Managing PV Installations with a Gantt Chart
Inside this Article
A project schedule is an important tool for managing a solar installation. A schedule allows the project manager to schedule deliveries, crews and subcontractors; to track progress and costs on a project; and to anticipate and prevent delays. It is much more than a mere list of activities with start dates on a calendar.
The most common format for a project schedule in the construction industry is the Gantt chart, or bar chart, named for its creator, mechanical engineer and management consultant Henry Gantt. This schedule in graphical form helps project managers and crews visualize timelines and interconnectedness for a project’s scopes and corresponding tasks. You can create it manually or with a computer program, but in either case you do so based on project-specific information.
Before creating a Gantt chart, you must develop the supporting information in the form of a work breakdown structure (WBS) and a network diagram. In this article, I use the terms schedule and Gantt chart interchangeably. I review the steps to develop a Gantt chart and describe how to use it effectively to manage PV installations.
Developing a Gantt Chart
Ultimately, a Gantt chart is only as good as the information you use to develop it. A WBS organizes and defines the scope of the project using a hierarchical structure, similar to an outline or an information tree with multiple levels. The network diagram identifies the relationships among the activities.
Step one: Establish a WBS. The top level of the WBS is the project, which represents the entire scope of work and all deliverables. The project consists of smaller, distinct scopes of work called work packages. The project manager often divides work packages according to responsibility, with the entire package assigned to an individual, department or subcontractor. Each package contains the activities—and sometimes smaller internal work packages—required to complete all work for that package. A work package is considered finished once the team has completed all of the activities and work packages inside it. You should assign a descriptive noun to each work package. Figure 1 shows the location by level for the three major components that make up a WBS.
Activities are the individual tasks the project team performs. Each activity has associated time and cost estimates. In Figure 2, graphical bars depict activities in the schedule, and thinner graphical bars with triangles at each end represent work packages. You should describe each activity with a verb, since activities consist of actions.
You can divide a work package into successively smaller work packages. The process of subdivision must follow the 100% rule, which dictates that the subitems of a work package must add up to 100% of the work package. You should define and organize activities so they do not cross over to other work packages. If an activity does not fit into one of the work packages, then you need to add another work package or redefine the existing ones to ensure that they capture all aspects of the project. The 100% rule ensures that nothing slips through the cracks and that you account for all time and resources when planning the project.
Refine the work packages and activities so you can practically track and manage each level. The structure varies depending on the project and the company doing the installation. Smaller, more refined work packages give the project team more control and visibility, but at the cost of greater overhead. In theory, you could refine work packages all the way down to individual PV modules, with detailed activities such as installing a bolt, tightening a nut and so on. However, you would then need to estimate, track and record the data for each of these activities. Trying to manage at this granular level would put too great a burden on the project team and would drive the installers crazy. With practice, you will find the sweet spot that offers the right level of management and control for your company.
Figure 3 shows the WBS for a 100 kW rooftop PV installation. The first level is the overall project; the second level divides the project into three work packages (project management, structural installation and electrical installation); and the third level shows the activities in each package.
Project managers may be tempted to forego the WBS and just define the project activities in a simple list. This approach limits the effectiveness of the schedule and does not allow you to use the planning and control methods described here.
Step two: Create a network diagram. The network diagram depicts the relationships among the activities. Relationships, or dependencies, exist when certain activities cannot start or finish until you start or complete other activities. Those earlier activities are called predecessors. For example, “Install ac system” is a predecessor to “Commission and inspect.”
A network diagram graphically maps the order and relationships of a project’s activities (see Figure 4). The relationships and sequence of activities vary from project to project, based on the type of system you are installing and the construction means and methods. Usually you need to perform activities in sequential order. Four types of relationships define the order:
Finish to start: An activity starts after its predecessor activity has finished.
Start to start: An activity starts after its predecessor activity has started.
Finish to finish: An activity finishes after its predecessor activity has finished.
Start to finish: An activity finishes after its predecessor activity has started (rarely used).
You can add a lag time or a lead time to any of the above relationships. A lag time creates a delay between the predecessor and successor activities in a relationship. For instance, if you need to install an inverter on a concrete pad, you will need several days of lag time after finishing the concrete pad to allow it to set before you can start installing the inverter. A lead time allows sequential activities to overlap. For instance, you must perform the activity “Install roof attachments” before you can start the activity “Assemble racking.” If you cannot wait to start assembling the racking on the sections of roof with attachments already installed, you can get a 1-day head start (lead) on the “Assemble racking” activity by using a finish-to-start relationship with a 1-day lead time. This signals the installation team to start installing racking one day before finishing all the roof attachments.
Lag and lead times are opposites. A positive two-day lag time is the same as a negative two-day lead time. Many software programs accept only a lag time input. If you need a lead time, you must enter it as a negative lag time. For the sake of clarity in our example, all of the relationships in the network diagram in Figure 4 are finish to start, with no lead or lag time.
Finally, you determine the duration of each activity. The duration is the number of calendar days or hours it takes to complete the activity, not the actual number of person-hours or person-days worked. Each activity box in the Figure 4 network diagram shows a duration. You may base the duration of the activity on historical data, industry rules of thumb or an educated guess. With experience, your duration estimates will become more accurate. The crew size affects the duration, so adding more crewmembers to the installation team or working overtime can shorten it.
Step three: Develop the Gantt chart. The WBS and network diagram contain the information you need to create a Gantt chart. With these two steps completed, you are ready to bring them together and create the schedule. If you wish to use all the features of the schedule, doing it by hand is not a practical option. Many computer programs allow you to easily accomplish in minutes what takes hours by hand. Microsoft Project and Oracle’s Primavera are two programs commonly used in the construction industry, but many others are available, and you can find one to fit your budget and needs. Assuming you use a program to develop the Gantt chart, the process is similar with most scheduling software.
Start by entering the WBS into the program, maintaining the project, work packages and activities hierarchy. Next, enter activity information from the network diagram, including durations and relationships. Work packages inherit the start and finish dates of the activities they contain.
Add milestones to the schedule as necessary. A milestone is a significant event in a project, such as the completion of a major deliverable. It is common to include contracted milestones, such as substantial completion and final completion. Milestones are similar to activities, but have zero duration and no associated resources. A milestone is an informational item only that helps the project manager understand what activities and work packages the team must complete to reach that milestone.
Once you have completed the schedule, it shows the start and finish dates for each activity and work package, as well as for the overall project.
Using the Gantt Chart
Now you can start putting the schedule to work as part of your project management plan. You can use the dates on the schedule to accurately schedule equipment deliveries, crews and subcontractors.
Updating the schedule. Since large projects rarely go exactly as planned, you should expect that you will complete some activities ahead of schedule while others fall behind. You therefore will need to update the schedule periodically to account for actual progress. If you change or update one date or duration, all the other activity dates automatically adjust according to the relationships you have defined. With just a few clicks, you can see how a delay in one activity affects the start and finish dates of other activities and the overall project, and you can then adjust the timeline accordingly.
Following the critical path. When managing your project, you must focus on the schedule’s critical path, the series of activities that determines the finish date. A delay of one day on a critical path activity results in a one-day delay in finishing the project. For example, “Install PV modules” is a critical path activity for PV installations. If rain delays the module installation two days, that has a two-day impact on the project’s projected completion date.
Some activities not on the critical path have float or slack time, which is the number of days you can delay a noncritical path activity before it becomes part of the critical path. For example, if an activity has 5 days of float, it can take 5 days longer than scheduled without affecting the overall project. If the activity is delayed past 5 days, it becomes part of the critical path, and additional delays push back the project completion date accordingly.
Since the critical path determines the project’s finish date, the project manager must understand which activities are on—or may become part of—the critical path and ensure that they finish on time. As the project progresses and activities finish early or late, the critical path can change, which is why updating the schedule is important.
Crashing the schedule. Just as a delay on the critical path can cause a delay in the schedule, if you complete the critical path activities a day early, the project can wrap up a day early. Project managers can use this to their advantage and crash the schedule, applying additional resources to an activity to complete it faster. You can assign more crewmembers or have the current crew work overtime to finish it earlier.
However, crashing is not usually the most cost-effective way to manage a project. Doubling the number of people on the roof probably will not cut the activity’s duration in half because crewmembers will be bumping into each other, which slows down productivity. In addition, overtime pay rates increase the project’s direct costs. That said, if the contract has a liquidated damages clause, the cost of crashing may be less than the cost of paying penalties for project delays.
If your project is behind schedule and you decide that the additional cost of crashing is warranted, it is essential to know the critical path and understand which activities to focus on. Only crashing the critical path activities will get you caught up. If you spend a weekend working on noncritical path activities, you increase the cost of the project but do not catch up to the schedule.
Earned value analysis. Scheduling software can take your schedule to the next level by analyzing project performance using metrics such as earned value analysis (EVA). Enter the actual project time and expenses into the scheduling software to compare planned to actual progress. You can determine whether you are falling behind schedule or going over budget. If an activity or work package is only 25% complete, but you have already used 75% of its allotted resources and funds, an EVA will show you that the project is going over budget and allow you to take corrective actions.
The Gantt chart is an essential tool for any solar installer or project manager. The schedule allows you to plan your crew assignments, schedule accurate material delivery dates, track actual against planned progress, give advance notice when a project is at risk of going over budget or falling behind schedule, and identify where you should focus your resources to correct issues or delays. At the end of each project, you can compare the actual schedule to your estimated schedule, and use that information to plan and estimate your next project more accurately.
Richard Ivins / Pure Power Systems / Hoboken, NJ / pure-ps.com