Inverters, What is the Formula?

[emmitdb]

Greetings All,

Any 'Sparky's' out there? Anybobody know what the mathmatical formula is?
What I'm after is finding out the drain on a battery when it's being used with an inverter attached.
I know that the bigger the appliance being used on 'Mains' then the bigger the drain, but it would be handy to know in advance exactly what the drain is.

Thanks in advance

 

[Parksy]

The formula given Here states:
'As a rule of thumb (leaning on the high side), an inverter will require 10% of its continuous wattage rating from a battery. Example: A 300 watt inverter providing 300 watts to an appliance will require 30 amps from the battery. The same inverter providing 150 watts to an appliance will require 15 amps from the battery. '

 

[ProfJohnL]

Hello Emmitdb,

Parksy's figure is a fair guide, but there area couple of other factors just consider:-

Most inverters produce what is called a modfied sine wave output. These are the most efficeint and are usually perfetcly satisfacctory for most equipment, but some items of hi-tech may not be totally happy with it. use 1A for every 10W of power needed.

For sensitive equipment you may need to look for a true sine wave output inverter. These are less efficient and you should allow about 1.5A input for every 10Watts of power needed.

 

[emmitdb]

Greetings once again,

Thank you for your replies. However, there is one thing that is still puzzling me (Sorry in advance if this seems 'stupid'.
Whilst I grasp the concept that I require 1 amp for every 10watts needed, how does that sit with the timeframe of how long the appliance is required?
If I require the use of, say a microwave for a maximum of 10mins, then surely that will require less capacity of a similar mains piece of equipment with a similar wattage that needs to be used for longer, or am I missing something from your recommendations.

 

[RogerL]

Watts and amps define the amount of power required - watthours and amphours take the time into account.
Your inverter will need to be rated for the microwave wattage whether its 1 min, 10 mins or 10 hours - HOWEVER - your battery capacity determines how long the inverter will run for at a given rate.

 

[ProfJohnL]

Hello again emmitdb

It certainly is not foolish to ask questions, The foolish person is the one who dosen't know but who chooses not to ask a question and goes on to make a mistake.

Rogers has given a good answer.

 

[Past_it]

Hi emmitdb, an inverter is not the way to go for caravanners, basically you need a good quality one to give a reasonable sine wave and they are expensive.
Most modern electronic equipment, including microwaves, will not give their full rating because of this and the drain on a 12volt battery would be enormous, of course damage could occur to the equipment.
Inverters are extremely inefficient, probably most efficient at about 60% full load and most inefficient when running light.
One use for an inverter is to warm the van while the battery lasts.
The only use I can think for a small inverter in a van is for an electric shaver !!

 

[RogerL]

Inverters are extremely inefficient, probably most efficient at about 60% full load and most inefficient when running light.
One use for an inverter is to warm the van while the battery lasts.

Isn't that out of date - it may have applied to older transformer-based designs but the modern solid-state ones have been around for a good while now.

 

[ProfJohnL]

Hello Past-it
Like your nick name your information is ---past -it.

As Roger states the modern switchmode transistorised inverters producing a modified sane way are in the region of 90 to 95% efficient. Full sine wave inverters are now about 70% efficient. Their efficency does not drop off significantly on part loads either, though they do tend to need between 0.5 and 1 A quiesetnt current.

However I do tend to agree that inverters are not perhaps the best way forward for caravans:-
Items like space heating, water heating and the refigerator can all be run on gas, and many LCD televisions already have a 12Vdc input. Attempting to run any of these items on an inverter would use a lot of battery charge, adn it certainly is not recomended.
Choosing what items to use when on holiday and selecting high efficencey designs (such as LED lighting) will usually provide most of what is needed in a caravan.

There are a few items that many people now choose to take away with them now, such as lap-tops and mobile phones which may need a mains supply to charge correctly, but these can be charged using a small plug in inverter in the car and charged when going out and about.

 

[ChrisUK]

The cheapo modified sine wave inverters can do so odd things as well. Charging one electric tooth brush the inverter got very hot indeed, even though the current was very low (Maybe the affect of the inductance in the TBrush charger) and another tooth brush charger was killed off by the inverter. The inverter will power my laptop battery, but the laptop power unit makes a loud buzzing. I bought myself a proper 12V to 19V converter instead.
I take alot of electrical stuff on holiday, but with the exception of the tooth brush, I have managed to find an approprate 12V input solution.

 

[Parksy]

We bought a reasonably inexpensive inverter from Argos, unfortunately it's in the caravan which is in storage and I can't remember the details but it has a thermostatically controlled fan and low battery power warning light and buzzer.
The inverter never becomes hot because of the fan and it will not damage the leisure battery because of the low voltage warning.
The inverter runs our led 16" flat screen tv from the 12v socket in the caravan and provided that there is sufficient charge in the leisure battery from the solar panel we could watch tv all evening whilst off ehu if we wanted to.

 

[Parksy]

I meant to add that the power requirement for a microwave oven would place excessive power demands on the leisure battery and inverter and I wouldn't attempt to power the fridge or electric heating with our inverter, it's just for the tv.