Install tip: Oversize MPPT controllers in off-grid systems for more ROI

This installation tip was provided by Douglas Grubbs, applications engineer at Morningstar

Some MPPT controllers can be sized well above the maximum operating output power rating without damaging the controller or having the charging current exceed the maximum output current rating of the controller. This means that array oversizing with off-grid systems can result in a positive return on investment, similar to the results commonly achieved with grid-tied inverter systems.

MPPT controller maximum nominal operating power depends on the rated charge current and the nominal system voltage. During the middle of a sunny day, there may be more power than the controller can use with an oversized array. However, an oversized array will often contribute more power than a smaller array in the early morning and late afternoon, or even during the middle of a cloudy day. Excessive oversizing of the array may show a diminishing return if the power limiting is more significant.

Some of the benefits of exceeding the nominal wattage ratings of an MPPT controller include:

• Daily maximum power levels can be much lower than the STC rated power of the solar array. Tilt, azimuth angle, time of day/year, weather, climate, dust and other factors reduce output power from the array leaving unused controller capacity
• Full charging potential of the MPPT controller can be used more often
• Better production early and late in the day
• Better production during cloudy weather
• On sunny days, excess power is not needed once the batteries are close to full
• Reduced maximum charge current for smaller battery banks
• More string-sizing options with higher power arrays

In addition to STC power, array voltage needs to be considered. The minimum Vmp should stay above the battery maximum Vb for consistent charging to occur, and the array Voc should never exceed the maximum voltage ratings of the controller. A string sizing calculator can provide minimum and maximum voltage levels for an array based on record low temperature and maximum average high temperature.

Operation at full power with sustained high temperatures may cause unnecessary stress on the controller’s electronic components. If oversizing is considered in hot climates, the TriStar MPPT controller can be programmed with a reduced maximum battery current limit so the controller will not get overheated as often.