Outback FX-R Series Technical Note on Input Modes

Application Note

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To size the inverter, we use the total watts column in Table 1. This is the maximum power the

inverter would need to deliver to the loads if they were all running at the same time. The

ambient temperature affects inverter conversion efficiency at 1% per degree C above 25°C. If

the worst-case temperature is 40°C, then the inverter needs to be derated by 15%. Some loads

also have power factor (see OutBack Power Factor Applications Note for more information) with

a common “rule of thumb” derate of at least 10% for power factor (PF). While many homes may

have a 90% PF or better, PF is not an easy thing to measure. Using 80% PF (20% derate)

could be a more conservative approach, especially if there are a lot of motor loads and high

energy electronic loads. So if the derate of 15% for temperature and 10% for PF is combined,

that gives a total derate of 25%. To get the minimum inverter size for this example, divide the

1700W from Table 1 by 0.75 (25% derate) which means a 2266VA inverter is the minimum size

for this application.

There are two Radian/FXR input modes for backup applications; Backup and UPS. The UPS

input mode is essentially the same as the Backup input mode, except that the power modules

never go into standby so they can be switched on faster with the transfer from utility power to

battery power.

OFFSET FUNCTION

An important thing to note about Backup and UPS modes is that they only become active with

the loss of grid input. Any renewable DC energy will go unused other than that which is used to

keep the batteries charged. The other input modes discussed below use some form of a built-in

“Offset” function which allows renewable or stored energy from the DC side to be used to power

loads or send energy to the power grid. With the Offset function, renewable energy will almost

always be fully utilized and not go to waste. However, all input modes will provide backup power

with the loss of an AC source whether that input mode uses the Offset function or not.

The main concept behind Offset is that it

blends renewable, or stored energy from the

DC side with power from the grid. The trigger

points at which the blending occurs is mostly

dependent on user settings, and by default, the

Offset itself is always enabled with the input

modes; Grid Tied, Mini Grid and Support.

However, Offset can be disabled by changing

the inverter Grid Tied mode (not the Input

Mode) to Disable. Firmware revisions after

version 001.005.000 change the setting name

from Grid Tied to Offset Enable to avoid

confusion with the Grid Tied Input mode which

is a different setting.

Figure 1 shows an example of how the Offset

function is inverting 20A from the DC side

Figure1 ‐ OffsetCurrentBlendingFunction