Determining Number of Solar Panels Needed

I. First, reduce your load!
The first thing you need to do (if you haven't already) is look at ways to reduce your household electrical needs. Big wasteful energy consumers are electric water heaters, stoves and heating systems. If possible, these systems should be converted to gas power - anything besides electrical. Usually the expense of changing out these systems is less than the added expense of having a solar/wind energy system to power these large electrical energy consumers.

Other obvious things you can do to significantly reduce your energy needs is to convert all incandescent lighting to full-spectrum fluorescent lighting. Again, the added expense of these lighting systems is easily compensated by the reduction in the cost of your needed solar/wind energy system.

Next you need to design as system that will meet your electrical needs. For the sake of simplicity, we're going to go over the basic steps you would take to design a solar electric system for a home that's occupied all year around (as opposed to a summer cabin, for instance). The difference being that a home that would be occupied all year round needs to be designed to produce enough energy during the low-sunlight times of the year.

II. Determine the number of solar panels you will need
1) Calculate your electrical load. Determine the number of watt*hours your location will use on a daily basis. For an entire home, this will take some busy work. You need to determine:
What appliances you are going to power.
How many Watts does each appliance consume?
On average, how many hours do you use this appliance per day? (Don't forget to include those things that you use on a weekly or monthly basis, like a vacuum sweeper or a blender).
Calculate your average daily watt*hour usage for your entire home. That is, multiply the watts of the appliance time the average number of hours used per day.

2) Determine the equivalent number of full-sun hours for your location for the month with the least amount of sunlight (typically December or January).

3) Divide your load calculation from step 1 by the number of full-sun hours from step 2. This is will tell you the number of Watts of solar panels you will need to provide you enough electricity in the lowest sun-light months.

4) Compensate for system inefficiencies. Every part of a solar powered system has some inefficiencies in it. The rule of thumb is if you are going to use an inverter (to produce AC) your total system inefficiency will be 30%. For systems that will be using the DC voltage directly from the battery bank, the inefficiency factor is 20%. So, to compensate for inefficiencies multiply your answer to step 3 by 1.3 (or 1.2, if there's no inverter). This answer is the number of watts of solar panels you will need to provide enough electricity for your loads.

5) Finally, to determine how many solar panels you will need, take your answer from step 4 and divide it by the rated power output (watts) of the solar panel that you have chosen.