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.
If you have a 40A MPPT controller with a 24V system then the “nominal max output power” would be ~ 1040 Watts but you can size your PV array with more power than this. For example, you could use four 320 Watt panels (2 strings of 2) and have a PV array power = 1280 Watts.
Most of the time the max power output of the array will not operate with 100% power but it will depend on the location and tilt of the array. For an RV the tilt is likely to be horizontal and will not receive direct sunlight so the max power of the array might not ever be able to put out more than 80% of the rated power of the panels.
Hi. Will you dumb this down a little more with laymen vocabulary? Are you saying:
“If you calculate (example) for a 40 amp MPPT, using an 80 amp MPPT may be beneficial.”
Is that the observation? Or am I over simplifying or thinking in the wrong direction? I’m in the early planning & budget phase of a 24v system on a converted bus (hopefully 8ea x 175watts = 1400watts). And I’m finding sizing tutorials a bit confusing.
Many thanks!
That is not what is being said.
The idea of an oversized array with an MPPT controller is that for most of the day you won’t be producing peak power. If you have a 40A solar controller and it is charging a 12V battery, it will require approximately 600W of solar input to charge at its maximum output. The problem is, if you install 600W of solar on the roof of your bus, then you will only be producing 40A for a very small window during the middle of the day (or you might never hit 40A if the weather is bad). If you are using a PWM controller, then you just have to live with the fact that it will be producing less than max output for most of the day. If you are using a MPPT controller, you can safely install extra panels in order to get a bit more output from the controller.
By installing a 720W array (20% oversize is good target to aim for), you can spend a larger portion of your day outputting 40A. It is likely some power will be wasted (anything more than 600W), but this way you can generate more power over the course of the entire day without needing to purchase and install an additional solar controller. It would obviously be better to just install a larger solar controller, but MPPT controllers are usually expensive and this method will get you more bang for your buck.