistic for an average output per day.  We typically see solar arrays producing only 50 to 70% of their “nameplate” ratings except for very brief periods of extreme cold sunny days, and blue sky noon summers days with mild temperatures.  For off-grid designs, this is a critical factor since you have no grid to fall back on.

Your battery bank requirements are not realistic and must have come from somebody reading a book.  For example, your daily energy loads total 8.9 kWh and you want 3 days of no-solar battery time with 50% discharge.  This means the battery bank would need have a capacity of 53.4 kWh, which would require four (4) deep cycle 1200 amp-hr batteries @ 12 volts each in series, which would have a combined weight of 4,640 pounds and would cost over $10,000 just for the batteries, plus a pot load of shipping and un-loading costs.

Finally, there are serious electrical design considerations when designing any solar power system, and this is even more serious for an off-grid system.   The National Electric Code has an entire Section 690 that addresses wire and fuse sizing for solar arrays and I strongly suggest that you either obtain the services of a solar designer that has experience with these systems or plan on taking several courses on the subject first.

Good luck, and be sure to include a good fire extinguisher with your installation if you do not follow my advice.

Jeff Yago

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We have a piece of raw land located in the upstate of South Carolina (zone 7, upper 90’s for a month or two to a light dusting of snow maybe twice a year) with a sloping southern exposure but no existing shade trees. We want a passive (and active) solar home designed specifically for the site, and situated properly to take advantage of a fabulous West to East summer breeze.

Laura:

Not sure why you want all this site data, as it is not normally required to design an alternative energy system. There are many low-cost solar calculators and several free on-line sites that can provide this information. If we know the site latitude, time of day, and month, you can easily find all three sun angles for ANY place in the world without the need to measure anything.

The same sources can provide weather data and solar data to help estimate available energy, although these are 10 year averages and will not be exact for a specific month.

There are also wind charts for each state that show prevailing wind directions and speeds by month, but wind tends to be much more site specific and height above ground is the real measurement you need, not ground level.

Yes, there are many meters available to record all this is you really want to, but they are expensive and all you will end up with is a record of the years you made the recordings, not future data as the weather does vary from year to year.

In the non commercial and smaller solar and wind applications, window and solar array orientation are far less critical, as all you are doing is changing the time of day the peak will occur. For example, a window or solar array facing 15 degrees west of due south will “see” the sun maybe 20 to 30 minutes later in the morning, but will still have direct sun 20 to 30 minutes later in the evening. If I am designing a solar home in an area with very overcast or foggy mornings but sunny afternoons and evenings I face them more south-west or even due west. I have had to install solar arrays facing due east at some locations where a large cliff or mountain totally blocked the afternoon sun. We typically locate bedroom windows on the north side (cooler), and bathrooms on the east (early morning sun). Large glass windows facing due south can really overheat a room if there is not lots of thermal mass, and all your furnishings will fade. We use very large roof overhangs to block summer sun.

There are several good solar books listed in the book store section of this magazine.

Good luck,

Jeff Yago

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Dear Jeff:

Samual:

Due to the many requests I receive each week for design assistance, we have only limited time to address each question and try not to get into specific sizing and equipment selection advice due to the many unknown factors for each specific site. However, I will give you some general comments that should point you in the right direction.

First, you say you want to limit costs, yet you start out with a very large solar array which represents about 70% of the total system cost. The limited number of laptop computers and lights you have indicated do not justify this large array, especially in a country with fairly good sun and mild temperatures year round. If you are not sizing this for larger future loads, my advice is to go with a smaller array and provide the ability to add more modules later as the electrical loads increase.

You cannot directly connect a solar array to a computer laptop battery or operate the computer directly from the solar array. There are many technical and safety reasons which I cannot go into in this limited forum, but don’t try. All solar arrays require a solar charge controller(s) which adjust the voltage and current of the array to the battery voltage and charge level to maximize the rate of charge. The solar array will be operating at much higher voltages than the 12 volt limit on most laptop computers operating directly from a car’s utility socket which is what they are designed for.

Your estimate of 65 watts per laptop seems high. Do NOT go by the nameplate data on any appliance, as these are for legal labeling requirements and are “worst-case” maximum values. Buy a $50 watt meter tester available from any solar dealer and actually test each light and computer for both their watt draw when turned “on” and also when turned “off”. Most will have a standby load when off, and this can be a major load since it will be for many hours of evening and night.

Your laptop computers should do OK with most inverters and power systems, but the printers are more sensitive to power quality and really need a “pure sine-wave” inverter. The Outback inverter is an excellent choice, but be sure to consider their “sealed” model for dusty climates. Years ago we always ran at least some DC lights and DC appliances directly from the batteries as this avoids the efficiency losses of an inverter. However, today’s inverters are extremely efficient and reliable, and its far easier to just use standard 120 VAC lights and appliances, as long as they are high efficient brands.

Your list indicates 40 watt fluorescent bulbs. My guess is you were planning to use standard 4 foot 40 watt fluorescent lights. Please note these actually draw almost 50 watts due to the ballast load and are not good light quality . You should use 4 foot T-8 style fluorescent lamps as these require only about 32 watts each to run, will produce far more light than a standard 40 watt tube so you can get by with fewer lights, and their electric ballasts are high frequency with no light “60 cycle flicker”.

I cannot tell what type battery you are planning to use, but suggest you give this a lot of thought as it looks like this also does not appear to properly match the loads and solar input.

Finally, the charge controller is the most important item in the system and you really want a good quality model as it can reduce the number of solar modules you need, but you did not even mention this. The Outback MX-60 is a good choice for this application, but you may need more than one with this many solar modules.

You all really need some engineering design assistance to get this right.

Good luck!

Jeff Yago

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