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Advice on Inverters
userMike Chapman
Posted: 20 January 2005 1:44 PM
Subject: Advice on Inverters

Can someone knowledgeable about 12v to 240v Inverters please give me some advice. I have no experience of buying or using an inverter. Thinking about using one to power a laptop, hi-fi, etc in the motorhome when a mains hook up is unavailable.

Is there a good article or website giving recommendations and advice?

The laptop will have a GPS based navigation program. Can it be powered from an inverter powered by the cab 12v socket while on the move? (being used by my wife). Will a smoothing device be required to remove voltage spikes?

With a 110 amp Leisure battery roughly how long will it be able to power the inverter before the low voltage warning is given and the battery need re-charging.

Which is the best model and what power would you recommend?

Your assistance will be greatly appreciated.


Mike Chapman
userWilliam R Dunstone
Posted: 20 January 2005 6:12 PM
Subject: Advice on Inverters

Hi Mike,
Some of your points are like how long is a piece of string?
I am sure one of the experts will be along shortly and give you an answer about inverters.
I would recommend to feed your laptop when under way to use a laptop power supply from someone like Action Replay at It has various settings for voltage (to match your laptops requirement), various sizes of tip (to fit your laptop) and costs about £30.
The advantage being that you are not getting the electrical losses of converting 12v to 230v and then back to c 12v again.

Posted: 20 January 2005 10:15 PM
Subject: Advice on Inverters

Today, Inverters are used where we want a “mains” type supply when there isn’t one available. Even a modern laptop computer will require more watts than the WW2 radio back pack. Some as much as 60 watts.

"Modified sine wave" do not have a 100% replica of a sine wave as you would get from the mains, but most devices I have tried will work from them. This includes some Toshiba laptop computers, phone and digital camera chargers and a charger for a cordless drill. Sometimes I get a little interferance on the TV. But they all work.
You spend a lot more money for "pure sine wave" inverters.

£19.99 for a 300 watt modified sine wave inverter from Maplin electronics is not expensive at all. I suggest that these are “as cheap as chips”.
These come with a DC input cable and providing your load is as you describe they should not take more than about 6 amps from your cigarette lighter socket. A suitable plug is fitted on one of the leads supplied and a 13 amp mains socket for the output. At this Laptop loading they have bags of spare capacity in hand. If you need to run something bigger, up to 300 watts then you will need to provide a supply capable of proving 30 amps to a 300 watt inverter.
Much bigger inverters are also listed on Maplins web site.
Modern high frequency inverters like these are quite efficient.
Posted: 21 January 2005 9:08 PM
Subject: Advice on Inverters

I seem to remember that laptops can be charged directly from a car's cigarette lighter, i.e. by 12v DC. Wouldn't that be a better solution?

It certainly seems crazy to buy an inverter to turn DC to AC so you can put it through a charger which contains a rectifier to turn it back into DC - which is what you need to charge a battery.
userMike Chapman
Posted: 21 January 2005 9:19 PM
Subject: Advice on Inverters

Hello Bill and Clive,

Thanks for the information will look up Maplins and Action-Replay.

Take your points about the laptop but I am considering a unit that will power other mains equipment such as TV's and Hi-Fi and sometimes other computer peripherals such as printers etc.

By the way Clive you forgot to tell me how effective they are at ALTITUDE!!!


Posted: 23 January 2005 11:19 AM
Subject: Advice on Inverters

Those that require a DC supply of 12 volts (or less) can easilly be run from the cigarette lighter via a suitable adaptor and filter.
My Toshiba for example requires 15 volts so its not suitable.
However as the Maplins offer is so cheep and modern inverters are suprisingly efficient it looks the lowest cost solution for the price. It comes with both cigarette lighter plug and 3 pin mains socket, all you need to do is plug it in. And no I don,t have shares in Maplin. It was A.N. Other who pointed this offer out to me.

Altitude - well, what can I say? At altitude the outside air pressure is less, so the difference between that outside and that inside a sealed container (tyre) will increase as you climb the mountain.
But its just a couple of PSI.
userMike Chapman
Posted: 23 January 2005 1:14 PM
Subject: Advice on Inverters

Hello Clive,

Thanks for the info. Have found a good write up on inverters (relates to boats but OK for motorhomes) on the Unique Power Systems Ltd website

This explains what I should have done originally which is to work out the total watts of the units I want/may want to power either singularly or together and calculate the maximum draw in watts. From this choose a unit that can supply this wattage as a minimum. Agree with you that Maplin's certainly do seem to provide units at a reasonable price.

Am going to replace this slow old beast (800 mhz) soon with a faster Portable computer so will take the power requirements into account.


Mike Chapman
userMike Chapman
Posted: 23 January 2005 1:48 PM
Subject: Advice on Inverters


The web site is NOT

Posted: 23 January 2005 5:03 PM
Subject: Advice on Inverters

My Laptop is slower than yours at 650 meg. I have a second (£50) which is 233 meg.
This main lash up box is only 1.3 Gig so I use my sons cast offs when it comes to computing!

Power in = power out less ineficiency.

Power = volts in X Amperes.

So If your Laptop required 60 watts, thats 5 amps into the inverter at 12 volts. Pluss a little for the inificiency of the inverter, hence my suggestion of 6 amps earlier.
Good luck
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