Sounds about right price-wise...
For an 8-hour work-day you'd have:
3 hours 'on' time: 14*3 = 42 kWh, 5 hours 'idle' time: 0.4*5 = 2 kWh, Total: 44 kWh/day
5 days a week: 44*5 = 220 kWh/week. At $0.13 that's $28.60 a week so your number for a monthly energy cost is not far off.
The idling power use is costing you money but in the scope of things it's still way smaller than the 'on' time usage. (about $1.30 a week)
An inverter machine would be more efficent on the in-use power use and may make more of a difference on the cost as that's where your main draw/cost is.
AC input power is a fickle beast
and depending on how you load an AC circuit you can get much better efficiency from the delivered power measured by your electric company than with a plain transformer.
To be able to weld at 300A you still have to feed any machine a sizable input current but if you look at inverters that are in that welding range you'd probably be looking at 8-10kVA input power which is quite a bit less than a transformer and would cut your electricity bill down.
Of course just buying an inverter just for the reduction in energy consumption is neat but probably not very economical unless the old machine is slated for replacement anyway or you need/want the features that inverters can offer. The cost saving in energy versus the price of a new machine means you'd take quite some time to break even on that alone. (if it allows you to do other/different work that pay mosre it may be more interesting..)
Bye, Arno.
