Simplified load evaluation

The load evaluation involves gathering information on loads which will serve as the basis for the rest of the system design process. Power ratings and usage estimates for necessary and potential loads are gathered into a table - one for direct current and one for alternating current. It is important to use maximum values for estimated usage to ensure that the system is adequately sized. If a system has extreme solar resource or load usage differences throughout the year, then it may be necessary to perform a load and solar resource comparison from the detailed design process.

When designing any system, it is almost always necessary to explore various different designs with varying loads and usage patterns to arrive at the best balance between cost and budget.

Additional considerations:

• If a system includes a refrigerator, it is necessary to follow the detailed system design process to properly account for duty cycle.
• The make, model, and power rating of any loads currently present at the site should be documented. Photos are very useful.
• If the system design is going to incorporate loads that have not yet been purchased, guidance should be provided about the value and importance of purchasing energy efficient loads.
• This process can be difficult in locations that do not currently have electricity or that have users that do not have experience with electricity beforehand. In these cases, there is a tendency to over-estimate and under-estimate appliance usage depending on the individual. The result is that more responsibility falls upon the person performing the load evaluation and the system designer to provide guidance and accurate estimates.
• Potential projects that do not currently have an electrical system will also require additional evaluation and design work to ensure that the building will have adequate outlets for (power receptacles) and lighting for the intended use of each room.

DC load evaluation

Typical direct current loads include lights, cell phones, and radios. If a system incorporates an inverter, it is important that its idle consumption is included in the DC load evaluation.

Step 1: Fill out DC load chart

#	Load	Quantity	Watts	Total watts	Hours per day	Average daily DC watt-hours
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• Load: The make and model or type of load.
• Quantity: The number of the particular load.
• Watts: The power rating in watts of the load.
• Total watts = Quantity × Watts
• Hours per day: The maximum number of hours the load(s) will be operated per day.
• Average daily DC watt-hours = Total watts × Hours per day

Step 2: Determine DC energy demand

Total average daily DC watt-hours	= sum of Average daily DC watt-hours for all loads

AC load evaluation

The AC load evaluation is important for determining the total amount of energy that is required for the system, but also to properly size the inverter so that it has sufficient power to be able to feed the loads.

Step 1: Fill out AC load chart

#	Load	Quantity	Watts	Total watts	Hours per day	Average daily AC watt-hours
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• Load: The make and model or type of load.
• Quantity: The number of the particular load.
• Watts: The power rating in watts for the load.
• Total watts = Quantity × Watts
• Hours per day: The maximum number of hours the load(s) will be operated per day. If the load has a duty cycle 24 hours should be used.
• Days per week: The maximum number of days the load(s) will be operated per week.
• Average daily AC watt-hours = Total watts × Duty cycle × Hours per day

Step 2: Deteremine AC energy demand

The total energy demand for all loads must be added up and then it must be multiplied by the average efficiency of the inverter to determine the actual total energy requirement. Inverter efficiency should be assumed to be .85 (85%). Some inverters will list higher peak efficiencies, but they will not achieve that level of efficiency under normal usage.

Sum average daily AC watt-hours	= sum of Average daily AC watt-hours for all loads

Total average daily AC watt-hours	= sum of Average daily AC watt-hours ÷ .85

Step 3: Determine AC power requirement

Total connected watts	= sum of Total watts

Total average daily energy demand

The total energy demand for the system is the added Average daily DC-watt hours and Average daily AC watt-hours for each time period.

Average daily watt-hours required	= Total average daily DC watt-hours + Total average daily AC watt-hours