Simplified minimum PV source size

The size of the PV source, which is determined based upon the simplified load evaluation and simplified weather and solar resource evaluation will determine the necessary size of the charge controller. The charge controller must be selected at the same time as the PV source as the charge controller type - PWM or MPPT - will also determine the possible configurations of PV modules.

In this phase of the design process, more than in any other phase, it is necessary to explore different designs using PV module, series and parallel wiring configurations, and charge controllers in order to achieve the highest performance at the lowest cost possible. This phase may have to be performed several times.

An off-grid PV system that depends upon the PV as its single charging source requires an array that is sufficiently sized to be able to generate sufficient energy to both meet the energy needs of the users and to recharge the energy storage system under less-than-ideal conditions. Any sizing decisions should therefore lean towards an oversized PV source.

Assumptions:

• The system is designed for a non-critical application, meaning that users can/will adjust their energy needs according to the weather.
• The PV source will have little to no shading from trees or mountains.
• The PV source will be cleaned at least every two months or as often as it becomes heavily soiled.
• A lead acid battery will be used.
• See Notes/references for more details.

Step 1: Determine minimum PV source size

A value should be chosen by using the Total daily Wh required and the Minimum monthly solar resource.

Step 2: Adjust minimum PV source size for maximum temperature

Temperature	Temperature adustement factor
25°C	1.00
30°C	.97
35°C	.94
40°C	.91
45°C	.88

Temperature adjusted minimum PV source size	= Minimum PV source size (Step 1) ÷ Temperature adjustement factor

Step 3: Determine charge controller type

There are two different charge controller types - pulse width modulation (PWM) and maximum power point tracking (MPPT) - each of which has advantages and disadvantages that are are detailed in Charge controller types. Two separate designs may be performed with each type of charge controller to determine the best system design. The current and voltage rating of the charge controller will be determined when in either Simplified PWM charge controller sizing and selection or Simplified MPPT charge controller sizing and selection.

PWM:

The PV source must be configured to operate at the charging voltage of the energy storage system and below the maximum PV source current rating of the charge controller.

• Nominal system voltage: 12V, 24V, 48V.
• Maximum PV source current: 6A-60A

MPPT:

The PV source must be configured to operate below the maximum PV source voltage rating of the charge controller, above the minimum PV source voltage based upon the maximum charging , and below the maximum PV source current rating of the charge controller.

• Nominal system voltage: 12V, 24V, 48V
• Maximum PV source voltage: varies up to 600V
• Minimum PV source voltage: depends upon nominal voltage and charge controller type
• Maximum PV source current: up to 100A+

Notes/references

• Location max temp = 40°C
• Mounting system temperature adder = 25°C
• Temperature power loss factor = -0.48%/°C
• Module degradation parameter = 0.9
• Shading loss parameter = 0.95
• Soiling loss parameter = 0.95
• Wiring loss parameter = 0.96
• Module mismatch parameter = 0.98
• Energy storage efficiency parameter = 0.75
• Charge controller efficiency parameter = 0.98
• Final total loss factor = 0.5