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
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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
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= sum of Average daily DC watt-hours for all loads
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AC load evaluation
There are additional considerations that go into performing an AC load analysis because this analysis is used for inverter sizing and selection, as well as PV source and charge controller sizing and selection and Energy storage sizing and selection. This chart therefore takes into account several other important factors:
Step 1: Inverter efficiency
The efficiency rating varies between different inverters. The manufacturers typically give a peak efficiency value that is above 90% (.9), it is recommended that a more conservative value like .8 or .85 be used.
Step 2: Fill out AC load chart
- 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
- Duty cycle = Rated or estimated duty cycle for the load. If the load has no duty cycle a value of 1 should be used. A load with a duty cycle of 20% would be inputted as .2
- Surge factor = Rated or estimated duty cycle for the load. Common values are between 3-5. If the load does not have a surge requirement a value of 0 should be used.
- Power factor = Rated or estimated power factor for the load.
- Volt-amperes (VA) = Total watts ÷ Power factor
- 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 ÷ Inverter efficiency (Step 1) × Hours per day × Days per week ÷ 7 days
Step 3: Deteremine AC energy demand
| Total average daily AC watt-hours (April - September)
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= sum of Average daily AC watt-hours for all loads for April - September
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| Total average daily AC watt-hours (October - March)
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= sum of Average daily AC watt-hours for all loads for October - March
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Step 4: Determine AC power demand
| Total VA
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= sum of volt-amperes (VA)
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| Total VA with surge watts
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= sum of Surge watts for all loads + Total VA
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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 (April - September)
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= Total average daily DC watt-hours (April - October) + Total average daily AC watt-hours (April - September)
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| Average daily watt-hours required (April - September)
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= Total average daily DC watt-hours (October - March) + Total average daily AC watt-hours (October - March)
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