A DIYer’s Guide to Solar Electric Power Systems
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A DIYer’s Guide to Solar Electric Power Systems

This article was written as a response to the question: How to get started in considering my purchase of solar panels for my home?

The purpose of this article isn’t to discuss the economical feasibility of converting your home to solar power but to discuss the technology involved in constructing a solar energy system. There are four major components involved in such a system, solar panels, a charge controller, a power inverter, and storage batteries. We will look at each of these four elements separately as we design our system.

The batteries

The storage batteries are the most important element in our system since they will supply the majority of the power we will be using. In order to decide on the number and ampere-hour rating of the batteries we will need we need to calculate the load that they will be supplying. To calculate the load we need to make a list of all the AC and DC loads that your system will be supplying and calculate the total ampere-hour rating of each load. To calculate the ampere-hour ratings of a load multiply its full-load amps by the number of hours it will operate during a 24-hour period. For motor loads and for most major appliance loads the FLA (Full Load Amperes) will be marked on the motor or appliance but for other load like lighting loads and small appliance loads you will have to calculate their Ampere ratings. To calculate Ampere simply divide the appliance’s wattage rating by its voltage rating I.e. to calculate amperes required by an 1800 Watt toaster oven we use the following formula: I = P/E where I = current in Amperes, P= power in Watts, and E = voltage. Therefore, for our toaster oven, I = 1800/120 = 15A. If you decide that the toaster oven will be used for 2 hours a day the Ampere-Hour rating of the toaster will be 30AH. Ampere-Hours = Amps X Hrs. Once you have calculated the AH (Ampere-Hour) rating of each load, add them together to get the total Ahs required of your battery pack then multiply that rating by 2 to get the total AH rating of the battery pack that you will need to construct. To construct your battery pack you will connect the batteries in a parallel circuit configuration. To ascertain the number of batteries that you will have to parallel to get the Ampere-Hours required divide the total AH Rating required by the AH Rating of one battery. For the sake of discussion, if the total AH Rating required is 900AHs and the AH Rating of 1 battery is 240AH then you would need to parallel 4 batteries (900AH/240AH=3.75=4). To parallel the four batteries simply connect all the negative terminal together and connect all the positive terminal together.

Depending on your budget, automotive batteries may be used but the best results are obtained when using deep-discharge batteries. A good source for deep-discharge batteries especially designed for renewable energy systems is the Trojan Battery Company. A downloadable specification sheet for their batteries is available here.

The solar panel array

The most important thing to keep in mind here is that the voltage that the panel produce has to be slightly higher then what the battery pack puts out. If you can think of voltage as pressure the reason for this will be easier to understand. If the pressure produced by the batteries is greater then the pressure produced by the array the batteries will try to force current into the array when we want the current to flow from the array into the batteries. To achieve what we want, the current flowing from the solar panel array into the batteries to recharge them, the array must produce a slightly higher voltage or electrical pressure. A 12-volt battery is considered fully charged at 12.7-volts so we need an array that puts out about 13.75-volts. Actually, we should be looking at a 14-volt panel. The charging regulator, which we’ll discuss in another section of this article, controls the charging rate and prevents the batteries from being over charged.

Solar panels are rated in watts. The panels have to generate the number of AHs required for a 24-hour period during the hours of sunlight. For the sake of discussion let’s say that the sun is visible for 8 hours a day in your area and the calculated AH load for a 24 hour period is 900AH therefore our array needs to produce 8.035A/hr (Amperes per hour).

For this calculation we divide 900AH by 8 hours of sunlight to get 112.5AH. We then divide 112.5AH by 14-Volts to get 8.035 A/hr. Since solar panels are rated in watts we need to convert from volts and amperes to watts. Watts = Volts X Amperes so we multiply 14-volts times 8.035 Amperes to get 112.49 Watts. We will need a solar array that will produce a minimum of 113-Watts of DC (Direct Current) power.

The charge regulator

The charging regulator of solar power system serves the same purpose as the voltage regulator built in to your automobiles charging system, it controls the amount of current flowing into you automobiles lead-acid storage battery. Basically the charging regulator senses the level of charge on the batteries and disconnects them from the solar array when their level of charge reaches 100-percent. The regulator also has built in fuses to protect the system against voltage spikes. Better to blow a $1 fuse then a solar panel costing several hundred dollars.

The DC to AC inverter

The last major component in your system is the inverter. The inverter serves two basic purposes, it converts DC (Direct Current) to AC (Alternating Current) that your household appliances and other loads require to operate. Inverters also incorporate a step-up transformers that increases the 12-volts AC to 110/220-Volts AC required by your household loads.

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