Frequently Asked Questions
Question: A friend of mine recently asked me how he should begin a small offgrid residential power system, using a very minimal budget. "How do I get started?", he inquired. He owns lake property and has a camper parked there for recreation. He would like to power some lights, radio, small tv, and maybe some refrigeration. The answer appears simple, but there are considerations and a few basics that must be addressed. This system will be 12 volts.
We will start on the outside, and work our way in. A solar module is the first item needed. Working with a minimal budget, this is also the first decision to be made. Module value is quantified as cost per watt of generation. A 60 watt module has a higher cost per watt than a 215 watt module. As a rule, start with the largest module budget will allow. Other factors include where and what type of mounting system will be used. Certain circumstances, and the budget, may require a 135 watt module. I got started in 2002 with one 130 watt module, placed just outside a window, leaned against the building. To prevent damage, I do not recommend this, but it is a rudimentary start. The close proximity placement to the battery eliminates line loss considerations. DC power is very sensitive to distance. The longer the distance between the module and battery, the larger the wire must be to carry the load safely.
The battery is the heart of any offgrid system. I began with two car batteries wired in parallel, the worst possible technique. One must use a "deep cycle" battery to even hope for success. A deep cycle marine battery, used on boats, is an inexpensive choice at a cost of about $80 in my area. The important factor here is the amp/hour capacity of the battery. This marine battery has a capacity of around 70 amp/hours. On the other hand, two 6 volt golf car batteries wired in series to yield 12 volts, will have 225 amp/hours of storage, but at a cost of about $225. The amp/hour capacity of the batteries will dictate how much power can be used at night, before being recharged the next day, and how many days of power are available until the battery is fully recharged, referred to as "days of autonomy." To achieve maximum battery life, it must be fully recharged every four to five days.
Any battery, or battery bank, must not be discharged below 50% of it's capacity. Doing this will drastically reduce it's life span. In my system, the marine batteries had a life of around 3 to 4 years. Golf car batteries can be expected to survive 4 to 5 years, with 6 years being possible, assuming proper maintenance. The reasons for this are, the golf car battery is of a more "heavy duty" construction, and has the larger capacity, so it gets less deeply discharged over time. We currently use three pairs of golf car batteries in our bank, which yields 675 amp/hours of storage. Using half that capacity, at our current usage levels, that equates to about 9 days of autonomy. But compared to most households, our usage is very low.
All this brings up the next point. Most people look at electrical usage in watts, or kilowatts. When offgrid, using batteries, one must look at power usage in amp/hours. These two are different measurements of the same thing. Volts x amps = watts. In this case, watts divided by volts = amps. Because batteries are rated in amps, usage must be calculated accordingly. One procedure would be to calculate all the loads, then design the system to supply the needed power. When keeping the budget as low as possible, as in this case, we will design the minimal system, then use whatever power is available. My starter system, with one 130 watt module and two car batteries, supplied enough power to run a car radio and a 13 inch crt (cathode ray tube) 12 volt tv. Daily run times depended on the amount of sunshine.
The last, and most important, item needed is a quality battery meter. One could use an automotive voltmeter, as I did, but this is not recommended. I would strongly suggest the installation of a Trimetric battery meter. It is high quality, supplies all relevant information, and is the only meter you will ever need. The batteries are the heart of the system, and the Trimetric is the heart monitor! At around $200, it is a fairly substantial beginning expense, but will not have to be revisited.
The short answer to this question is: For this most basic system, one will need a module (about $400), a battery (about $80), and a meter (about $200), with a combined total expenditure of around $680. I do not mention a charge controller because, as in my case, one will rarely have to be concerned about the battery overcharging. If this system were left unattended for periods of time, a charge controller would be a must. I was constantly present, and monitored the batteries closely, lessening the need for a controller.
Question: A local successful businessman asked me,"What would it cost to take all of my employees offgrid?
This question has no accurate answer. What was his employees' current usage? Some of the families had children, some did not. Without scrutinizing each employee's usage, and assuming the average, it would be very safe to say that this would cost millions, depending on the number of employees. His approach was to spend x amount of money, generate x amount of power per family, and they would have to live with the result. He was prepared to spend $100,000 initially, then up to one million dollars, to accomplish this. These families would have to alter their lifestyles to fit the amount of power their system generated. To me, this was unacceptable. The employees would be dissatisfied with the results, and with unhappy employees, the businessman would also not be satisfied. I refused the project.
Question: A local landowner has several properties on the lake and wants advice on installing wind and solar electricity generation to provide income.
In this situation, my first advice is to eliminate wind because of maintenance & repair costs. Even at the lakefront, the existing wind resource is not plentiful. Using solar, which has the greatest tax benefits, is not feasible in this instance because TVA requires present electrical usage at each site, and then will only approve a system that will offset usage. If I am trying to sell a product to only one customer (it is illegal to sell electricity to anyone in the TVA service area, except to TVA), and that customer currently holds in excess of $30 billion in debt, this does not appear to be a sound business plan!
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