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====== DIFF() Function ======

The DIFF() function can be used in a variety of ways to get the difference (end-start) between a value at two timestamps.  This is typically used for incrementing counters such as kWh or total liters.

Usage:

''DIFF(variable, options, [days],[minutes],[min],[max])'' - see below for description.

Parameters:

variable - Variable name to use

options - Type of diff (see below)

days - optional number of days to lookup  before selected day

mins - optional number of minutes to offset by

min - minimum value, will result in 0 if it is below this value

max - maximum value, will result in 0 if it is above this value


===== Generic Variables (per device type) =====

''DIFF('INC1_AE', 5)'' 

Get the difference for the selected period

===== Device - Specific Variables =====

For device specific variables use a . to separate the device name:

''VALUE('My Device.My Value',1)''

Get the difference between current value and start of today for the variable 'My Value' in the device 'My Device'

===== Option Parameters =====

The second parameter, [options] is a number and can be any of the following

|0 | latest value (Default) |
|1 | DAY - Difference between start of day and latest value |
|2 | WEEK - Difference between start of week and latest value |
|3 | MONTH - Difference between start of month and latest value |
|4 | YEAR - Difference between start of year and latest value |
|5 | PERIOD - Difference between start and end of selected date range |
|6 | AT_START_DATE - value at the start of the selected date range |
|7 | AT_END_DATE - value at the end of the selected date range |
|8 | PERIOD_CUMULATIVE - value between start of period and range element (in a graph) |
|9 | YEAR - Difference between one year ago (365 days) and today |
The third parameter is an optional one and lets you specify a number days.  This is subtracted from the currently selected period to allow you to graph historic data alongside current data.

===== Example =====
Show a cumulative value for data within a period:

''(DIFF('delta0_kwh',8)''

The graph for cumulative values since period start looks like this:

{{:wiki:diff_cumulative.png?500|}}

''(DIFF('delta0_kwh',5)''

The graph for period for the same variable looks like this:

{{:wiki:diff_period.png?500|}}

Get the solar generation for one year ago (365 days):

''(DIFF('inverter1_AC_Total_kWh',0,365)''
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