# Microwave oven and inverter



## HARRYH

Is it practical to power a 500w microwave oven that pulls 850w at startup via a 1000w inverter in a motorhome?
Will a 100a/h leisure battery cope? 
Given fully charged battery how many minutes could it run (say 600w) before the battery was half discharged?
Wire size, location, switching, are all considerations.
Has any one experience of this, or is there a 12volt microwave oven that an alternative option.


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## DABurleigh

Well it's practical some of the time off hookup. Is 600W the consumption on the appliance label or from your estimate of its efficiency? 

On the face of it, as you will be consuming 55amps after start-up (I've assumed inverter efficiency of 90%), you might think you can run for just under the hour. In practice, Peukert's formula is a killer, so your battery capacity will be far lower than 100Ah when you are discharging at those high rates.

In winter, you might well find your battery voltage diving down below 12V, and perhaps below the voltage the inverter needs to operate, after only a few minutes. So you would then have to resort to firing the motorhome engine up. Not what you would want to do to cook your breakfast on a quiet rally morning.

In summer, and if your other battery needs are light (not hours of TV, say, or hairdryer use) and if you will recharge the battery by moving on each day (especially if you have a Sterling B-to-B charger to make the most of your alternator), you may find your microwave to be a real convenience.

Stick the inverter as CLOSE to the batteries as you can, and connect them with THICK welding cable, the shorter and thicker the better. Connect to +ve of one battery and -ve of the other, if you have two in parallel.

Dave


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## C7KEN

Hi Harry I can't exactly answer your question but I can tell you what we have, one 4kw peak invertor running off two 85amp batteries which are dedicated to the microwave which is a variable up to 800watt unit, this setup has been extremely successfull for the past two years but was designed to handle two quick meals for myself and son when we were travelling to race meetings, we would get about 10 -11 minutes before the low voltage sound came on which was ample for our needs, however if we hads friends with us and needed more I would just run the engine which increased the operational time and we could do another meal, the microwave has been a handy extra for us as its other use is to cook the bacon for my bacon sarnie at breakfast time which it handles with ease, I think two batteries would be a better bet for your use assuming that the inverter is big enough for your microwave, no doubt someone will come along with a the techie stuff but after the glasses of red I consumed today in sunny 20deg spain I dont have the inclination to work it out


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## richard863

Hi HarryH 
What is the make and model of your 500w Microwave? I have been searching for one. 
Dave when using 2 batteries in parallel and connecting across the pair isn't it essential to have the joining leads between the batteries made of the same thickness welding cable?
Kind regards


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## DABurleigh

"Dave when using 2 batteries in parallel and connecting across the pair isn't it essential to have the joining leads between the batteries made of the same thickness welding cable?"

Richard,

Certainly good practice, and this:

"Stick the inverter as CLOSE to the batteries as you can, and connect them [the inverter to the batteries] with THICK welding cable, the shorter and thicker the better. Connect to +ve of one battery and -ve of the other, if you have two in parallel."

is entirely consistent with that, as it says nothing about the cables actually connecting the two batteries.

Dave


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## C7KEN

Wiring up our inverter was really easy as there are two neg terminals and two postive so each battery has its own connections, the leads were supplied with the invertor and are heavy duty welding type, the invertor is about two foot away from the batteries it cost £150 and was bought of a firm in High Wycome the manufacturer is Ring (fan cooled)and as i already said no problems whatsoever, it is never used with the alarm sounding voltage drop down to 10.5v , if this sounds then the engine is run or the microwave switched off until the batteries have a top up charge , the batteries were not expensive around £30 each I think from touchwood sports in Oxford, the other main use for this invertor is to power a PS2 or Satellite with flat screen TV although it will power a toaster, vac cleaner, hair dryer and such like for long enough to do the job, I tried to get in touch with this firm when last in UK but at that time they had no stock so maybe I will try again and post on here if they have anything


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## HARRYH

*Thanks, very quick answers*

The microwaves a small Samsung M308 11"x11"x!3"high, rated start input 800w at 230v Output 470w at 230v

Its proved useful on hook up.Bought it at a carboot last year for a few quid as it fit nicely into an over worktop high cupboard.
1000w continous run inverters seem to be available on e-bay at about £70. and it would power the tv/video & small vac at other times.Could be useful.

The scary bit is that 12v wiring load. Safety & also Motor insurance donst cover electrical fault fires, are to be concidered.


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## DABurleigh

Personally I wouldn't use a 1kW inverter to run just TV/video on a long-term basis. An inverter's efficiency is low at a fraction of its rated load, increasing to a maximum of up to 95% at half-rated load, and dropping back down to a still healthy 90% at full load. So your inverter overkill would be wasting 12V juice.

Dave


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## HARRYH

Your sayig horse for coarses?
A small inverter for small load a big inverter for big load?
Fit two units one for the microwave use and one for tv etc?
Getting complicated ?????


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## DABurleigh

Not complicated at all. I have a 1.6kW sine-wave inverter for aircon, hair-drier, etc., powering all mains socketes when not on hookup, and a 150W dedicated to satellite controller, digibox and TV. The latter only costs £20 from Maplin.

Dave


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## DJP

> So your inverter overkill would be wasting 12V juice.


Hi Dave

So, does a 500w inverter runing a TV, Camos and Sky box use more "juice" than a 1000w inverter running identical items?
If so how much more? I know how long is a piece of string but worst case and all that?
To assist. The spec for the 500w is
Maximum power rating (watts) 500 
Peak power (watts) 0.01 seconds 800 
Stand by cutrrent (amps) <0.3 
Waveform Modified Sine 
Efficency 90%

and the 1000w is
Maximum power rating (watts) 1000 
Peak power (watts) 0.01 seconds 2000 
Stand by cutrrent (amps) <0.5 
Waveform Modified Sine 
Efficency 90% 
The stand buy current differs by 0.2 of an amp. Neglible?
If you go for a 2000w the standby is <0.8 of an amp.
Thanks
Dennis


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## DABurleigh

"So, does a 500w inverter runing a TV, Camos and Sky box use more "juice" than a 1000w inverter running identical items?"

No. The other way around, yes, but the difference would not be enough to worry much about. Unfortunately just looking at the no-load/standby current is insufficient, as many inverters have special circuits to keep that low, but as soon as you have any load, the true quiescent current kicks in. The shape of an inverter's efficiency curve is as shown on page 5 of:
> Victron technical leaflet <
Bear in mind this curve applies to a quality design where special measures have been taken to hold the efficiency up at low loads; many inverters are much less efficient at small loads.

The very cheap 150W inverter I use hard-wired has a convenience utility, too. Just a single master switch by the TV connects the cheap inverter to the battery at whch point all the satellite dish, controller, digibox and TV turn-on simultaneously.

Dave


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## DJP

> "So, does a 500w inverter runing a TV, Camos and Sky box use more "juice" than a 1000w inverter running identical items?"





> No. The other way around, yes,


That's what I meant to say. Too much Sherry yesterday me thinks.

Thanks again for your help Dave.
Happy New Year

Dennis


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## HARRYH

thanks to all.
I think Ill leave inverters to others to play with. Use a 12volt tv & make do with out the microwave off sites. That 12volt inverter load seem to much fire risk.
Thanks again 
Harry


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## Yorky

Dave
Do you have the two invertors on separate circuits or do they feed into the mains generally?
If they feed into the van mains do you have to isolate one from the other when running, ie will power from the running unit feed back into the idle unit and damage it?
Regards Eddie


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## DABurleigh

Eddie,

They are separate. The 1.6kW sine wave inverter is in the same black box as a 50A mains charger. This makes integrating the inverter into the van mains system easier (otherwise your battery power is generating mains which is charging your batteries ....). Also the mains power from the batteries can be synchronised with limited power from hookup to run a total instantaneous mains power considerably in excess of the hookup capacity.

The 150W system is then dedicated JUST to TV, satellite dish, controller and digibox. All of these are permanently switched on and I just power the inverter on and off to switch them all simultaneously. It doesn't feed mains into the van's mains circuits.

Dave


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## richard863

Thanks HarryH for the info on the Samsung. My EL cheapo microwave while being 800W actually drew 1248 W on load. This made noises one could only equate to a pair of skeletons copulating on a nissan hut roof, when powered with the original 3KW modified sine wave inverter. Since I changed the inverter to a 3KW pure sine wave inverter, all works normally now even charging the phone and laptop is quicker. 

Kind regards


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## Yorky

Hello Dave
I have a Moor's 1800 inverter fitted wired into the vans main circuit with an changeover switch fitted which isolates the invertor from the mains circuit when I connect up to mains power. This system allows me to use all the sockets in the van when on inverter power.
The only thing I have to remember when I been connected to the mains running everything, ie. charger, fridge, water heater is to turn everything off at the switches before turning the inverter on again.
During the evening this big inverter is only running the TV and sat unit so I might re fit my old 300 unit just for the TV system as you recommend.
Thanks for the advice.
Regards Eddie


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## 109601

*Peukert's formula*

Hello Gents

I would like to have some advice on my upcoming battery purchase and get your thoughts as to whether my logic is correct or wrong on the following:

My need: To be able to run household equipment that might draw a total of say 2,000 Wh for 2 consecutive hours every day.

My problem: Having studied Peukert's formula I think I'm making the following assumption correctly but I'm not sure:

*Scenario 1*
Peukert's equation shows that If I'm going to draw 2 Amps from a given system, the actual rate of battery depletion will be as if I draw 2.3 Amps - therefore 15% more - therefore the battery needs to have 15% more capacity - and this should theoretically result in a *small* increase in capital outlay for the battery - say also 15%.

*Scenario 2*
Peukert's equation shows that If I'm going to draw 100 Amps from a given system, the actual rate of battery depletion will be as if I draw 251 Amps - therefore 151% more - therefore the battery needs to have 151% more capacity - and this should theoretically result in a *huge* increase in capital outlay for the battery - say also 151%.

*So here is my question: Is the following a possible solution or am I simply fooling myself:*

My plan is to connect 4 number 12 Volt batteries in series in order to have a 48 Volt battery. I then will buy a 48 Volt inverter (instead of the 12 Volt model). Exact battery capacity is not determined yet. For now I'm just addressing the principle - please bear with me.

The 230 Volt AC equipment will draw (plus minus) the same amount of Watts - whether I use a 12 Volt battery system or a 48 Volt battery system because : Amps x Volts = Watts.

My logic tells me that with a 48 Volt system my Volts are 4 times higher (than with a 12 Volt system) and therefore my Amps will be 4 times lower.

Taking Peukert's formula into consideration I derive that I could get away with a smaller (and cheaper) battery system if I go for the 48 Volt system.

*Question 1:* Am I correct?

*Question 2:* Should I go with a Pure Sine Inverter if I want to have the option to run my microwave and/or expensive LCD TV and/or PC and/or laptop etc., or could I get away with the much cheaper Quasi (or similar) inverters?

*Question 3:* Might a Quasi inverter with "filters and goodies" do the job with "acceptable" risk of equipment damage? In other words are they significantly better than the normal Quasi inverters?

*Question 4:* What Voltage system do you recommend for my proposed 2,000 Watt per hour x 2 consecutive hour needs?

All responses welcome


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## richard863

*Peukert's formula*

Hi Meeperd
To assist you, I think if I understand your problem. You intend to need 4Kw in the 2 hour period.
So therefor as you say 4Kw is 4Kw. In theory this will be a drain of 334 Amps on 12v, 83.4A on a 48v system.
Whilst you save money in theory on 4 smaller cells. What I don't think you have taken into consideration is 1) the extra weight. 2) Having 4 cells in series you will still need some large cells as each cell will have to cope with 83.4A, so really you will need 4 x 160Ah batteries. heavy man, thats 160Kgs,ish
:lol: .
What also you must think about is if 1 battery goes down you have got to find that one? 
With regards to the pure or square wave inverter IMO the my equipment motors runs quieter and smoother with a pure sine wave, mind you a electric kettle doesn't care which system.
If this all is to replace a household system, which could be housed in a barn, no problem, but in a motor home I have my doubts. Sorry

  
Kind regards


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## DABurleigh

Answered meeperd's post and Skype call last night. Sorry I wasn't around when you called again at 4am this morning :roll: 

Dave


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## richard863

*Battery theory*

Hi Dave
Could you PM me your answer to Meeperd so I can see if I was on the right track with my answer. All in the quest for knowledge
Kind regards


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## carol

Dave we could all learn - please


It sounds a lot of weight 4 batteries, but then we don't know what mh it is for. If an RV then it is a completely different scenario....


Carol


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## johng1974

> If an RV then it is a completely different scenario....


not so in mine Carol 

to close for comfort...


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## DABurleigh

I forgot to come back to this.

The application is a domestic mains power supply when the electricity goes off for 2 hours every 24 hrs. During the intervening period, it is desirable to provide continuity of supply taking the energy from lead acid batteries, as noise would be an issue with a genny.

I had thought it was 1kW maximum power, making the stated 2000Wh in 2 hrs. In fact it is ~2kW for 2 hours. For simplicity, let's say that requires 200A from 12V for 2 hours, given inverter losses etc. Probably an average draw would be less than 2kW, but if not, we are talking 800Ah so as not to go below 50%, so allowing for Peukert efficiency losses cf capacity at rated draw, we are talking ~10 batteries of 100Ah, not unreasonable if you picture the convenience a UPS would give you in that irritating scenario.

Now to the question of whether connecting the 12V batteries in series or parallel would be better. That is, go for 12V system, 24V system, 48V, ....? From a Peukert point of view, it doesn't matter. Think about it and you'll see that the current through each 12V battery is identical in series or parallel configurations.

24V is popular in lorry electrics, marine electrics etc., and it, or even higher voltages, will be coming to your cars before too long as electrical demands increase. The reason for this is not Peukert, but a combination of reducing ohmic losses at the higher voltages, lower currents, as well as cheaper components, such as switches, etc. But for this particular application, none of this applies, so go with the cheapest inverter to do the job, whether 12V, 24V or 48V to 230V. Of course you also need to consider the charger, so it's the cheapest combination. And ideally you need UPS functionality, so it makes sense to go for an integrated inverter/charger/UPS.

All of which is the longest plug I've made for Victron gear, which does just this, and is in the van 
http://www.victronenergy.co.uk/inverters-chargers/multiplus-12v-24v-48v-800va-3kva/

The pros and cons of modified vs pure sine wave inverters has been done to death on MHF, so I won't repeat it here.

I hope that's clear.

Dave


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## 123107

how much current can you draw from a leisure battery before it damages it there not desinged for high current discharge


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