Creating Lua functions in the Formulas script editor will allow the functions to be used in a formula register.
For example, here are two functions made in the Lua Formulas script editor that will return the min or max of two numbers:

-- function returning the larger of two numbers
function lua_max(num1, num2)
 if (num1 > num2) then
    result = num1;
 else
    result = num2;
 end
 return result;
end

--function returning the lesser of two numbers
function lua_min(num1, num2)
 if (num1 < num2) then
    result = num1;
 else
    result = num2;
 end
 return result;
end

They may then be used in a formula register:

And we may see the registers work as defined by the script functions:

Advanced time-of-use and tiered billing may be performed in this Lua script.

A tariff script should provide (at least) a cost(register, negate, schedule) function which calculates the incremental cost based on the energy-use recorded by register. negate can be set to true if the register's value counts down for power consumption. Schedule is optional and can be set to the (non-default) name of the schedule to use when calculating the cost.

A formula register of type "monetary" would be used with the cost() function.

For an example billing script, from the classic interface:

1. Navigate to Settings → Billing.
2. Choose Xcel Colorado as the tariff provider.
3. Click "OK" to save and go back to the main settings page.
4. Go back to Settings → Billing.
5. Change the tariff provider to “custom”
6. Click the "Customize tariff script" button, which will open a copy of the Xcel Colorado billing tariff in the Lua script editor.

Alerts scripts work the same as Formula scripts, but are used in eGauge meter alerts, configured in Setup → Other settings then Alerts at the left.

Control scripts can be used to control supported equipment such as the eGauge Power Relay Module (PRM3).

For example, the following script reads the instantaneous value of a register called "Temperature" and controls a PRM3 relay contact. If the temperature is lower than 21 C, relay number 0 of the PRM3 is closed (activated), otherwise opens (turns off) relay number 0. It then sleeps for 15 minutes before checking again.

In the real world, the control script should be more advanced

dev = ctrl:dev({interface='relay'})
  relay = dev:interface('relay')

  while true do
    print("Temperature is currently: " .. __r("Temperature"))
    if __r("Temperature") < 21 then
        relay:close(0)
    else
        relay:open(0)
    end
    sleep(60*15)
  end

See the main Lua Scripting Overview section on Persistent variables.

Persistent variables are variables that are preserved between reboots or power cycles.

The following Formula script creates and updates a persistent variables with a given name and number passed to it in a formula register, and prints debug to the output log:

function persistent_variable_example(name, number)
  
     obj = persistent:new(name, number, "Variable stored by formula function persistent_variable_example")
  
     current_value = obj:get()
     print(name .. " currently has value " .. current_value)
  
     print("updating " .. name .. "to new value " .. number)
  
     obj:set(number)
  end

A register is configured to run the formula script:

This creates or updates a persistent variable called variable test 1 with the current time.

The Formula script editor shows the print debug as the variable is updated once a second as the formula register is run:

15:26:28.168 variable test 1 currently has value 22.043333333333
15:26:28.168 updating variable test 1 to new value 22.043611111111
15:26:29.171 variable test 1 currently has value 22.043611111111
15:26:29.172 updating variable test 1 to new value 22.043888888889
15:26:30.168 variable test 1 currently has value 22.043888888889
15:26:30.168 updating variable test 1 to new value 22.044166666667
15:26:31.168 variable test 1 currently has value 22.044166666667
15:26:31.168 updating variable test 1 to new value 22.044444444444

Although this specific example may seem trivial, persistent variables can be utilized in any Lua script. Control scripts run continuously, so there’s no need to create a formula register to execute them.