*Product: HOMER Legacy and HOMER 2*

**Do the number of starts affect the modeled generator life in HOMER and is that a variable that I can adjust or minimize?**

No, the number of starts does not affect the generator life in HOMER. We assume that the generator will operate for a certain number of hours before it requires replacement, regardless of whether it operates continually or it turns on and off frequently.

We made this decision (to ignore the number of starts when calculating the generator life) several years ago when HOMER only allowed hourly time steps. Another researcher at NREL had determined that to start a diesel generator caused something like 5 minutes worth of wear on that generator, so we decided that we could ignore that since the generator would run for at least one hour at a time. Now that we allow shorter time steps, we may need to revisit this simplification.

**Is there any other way to minimize the number of starts on a HOMER modeled generator? I want to somehow model the battery taking over from the PV generator for periodic changes in the solar resource, while minimizing the wear on the generators.**

You can use the setpoint state of charge to minimize the number of generator starts. In the attached file, I have done a sensitivity analysis on the setpoint state of charge to show its effect. If you plot the number of diesel starts versus the setpoint state of charge on the Sensitivity Outputs tab, you will see the following graph:

Time series graph show the effect of the setpoint fairly well. Here is a two-week segment of the simulation results with a setpoint of 50%:

Here is the same two-week segment with the setpoint raised to 85%:

That shows that the higher setpoint forces the generator to run longer because it can’t stop charging the battery bank until it reaches the setpoint state of charge. As a result, the battery can handle the load for a much longer period, so the generator can stay off for days at a time.

Unless you force it to do so by specifying a high setpoint state of charge, HOMER doesn’t operate the generator in this way because it sees no economic benefit to doing so. In real life there is an economic benefit to doing so, since it reduces generator starts and prevents the battery from spending long periods at low states of charge, but HOMER does not consider those effects. By default, we set the default state of charge to 80%.