Let's consider taking a group of entertainment center components and powering them with a stand alone, off-grid solar RE system. We will gather load data and usage patterns. We'll incorporate requirements to power the entertainment center via the RE system's batteries through a duration of little sun. We'll choose and evaluate the RE system's major components to meet the requirements and expectations of operation.

First we will perform a load calculation. Gather data from all the components you wish to power. The running watts information is typically found near the power cable coming out of the unit. "Running Watts" is the power the unit is using when it is turned on. For example, when you turn on a 60 watt light bulb, 60 watts is the power being consumed. "Phantom Load Watts" is the power consumed when the unit is turned off. These Phantom Loads can be significant because they are using power at all times and can add up on your electric bill. Add up the hours per day and days per week that the individual components of the entertainments system is being used. In this case there are no surge watts. The more accurate and complete the load analysis is, the more efficient and effective the resulting RE system. The design of the RE system will be sized appropriately for operatiional expectations, and the cost of the RE system will be kept at a minimum. Don't buy more than you need, but buy what the load and the usage behaviour demands. That, is a well designed and balanced RE system.

Home Power magazine has the spreadsheet we are using and a How-To document describing it's operation. Look for them in their download section as loadcalc.xls

Total Average WattHours per Day - 6298

Largest AC Appliance Wattage - 450

Inverter Priority Wattage - 857

Using an electrical utility company rate of 13.3 cents per Kilowatthour and multiplying that with the average watthour used per day of 6.298KWH, we are spending $0.84 per day, or $25.97 per month, or $311.60 per year to power this particular entertainment system given it's usage pattern. We will use these power company incurred operating expenses as a comparison to the capital investment of the RE system and give us an idea of payback time and a sense of our return on that investment. Using the US Government's EnergyStar conversion ratios associating KWH energy use to carbon emmisions we can determine how many pounds of CO2 that are avoided per year.

Operating this entertainment center, on average, requires 6298 watthours per day. We need to figure out the total wattage of the solar panel array that will provide that power. We divide total watthours by the sun-hours available at our location. For the DFW area Solar Insolation (sun-hours) is a high of 6 hours in the summer time, 4.8 hours during winter, and on average we can use 5.43 hours. Insolation is a measure of solar energy received on a given surface area in a given time.

In the above equation we happen to choose a 170 watt solar module. Rounding up results in 8 solar modules for this renewable energy system.