Electrical Question

[[10753]]

If I have a 240v 120w electrical appliance, that would draw 0.5 amps.

If I then use an inverter and run the same appliance on my 12v system does that mean it would draw 10 amps ?

 

[Jean]

Yes, nominally 10 amps from batteries, plus the losses from the inverter, and effects of the type of waveform produced by the inverter and load impedance characteristics, which may add a bit more.

 

[rafiki_]

If I have a 240v 120w electrical appliance, that would draw 0.5 amps.

If I then use an inverter and run the same appliance on my 12v system does that mean it would draw 10 amps ?

Yes ish, but you might need to factor in approx 10% additional due to sine wave angles being slightly different etc.

 

[ghostlymoron]

Whatever voltage you use, it still requires 120watts to power it (not including conversion losses).

 

[CreakyDecks]

It depends how efficient your inverter is at 120W. One designed to give out 200W would be more efficient at 120W than one designed to give out 2000W. The efficiencies might be 90% or 70%. At worst I think you would be drawing about 15A.

 

[noelex]

The stickers on 240v appliances are sometimes for the worse case situation at start up etc. So a 120W appliance will often only use 90-100w in real life. So you may be pleasantly surprised and find even with inverter ineffiency the average draw may be a bit under 10A

Others have given you good advice, but its worth remembering batteries get less efficient at a high current draw. This is not much a factor at 10A, but it can be a big factor for larger equipment. The battery inefficiency will not change the amp draw, but impacts how long the batteries will power larger appliances.

 

[savageseadog]

There's often a very high inrush or surge current on many appliances when switched on. Worst cases are devices with motors of an kind followed by transformers and power supplies with capacitors to charge. In practice a nominal 0.5A could have a 2-5A momentary surge, something that inverters don't like.

 

[Jean]

Good point savageseadog, one of the reasons perhaps for buying a quality softstart inverter, overrated for your application, ideally pure sine too as all those horrible harmonics just dissipate as heat (and perhaps noise) in motors and transformers.

 

[pagoda]

Yes, nominally 10 amps from batteries, plus the losses from the inverter, and effects of the type of waveform produced by the inverter and load impedance characteristics, which may add a bit more.

120W is not a huge amount for a mains device. If it is to power a laptop - you would be better off using a dedicated 12V DC > 18.5V DC converter (or whatever DC volts the laptop gets from it's own power supply)
They seem to be more efficient, and many come with an assortment of plugs & voltages to suit different laptops. We sometimes use one to play films on the laptop. The inverter was nominally capable of 400W, but struggled at start up on a regular basis.
The DC-DC converter has no difficulties we're aware of.
Curling tongs and the like -- can't answer for

 

[William_H]

If I have a 240v 120w electrical appliance, that would draw 0.5 amps.

If I then use an inverter and run the same appliance on my 12v system does that mean it would draw 10 amps ?

If you are just musing on electrics then yes you are correct however as said there are all sorts of horrible inefficiencies that could run to 40% more current.
You do not say what the appliance is. This makes a lot of difference. Heating elements are very efficient and will run on any waveform. Induction motors don't like anything but sine wave while universal motors as in electric drills (with brushes) are fairly tolerant. Electronic power supplies seem to be a mixed bag.
good luck olewill

 

[Heckler]

If I have a 240v 120w electrical appliance, that would draw 0.5 amps.

If I then use an inverter and run the same appliance on my 12v system does that mean it would draw 10 amps ?

The rule of thumb is, watts is watts, basically, power consumed. So the higher the voltage the less the amps, THE SMALLER THE WIRE NEEDS TO BE. Thats why power transmission lines run at 11kv or more.
So divide the wattage by the volts USED to get a rough idea.
Stu

 

[RichardS]

120W is not a huge amount for a mains device. If it is to power a laptop - you would be better off using a dedicated 12V DC > 18.5V DC converter (or whatever DC volts the laptop gets from it's own power supply)

Unfortunately some recent laptop models use a third contact on the input which detects whether the power supply is the "approved" type!

The manufacturers argument is that the third connector is used to ensure intelligent charging but I think this is rubbish. It's all down to after-sales.

There are sometimes ways around this but you then find that although the laptop will run directly from a universal power block, it will not charge the laptop battery so you have to keep it plugged all the time.

My youngest son's Samsung is OK but with the Dell and the HP the only sure way is to use the proper mains power supply and the inverter (or buy the manufacturer's 12v step-up converter!).

Richard