How to Calculate Solar Energy Needs

Going solar -- moving your entire house to solar energy -- is a huge commitment. In general, before making the transition, households do everything they can to increase energy efficiency, thus reducing the size and cost of the solar installation necessary to power the house. As a starting point, however, you will need to calculate the energy consumption of your house in order to design a solar system or set targets for energy reduction in order to install a more realistic system. Fortunately, calculating the energy consumption and solar needs of your household is fairly easy and a good estimate requires only your latest electric bill. You can use the same methodology to calculate the solar power needs of any appliance or system by starting with its watt use in the product's specifications.

Things You'll Need

• Electricity bills for summer and winter months
• Calculator or scratch paper

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Instructions

• 1

  Identify the total monthly consumption of your house in kilowatt hours. This number is easy to find on your utility bills and is usually toward the bottom of the bill right before the usage number is converted to a price.

  Because your energy use can vary considerably between summer and winter, find your total monthly use in both seasons. You can either work only with the higher number in order to design a system that will work year-round and produce some excess power during half the year; or use the smaller number to design a system that will meet your energy needs during half the year, but require some grid power during the remaining months. It is also a good idea to look at several bills to make sure that the figures you are using are representative of your house's average energy use during that period.

• 2

  Divide the total monthly consumption by the number of days in that particular month. Some power companies provide a day-by-day breakdown of power use, but, if not, you can easily determine an estimated daily use with some division.

  For instance, if your home consumes 300 kW/h per month in a 30 day month, your home consumes, on average, 10 kW/h per day.

  Dividing the total use by days is important, as solar systems will only produce significant power during the day and you need to be ready to meet your daily power needs. Of course, some energy can be stored in batteries for evenings and cloudy days, but usually not enough to power a home on a monthly basis.

• 3

  Divide your daily need by the number of hours of sunlight in a day. The amount of sunlight varies from region to region, but returning to our example and assuming 10 hours of sunlight in a day, the result is:

  10 kW/h / 10 hours = 1 kW per hour

  In other words, in order to produce 10 kW/h in a day with 10 hours of sunlight, your system will have to produce one kW every hour.

• 4

  Convert that figure to watts per hour by multiplying by 1000 (there are 1000 watts in a kilowatt).

  In our example, 1 kW/h x 1000 = 1000 W per hour.

• 5

  Add a 30 percent uncertainly increase to adjust for cloudy days or other obstructions to your solar panels. In the case of the above example, 1000 W/h would become 1300 W/h (1000 x 1.3).

• 6

  Double the last figure. Due to the different amounts of sunlight during the day, seasonal differences and several other factors, current solar technology requires that you double your expected installation capacity in order to consistently meet your needs. The good news is that this adjustment will guarantee that you always have enough solar electricity to power your house.

  In the example, the total installation capacity needed would be 2600 watts, meaning that an installation of that size would be able to consistently cover the needs of the household at its current energy use rates.