Step 1 - Estimate the critical loads that need to be supported

Appliances that operate on electrical power are also referred to as loads. Making an accurate estimate of the loads that need to be supported by the PV system is the single largest influence on the size and cost of the PV system. Take all steps to conserve energy before designing your PV system, for example use Florescent lighting where ever possible in place of incandescent bulbs. Water and space heating can be more efficiently done with Solar thermal systems and are not recommended loads for a PV system.

A sample of typical household loads and their requirements are listed below:

An easy way to estimate the appliance characteristics for load is by using a "Kill a Watt" meter. You can get more information on the "Kill a Watt" meter by searching for it on Google. It is available in many online stores and you can find one here: P3 International Kill-a-Watt Electricity Usage Monitor.

When estimating the system size, list all the appliances that need to be supported by the system and estimate the overall usage of the appliances over the course of a week. How long the appliances are used in a day and how many days in a week? Staggering appliance usage over the course of a week helps optimize system size and lowers the overall cost of the system.

A table to help analyze the load, created by Benjamin Root and later modified by John F Robbins can be found in the 58th issue of Home Power magazine. (www.homepower.com) The goal of Step1 is to estimate the average power consumption per day and the total "Run Watts" or power required on a daily basis. It is also important to calculate the surge power required and ensure the system is capable of supporting the surge power requirements. The average power consumption will help in designing the battery bank required to support it as well as the PV Panel array required to generate the power requirements and the "Run Watts" help in selecting the Inverter. Most households also have "phantom" loads present all the time that need to be taken into account. "Phantom" loads are loads that are almost invisible but present all the time, for example the clock on the Microwave or the TV in stand-by more etc. You can calculate the "Phantom" loads that need to be supported or use an estimated 200-300Watts per day to ensure the system can support the additional "Phantom" loads.

To begin the design of your PV system, complete the table below and estimate the average power requirements per day and the total 'Run Watts' that need to be supported. It is also important to calculate the "Surge Watts" that need to be supported by the system. Inductive loads like Motors and a few other appliances like fluorescent bulbs draw a surge of power at start time, to overcome inertia which needs to be supported by the system. A rule of thumb for Surge power is 3X the standard power consumption.

The design section of this site has easy to use tools to help calculate the overall requirements for the system including daily average power consumption, Run Watts and Surge Watts, the key inputs into your system design.

Electrical Loads			RUN	Hours	Days	Surge	Ave. WH
	Qty	Volts	Watts	/Day	/Wk	Watts	/Day
Appliance 1	1	120	1500	5.00	7	4500	7500.0
Appliance 2	1	120	350	1.00	2	1050	100.0
Appliance 3	1	120	30	2.00	7	45	60.0

The Average Watt Hours per day can be calculated by using the formula below:

Ave WH/Day =	Run Watts * Hours/Day * Days/Week
	7

Also note the voltage requirements for your loads. You can support both AC and DC appliances with your PV system. DC appliances can be supported directly by the batteries while the AC appliances need to be connected to an Inverter which converts the DC power supplied by the batteries into AC current.

If you want to run AC appliances which require 2 phase or 3 phase power supply you will need to choose an Inverter that can support that requirement. Some inverters can also be connected in Series/Parallel to achieve the required voltage needed to support the appliances.

• Step 0: Determine the Solar Hours of your location
• Step 1: Estimate the critical loads that need to be supported
• Step 2: Determine the days of Autonomy for the system
• Step 3: Select a "Nominal" DC side voltage that the design will be based on
• Step 4: Select the PV Panels to be used for the system
• Step 5: Select the Battery manufacturer and model to be used in System
• Step 6: Select an appropriate Charge controller
• Step 7: Select an appropriate Inverter
• Step 8: Calculate and select BOS components