Determining Electrical Load for Stand-Alone PV System Sizing

Determining electrical loads is a crucial aspect when sizing stand-alone photovoltaic systems. It involves assessing the power requirements of different AC and DC devices to ensure the system is appropriately sized to meet demand efficiently. Factors such as duty cycles, phantom, and surge load must be considered during design to ensure the system operates effectively under varying conditions. This comprehensive evaluation helps optimize the performance and reliability of the photovoltaic system in diverse applications.

 

Image used courtesy of Science Direct

 

Equipment that uses electricity to operate is called a load. Loads are the largest single influence on the size of a PV system. It is better to supply some loads with power from other generating means to limit the size of a PV system. For example, powering an electric range in a home with a PV system can be cost-prohibitive. A natural gas or propane-powered stove would be more appropriate. This is called load shifting.

 

AC/DC Loads

In residential applications, many electrical appliances should be avoided in a stand-alone PV system because they use electricity to produce heat and, as a result, consume large amounts of electrical power. These appliances include electric space heaters, electric water heaters, clothes dryers, dishwashers, and electric ranges. These appliances should also be avoided in commercial applications. Still, several electrical appliances, like photocopiers, must be used in a commercial application with a stand-alone PV system. This means the PV system must be sized large enough to handle whatever the electrical load is.

 

Image used courtesy of Pexels

 

In certain applications, a PV system designer could use only direct current loads, so an inverter would not be needed. Because inverters are not 100% efficient, this helps minimize a stand-alone PV system's overall size and cost. Of course, this is not very practical because alternating current electrical loads are more plentiful and are much cheaper to purchase.

 

Calculating Electrical Loads

To calculate the electrical load, you need the specific loads’ wattage or volt-amp rating. This information is available from the manufacturer’s literature or the equipment’s nameplate. If the wattage or volt-amp rating is not found, the voltage rating and current draw will be available. For example, a nameplate may read 120 VAC @ 10 amps. From this information, you can calculate the watts or volt-amps by multiplying the volts by the amps. For example: ‍‍‍‍V×I=‍watts;120‍volts×10‍amps=1200‍watts . Treating volt-amps and watts the same when calculating the building’s electrical load is common practice. Many designers use the information for common electrical loads from a table like the one shown in Table 1. Most electric utility companies also have this information available on their websites.

 

Table 1. Common Electrical Loads and Their Power Consumption in Watts

Load	Watts	Load	Watts
Central Air Conditioner	5000	100-watt Incandescent Bulb	100
Electric Clothes Dryer	3400	25-watt CFL Bulb	28
Oven	3000	50-watt DC Incandescent	50
Hair Dryer	1538	40-watt DC Halogen	40
Dishwasher	1200–1500	20-watt DC CFL	22
Coffee Machine	1500	CFL Bulb (60-watt incand. equivalent)	18
Microwave	1500	CFL Bulb (40-watt incand. equivalent)	11
Popcorn Popper	1400	CFL Bulb (75-watt incand. equivalent)	20
Toaster Oven	1200	CFL Bulb (100-watt incand. equivalent)	30
Hot Plate	1200	Engine Block Heater	150–1000
Iron	1100	Portable Heater	1500
Toaster	1100	Waterbed Heater	400
Microwave	500–1500	Stock Tank Heater	100
Room Air Conditioner	1100	Furnace Blower	300–1000
Vacuum Cleaner	500	Clothes Dryer—Gas Heated	300–400
Water Heater	479	Well Pump (⅓-1HP)	480–1200
Sink Waste Disposal	450	Hedge Trimmer	450
Espresso Machine	360	Weed Eater	500
Dehumidifier	350	¼″ Drill	250
Blender	300	½″ Drill	750
Humidifier	300–1000	1″ Drill	1000
Video Game Player	195	9″ Disc Sander	1200
Standard TV	188	3″ Belt Sander	1000
Monitor	150	12″ Chain Saw	1100
Computer	120	7¼″ Circular Saw	900
Portable Fan	100	8¼″ Circular Saw	1400
Ceiling Fan	100	Refrigerator/Freezer 17 cu ft	500
Can Opener	100	Freezer 15 cu ft	335
Curling Iron	90	MP3 Player—Recharge	0.25–0.40
Stereo	60	42″ Plasma TV	339
Cable Box	20	42″ LCD TV	213
Clock Radio	7	25″ Color TV	150
Clothes Washer	500	19″ Color TV	70
DVD Player	50	12″ Black and White TV	20
Electric Blanket	200	Stereo	10–30
Shaver	15	Satellite Dish	30
Waterpik	100	Cordless Telephone—Receive	5
Well Pump (½–1 HP)	480–1200	Cordless Telephone—Transmit	40–150
Laptop	60–250	Cell Phone—Recharge	2–4

 

Duty Cycle Loads

A duty cycle is when an appliance is on and drawing current. Examples are refrigerators, freezers, electric heaters, and electric cooking appliances. These appliances automatically turn on and off as necessary. A standalone PV system designer needs to consider the duty cycles of electrical equipment so that when an appliance is ready to turn on, the PV system will have enough power available.

 

Phantom Loads

A phantom load is a load type that draws a small amount of current, even when the load is OFF. Examples of phantom loads include TVs, DVD players, and small plug-in transformers used to charge equipment like cordless telephones. Table 2 shows several phantom loads and the amount of electrical power they consume. Stand-alone PV system designers need to account for the phantom loads in a building when they size a PV system.

 

Table 2. Common Phantom Loads and Their Power Consumption in Watts

Item	Watts Average	Watts Min.	Watts Max.
Battery Charger	2.0	0.2	3.5
Bread Maker	1.0	1.0	1.0
Cable TV Receiver Box	12.5	5.0	20.0
CD Player	1.0	1.0	1.0
Cell Phone Charger	2.4	0.6	5.0
Central Vacuum	0.8	0.8	0.8
Clock	3.5	2.0	5.0
CO Sensor	1.6	1.6	1.6
Computer—Inkjet Printer	7.6	7.6	7.6
Computer—Laptop	3.5	1.5	6.2
Computer—Laser Printer	0.2	0.2	0.2
Computer—Modem	3.5	1.4	7.1
Computer—Monitor	6.9	6.9	6.9
Computer—Zip Drive	8.0	8.0	8.0
Computer Desktop	7.5	6.5	8.5
Computer—Sound Sys	13	05	12
Cordless—Phone	2.5	0.6	4.0
Dishwasher	1.8	1.8	1.8
Doorbell	5.0	5.0	5.0
DVD Player	5.3	4.5	6.0
Electric Shaver	0.6	0.6	0.6
Electric Toothbrush	1.6	1.6	1.6
Fax Machine	9.3	2.0	14.0
Flashlight (rechargeable)	2.9	2.9	2.9
Garage Door Opener	2.5	1.0	4.0
Generator—Standby	25	5	45
GFCI Receptacle	0.8	0.8	0.8
Microwave	3.0	2.0	5.0
Oven	3.0	3.0	3.0
Answering Machine	3.5	2.0	4.6
Power Cubes	3.4	2.3	5.0
Radio Clock/Alarm	3.3	3.2	3.3
Radio Portable	3.1	3.1	3.1
Radio Stereo	1.6	1.6	1.6
Radio Stereo Mini System	16.2	3.3	29.0
Satellite Receiver	26	26	26
Stove	2.6	2.6	2.6
Surge Protector	0.3	0.2	0.4
Timer	1.5	1.2	1.8
TV	8.0	6.4	10.0
Washing Machine	4.8	4.8	4.8

 

Surge Loads

Another type of load that must be considered is a surge load. As you know, electric motors draw more current to get started than to run. As a result, a way to momentarily supply the surge power needed to get electric motor–driven equipment started must be built into the PV system. PV system designers like to use manufacturers’ specific equipment surge requirements. Because this information is often hard to find, they often use a rule of thumb to calculate the minimum surge requirements. The rule of thumb is simply multiplying the required load watts by 2.5. For example, a motor-operated appliance with a rating of 1000 W is listed as 2500 W in the load analysis ‍‍(1000 ‍W ×2.5 = 2500 ‍W). Certain designers opt for a larger value in their rule of thumb for surge loads. This approach accommodates the system's sizing for potential future additions of surge loads, although it increases the size and cost of a stand-alone PV system.

 

Understanding Electrical Loads

Understanding and accurately determining electrical loads for stand-alone photovoltaic systems is crucial for several reasons. First, it ensures the system is appropriately sized to meet the power requirements of various devices, optimizing its performance and efficiency. Second, engineers can effectively manage power consumption and ensure the system's reliability and longevity by identifying and accounting for factors such as phantom loads, AC and DC loads, duty cycles, and surge loads. Additionally, considering these factors helps make informed decisions regarding load prioritization and power management strategies, ultimately contributing to the success of stand-alone photovoltaic systems across diverse applications and environments.