# rulesDSL maths

**URL:** <https://community.openhab.org/t/rulesdsl-maths/133685>\
**Category:** Beginners\
**Created:** [February 25, 2022, 7:16am UTC](https://community.openhab.org/t/rulesdsl-maths/133685 "2022-02-25T07:16:11Z")\
**Posts on this page:** 1\
**Showing post:** 5

<div class="post-metadata">

**Author:** ![jimtng](https://community.openhab.org/user_avatar/community.openhab.org/jimtng/32/98543_2.png) [@jimtng](https://community.openhab.org/u/jimtng)\
**Post date:** [February 25, 2022, 11:09am UTC](https://community.openhab.org/t/rulesdsl-maths/133685/5 "2022-02-25T11:09:32Z")

</div>

I am so glad I no longer need to do such type gymnastics. In [jruby](https://community.openhab.org/t/jruby-openhab-rules-system/110598):

```auto
rule "solaredge.selfconsumption" do
  changed SolarEdge_Live_Consumption
  run do
    self_consumption = [SolarEdge_Live_Production - SolarEdge_Live_Consumption, 0].max | 'kW'   
    SolarEdge_Live_Self_Consumption.update(self_consumption) 

    logger.info("self consumption: #{self_consumption}")
  end
end

```

Basically you just use the item name. It is automatically inferred that they’re a numeric type and you can perform arithmetic as if they’re a number, but you can also treat them like a first class openhab items, with so many capabilities, such as:

- `SolarEdge_Live_Self_Consumption.meta['namespace']` to access its metadata,
- `SolarEdge_Live_Self_Consumption.persist` to tell it to persist to the database,
- `SolarEdge_Live_Self_Consumption.average_since(24.hours, :influxdb)` to get the persistence average data,
- `SolarEdge_Live_Self_Consumption.update '12kW'` to update it with a string with UoM, and many more.

Note if your Production is less than Consumption, your code will give you a negative self consumption. I’m not sure that’s what you wanted? My code above will set the self consumption to zero in this case.

A bit of note if you’re not familiar with ruby:  
`self_consumption.zero?` is just a preferred way of writing `self_consumption == 0` or `self_consumption == '0 W'`. You can use any of the 3 variations.

`[x, y, z]` is an array, in this case `[SolarEdge_Live_Production - SolarEdge_Live_Consumption, 0]` is an array containing two elements: the calculation result and zero. Then you call `[array].max` to get the max of the elements. It’s the same as the traditional way of doing max(x, y) in other languages.

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_[View the full topic](https://community.openhab.org/t/rulesdsl-maths/133685)._