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 tables - 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. It may be necessary to perform an estimation of this sort for each month of the year if loads, estimated usage, or solar resource vary frequently throughout the year. This process is covered in Load and solar resource comparison and values that result from this process should be used to design the system. In the tables below usage is estimated on a weekly basis. This process works well for loads that are used every day or large loads that are only used occasionally, but it is necessary to be careful in two cases:

1. With loads that are used for multiple consecutive days during the week. If a load is used during all 5 business days of the week it should be treated as operating 7 days out of the week as removing 2 days a week will reduce the average energy requirement, but in reality the system will be required to generate and store the full energy requirement on most days.

1. With systems that see heavy usage only 1 or 2 days out of the week. Inputting 1 or 2 days for all loads will lead to a system with PV source and energy storage system that are undersized. In this case, it is recommended that the days of usage be set to 4 or 5 days.

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

• 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, radios, and DC refrigerators. 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

						April - September			October - March
#	Load	Quantity	Watts	Total watts	Duty cycle	Hours per day	Days per week	Average daily DC watt-hours	Hours per day	Days per week	Average daily DC watt-hours
1
2
3
4
5
6
7
8
9
10

• 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
• 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
• 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 DC watt-hours = Total watts × Duty cycle × Hours per day × Days per week ÷ 7 days

Step 2: Determine DC energy demand

Total average daily DC watt-hours (April - September)	= sum of Average daily DC watt-hours for all loads for April - September

Total average daily DC watt-hours (October - March)	= sum of Average daily DC watt-hours for all loads for October - March

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:

• Inverter efficiency
• Surge loads
• Power factor

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.

Inverter efficiency	.85

Step 2: Fill out AC load chart

										April - October			March - September
#	Load	Quantity	Watts	Total watts	Duty cycle	Surge factor	Surge watts	Power factor	Volt-amperes (VA)	Hours per day	Days per week	Average daily AC watt-hours	Hours per day	Days per week	Average daily AC watt-hours
1
2
3
4
5
6
7
8
9
10

• 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 surge factor 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)	= sum of Average daily AC watt-hours for all loads for April - September

Total average daily AC watt-hours (October - March)	= sum of Average daily AC watt-hours for all loads for October - March

Step 4: Determine AC power demand

Total VA	= sum of volt-amperes (VA) of all the loads

Total VA with surge watts	= sum of Surge watts for all loads + Total VA

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)	= Total average daily DC watt-hours (April - October) + Total average daily AC watt-hours (April - September)

Average daily watt-hours required (April - September)	= Total average daily DC watt-hours (October - March) + Total average daily AC watt-hours (October - March)