Having just bought a Halfords 12v Car Battery for £20 on offer, I am resurrecting my two DC to Mains Inverters, 12v to 240v, 150 watts, and 300 watt.

I have done a lot of Internet searching & reading.

I would like anyones PRACTICAL EXPERIENCE of using an inverter, MSW to power at my main socket.....

My SKY+HD Box

Another Freeview PVR

My 32" TV

Also supplied by this one socket, via surge suppressed mains extension 6 ways, are

Home Wireless Phone.

WiFi Router.

TP Link Whitebox.

Additionally, probably also one compact flourescent Energy saving light.

This is to be ready for any power cuts, although we don't get many, but it is also a little project.

Now to measured figures...

Everything except TV and CFL 38 watts 0.55 amps

Switch on 32" TV, now 108 watts 0.65 amps

Add in CFL on floorstand, now 128 watts 0.70 amps

Much less overall consumption than I had thought, with everything on, only 128 watts, so within 150 watts and 300 watts inverters capabilities.

So help please, especially perhaps from Caravanner's, is it OK to power my SKY+HD Box with a MSW (Modified Sine Wave) inverter ?

Same question, for my TV ?

Any noticeable loss of quality, picture disturbance etc.

SKY+HD Box would be used to view, record & Play

.TIA

There really shouldn't be any concerns driving anything that uses a switch mode PSU, as you can even feed them from DC - but really the only way to know is to try, and with the exact unit you're wanting to use.

As you've already got the inverters, try them and see.

I have tried both inverters with a secondary system, of 19" TV, and CFL.

No problems.

Both inverters coped well. Neither got warm.

The 150w has no fan, & is quiet, the 300w one has a small fan, but un-noticeable once the TV is on.

Before I plug in my SKY+HD Box, I am a little wary, so if anyone has done this, please let me know.

Also, I have an iMac, anyone powered this or a PC from 12v via an inverter ?

The main problem is how long the battery will hold out. 128 watt load will take10 amps from the battery. A typical car battery will be 40 amp hour at the 20 hour rate, ie when drawing 2 amps. At 10 amps you won't get 4 hours as you might expect but maybe 1 1/2 hours. These inverters switch off once the battery voltage falls to around 10.5 volts.

Then there is the problem of keeping the battery charged and in good condition until your next power cut in maybe 10 years time.

Last time I had a power cut I didn't bother with TV, instead took the missis down the pub where the power was still on.

Note: Watts measured is at 240v.

In my view, more relevant is 0.65 amps.

For a 40AH battery, that equates to 60 hours !!

We shall see, I will do a RT test once my 24 hours measurement cycle is over.


Power cuts only average two per year, but several hours each.

Main concerns are TV recordings, and using my iMac.

And the 12v battery can also be used for NiMH battery & mobile phone charging, I've done tests using various devices, all can be charged from the 12v battery, with adaptors I already have.

Winston is correct - more amps at the low voltage end. If the converter is 100% efficient (doubtful) 120 watts output means 10 amps at 12 V or 0.5 amps at 240V. 0.5 amps at 12V is 6 watts. Sorry dude, but it's physics.

That is 0.65 A at 240V. The battery is 12V so the current drain on that will be 20 times higher as the voltage is 1/20th that of the mains.

Volts x Current = Watts.

If you take 120W out of the inverter at mains voltage then you have to put at least 120W at battery voltage into the inverter. In fact the inverter won't be 100% efficient so the drain on the battery will be higher.

So the mains current may be 0.65 but the battery current will be 13A. So your 40AHr battery will last rather less than 4hrs. Maybe not even close to that.

I'd also be interested in how the OP measured AC watts and amps. I know the old school AVO did AC current but they are expensive and a bit rare. I'm guessing he got the info off the back of the devices . . .

You can buy cheap and simple units that measure consumption accurately - I've got one I bought from Maplin - handy little tool.

Avo's wouldn't be any good for doing the measurement because they aren't true RMS which is essential on anything modern and switch-mode.

I do indeed use the Maplin device, it's stood me in good stead for many years now.




http://www.maplin.co.uk/plug-in-main...-monitor-38343

Pretty well all meters will measure ac current. Measuring power in an ac circuit requires knowing the phase angle difference between the current and voltage. Power in an ac circuit is Current x Voltage x Cosine of Angle between voltage and current.

Only when the current and voltage is in phase (Cosine Zero degrees is 1) is power current x voltage. If you feed a pure capacitance with ac current the current leads the voltage by 90 degrees and the power is zero (Cosine 90 deg is zero).

The angle between current and voltage is known as the power factor.

Perhaps a bit 'over the top' for a TV forum , and ignoring the most important thing - the VAST majority of multimeter's aren't true RMS, and only read 'average' based on a sine wave input.

Anything that uses a switch-mode PSU only takes short pulses of power (on the mains peaks), and makes an average reading meter completely useless.

But as the poster mentions, he's reading an actual power meter, which gives the true values (within it's tolerances of course).

I stand corrected on the power measuring devices. My £5 DVM doesn't do AC current. That just leaves the OP with the problem of drawing 10A + from a car battery.

All the OP needs to measure the power is to read the battery terminal voltage when delivering the power to the inverter and the DC current consumed.

Gets rid of any argument as to the efficiency of the inverter and any deviation on it's output voltage from a pure sine wave point of view and also the fact that it's not delivering a pure sine wave ac current. Any domestic plug in power measuring device is likely to be very inaccurate when used in this way, for exactly the same reasons you quoted.

I applied a more realistic profile to the Run Time test.

I turned off the TV at 2 hours, and continued with the SKY+HD and other PVRs being powered, ie recordings.

At 8 hours, I terminated the test, with the battery voltage under load being 12.08v