Sizing a photovoltaic system for a stand-alone photovoltaic power system involves a five-step process which will allow the photovoltaic system designer or user to accurately size a system based on users projected needs, goals and budget. These steps are:
a. Estimating the Electric Load
b. Sizing and Specifying an Inverter
c. Sizing and Specifying Batteries
d. Sizing and Specifying an Array
e. Specifying A Controller
1. Estimating the Electric Load
The first task for any PV system design is to determine the system load. The load determination is straightforward. Make a list of the electrical appliances and/or loads to be powered by the PV system. The power required by an appliance can be measured or obtained from the label on the back of appliance which lists the wattage. Once you have the wattage ratings, fill out the load sizing worksheet (refer below). The power requirements are calculated by multiplying the number of hours per day that specific appliances will operate each day. For existing buildings, other alternative is to get consumption figures from your utility invoice; it shows actual usage over a 12-month period. 
The load is determined by listing all appliances with their power ratings and operation hours then summing it to obtain the total average energy demand in watt-hours or kilowatt-hours. Worksheet below gives ideas of how to estimate the load.
2. Battery Sizing in Sizing a photovoltaic system
Batteries for stand-alone systems are sized to store energy produced by the array for use by the system loads as required. The total amount of rated battery capacity required depends on the following:
a. Desired days of storage to meet system loads with no recharge from PV
b. Maximum allowable depth-of-discharge
c. Temperature and discharge rates
d. System losses and efficiencies
e. The system voltage defines the number of series-connected battery cells required.
f. The total capacity needed defines the number of parallel battery strings required.
2.1 Days of Storage or Autonomy
a. Autonomy is the number of days that a fully charged battery can meet the system loads without any recharge from the PV array.
b. Greater autonomy periods are used for more critical applications and increase system availability, but at higher cost due to the larger battery required.
Important: Batteries should be capable of meeting both the power and energy requirements of the system. As a rule of thumb, the minimum autonomy should be kept as 3 days for regular loads. For critical loads autonomy should be more than 3 days based on weather conditions of the particular area.
2.2 Factors Affecting Battery Sizing
a. The specified autonomy and maximum allowable depth of discharge (DOD) defines the total amount of battery capacity required for a given system load.
b. Greater autonomy periods increase the size of the battery and increase availability and decrease average daily depth-of- discharge.
c. Greater allowable DOD provides greater system availability, but at the expense of battery health.
d. Rated battery capacity is affected by temperature, discharge rate and age of the battery.
Sizing the battery bank for the worst case is not only important for ensure that the PV system can cover the loads of the building under all conditions, but also because to increase the chances of minimizing the seasonal battery depth of discharge. In addition, you should also consider your usage pattern and the criticality of your application.
Note -1 (Refer Item B4):
Once the required number of amp-hours has been determined (B3), batteries or battery cells can be selected using manufacturers’ information. Figure below shows the extract of industrial grade battery for different day rates. Since battery capacity may vary with the rate of discharge, the amp-hour capacity that corresponds to the required days of storage should be used.
2.3 PV Sizing
Solar array size is determined by the following parameters:
a. The PV array for stand-alone systems is sized to meet the average daily load during the critical design month.
b. Solar insolation received in the site
c. System losses, soiling and higher operating temperatures are factored in estimating array output.
d. Characteristics of the PV modules
e. The system voltage determines the number of series-connected modules required per source circuit.
f. The system power and energy requirements determine the total number of parallel source circuits required.
The array is sized to meet the average daily load requirements for the month or season of the year with the lowest ratio daily insolation to the daily load. Using module power output and daily insolation (in peak sun hours), the energy (watt- hours or amp-hours) delivered by a photovoltaic module for an average day can be determined. Then, knowing the requirements of the load and the output of a single module, the array can be sized. Higher system availability can be achieved by increasing the size of the PV array and/or battery.
Note: The design method for the PV array often uses current (amperes) instead of power (watts) to describe the load requirement because it is easier to make a meaningful comparison of PV module performance. For example, it is far convenient to compare performance, physical size and cost when specifying PV modules that will produce 30 amperes at 12 volts specified operating temperature rather than try to compare 50-watt modules that may have different operating points.
2.3 Selecting an Inverter
Inverter is required to convert direct current to alternating current. Stand-alone inverters are typically voltage-specific, i.e. the inverter must have the same nominal voltage as your battery. The inverter is rated in Watts. The input rating of the inverter should never be lower than the total watt of the appliances i.e.
Inverter Capacity > A12 Watts
Important: The size of an inverter for standalone system is measured by its maximum continuous output in watts and this rating must be larger than the total wattage of all the connected AC loads. Also, electrical appliances such as washing machines, dries, refrigerators, etc. use electric motors, which require more power to start. This high starting power consumption can be more than twice the normal power consumption so the input rating of the inverter should be ideally 25-30% bigger than the rated wattage of your appliances.
2.4 Sizing the Controller
The function of a charge controller is to regulate the charge going into your batteries bank from your solar panel array and prevent overcharging and reverse current flow at the night. Most used charge controllers are Pulse width modulation (PWM) or Maximum power point tracking (MPPT).
The voltage at which PV module can produce maximum power is called maximum power point (or peak power voltage). Maximum power varies with solar radiation, ambient temperature and solar cell temperature. Typical PV module produces power with maximum power voltage of around 17V when measured at a cell temperature of 25°C, it can drop to around 15V on a very hot day and it can also rise to 18V on a very cold day. When a MPPT solar charge controller notices variations in current-voltage characteristics of solar cell, it will automatically and efficiently correct the voltage. it forces PV module to operate at voltage close to maximum power point to draw maximum available power.
MPPT solar charge controller allows users to use PV module with a higher voltage output than operating voltage of battery system. For example, if PV module has to be placed far away from charge controller and battery, its wire size must be very large to reduce voltage drop. With a MPPT solar charge controller, users can wire PV module for 24 or 48 V (depending on charge controller and PV modules) and bring power into 12 or 24 V battery system. This means it reduces the wire size needed while retaining full output of PV module.
The charge controller current input rating is equal to the product of the short circuit current of the PV module, number of PV modules in parallel, and safety factor, where safety factor 1.25. I Rated = (N PV-parallel x I sc) x 1.25
Where:
• I Rated = Solar charge controller rating
• I sc =Short circuit current
• N B-parallel = Number of PV modules in parallel
• 1.25 is safety factor
2.5 Cable Sizing
The purpose of this step is to estimate the size and the type of wire in the following loops:
a. Cable between PV modules and Batteries
b. Cable between the Battery Bank and the Inverter
c. Cable between the Inverter and Load
The equation below can be used to determine the cross section of copper wire.
\[A = \frac{{p*L*I*2}}{{{V_d}}}\]
Where:
• p=resistivity of wire —– [For copper p=1.724 x 10-8 Ω ⋅m]
• L = length of wire (in m)
• A = cross sectional area of cable in mm2
• I =the rated current of regulator, amps
• Vd =Voltage drop, volts
In both AC and DC wiring, the voltage drop is taken not to exceed 4
\[{v_d} = V*4% \]
The voltage V is typically,
a. Cable between PV modules and Batteries = 12V, 24 V or 48V
b. Cable between the Battery Bank and the Inverter = 12V, 24V or 48V
c. Cable between the Inverter and Load = 110 V