# DC to AC inverter calculations [duplicate]

I have purchased a 300W Modified Sine Wave Inverter. I am also connecting a 12V 18A/hr SLA battery to it.

I am using it to power an RFID reader. The input power characteristics of the power block are: 100-250V, 50-60Hz, 0.5A.

The output characteristics are: +48V DC, 0.4A. Please note the power block connects to the RFID reader via an ethernet cable only. Hence I cannot just connect a DC supply directly to the RFID reader.

How would I calculate how long the battery will last before I need to charge it, do I use the input or output characteristics of the power block? (Is there an equation I could also use).

Also, I would also like to connect my laptop charger to the inverter while the inverter is powering the RFID reader. The input characteristics for the laptop charger is: 100-240V, 50-60Hz, 1.2A

Output characteristics are for the laptop charger are 19V DC, 2.73A.

Will it be okay to connect the laptop charger and the RFID reader at the same time, or will this exceed the limitations of the battery and the inverter?

I know how to calculate the duration of a battery given a DC to DC setup. But know I am going from DC to AC to DC again. Do I use the input characteristics of the power supply blocks i.e. the currents or the output characteristics of the power blocks?

• this is totally overkill. Your RFID reader's "block" is DC supply with a voltage that's probably lower than 12V. A simple buck converter would have powered it directly from the battery with a fraction of the cost and effort of your inverter. (and a way, way higher efficiency) Dec 19, 2019 at 21:09
• Other than that, this question has really been asked a hundred times here. Dec 19, 2019 at 21:10
• and that's really just one of very many questions. Dec 19, 2019 at 21:10
• @MarcusMüller firstly a simple buck converter will not work due to the RFID reader setup. It can only be powered from the block which only accepts an AC source. Indeed I wished it could have been powered from a buck converter. Secondly no it does not answer my question. Dec 19, 2019 at 21:13
• This is no answer, and I'm not trying to belittle you. Power is power. It's clear where it has to come from, and it's clear how much you need of that and where, so I really don't understand what you're asking here, sorry. Dec 21, 2019 at 15:55