Tracker Posted July 27, 2012 Share Posted July 27, 2012 We fancy using a microwave and even maybe a toaster off 230 volts ac without ehu and as we already have 2 110 ah batteries and a solar panel how hard can it be? I'm getting conflicting advice from different sellers. Some say that a m/wave and/or a toaster, or any other heat generating device will only work on a pure sine wave, some that it will work on a modified sine wave but less efficiently (meaning, I presume, longer coking times, and one firm stated categorically that a modified sine wave inverter would damage or burn out a microwave? I don't know who to believe but other folks must have done this before me - I just hope they get to see this query and will allow me the benefit of their own experiences please! I have in mind to get a 1000 watt continuous inverter and use only a 700 watt microwave and a 700 watt toaster - but not simultaneously! I reckon, ignoring the startup surge, that a 700 watt rated appliance will probably consume about 800 watts and that the inverter itself will absorb another 100 watts or so and on that basis using 10 minutes of toaster and say 10 minutes of microwave will knock about 25 ah out of my batteries. If those who understand these things better than I can advise if Is this a reasonable or a flawed projection I would be obliged please? The solar panel 60 watt input on an average day of say 10 hours brightish daylight / sunlight (not in the UK generally!) should bung around 300 watts back in which I reckon is more or less the 25 ah consumed? So there we have perpetual motion - unless I've got it all wrong? If I wire the inverter output straight into the ehu mains inlet will the inverter sense when mains is hooked up (only usually at home) or is there a risk of damage, or even fire, if 230 volts is backfed into the inverter - or should I isolate the inverter before hooking the van up? I am well capable of wiring the inverter with adequate cables and fittings but I just need to clarify the theory before executing the practical bits! Better safe than sorry! Thanks folks. Link to comment Share on other sites More sharing options...
Frankkia Posted July 27, 2012 Share Posted July 27, 2012 Hi Rich. A 700w output microwave typically consumes about 1100w of power - The rest is lost in heat in the magnetron. Some microwaves have been known to throw a wobbler on anything but pure sine wave and others seem to work if the inverter has a very generous output. As for a toaster - what you propose is a very power hungry method of incinerating a piece of bread. The most efficient 1 we have found (and for a lot less dosh) is this :- http://www.amazon.co.uk/Gelert-CUT118-Folding-Camping-Toaster/dp/B000QH2V40/ref=sr_1_5?ie=UTF8&qid=1343422635&sr=8-5 Cheers Link to comment Share on other sites More sharing options...
Tracker Posted July 27, 2012 Author Share Posted July 27, 2012 Thanks Bernie - we have been using two of they there toasters for years and very good they are too - but you have to have your wits about you when using them. So seeing as I am going to get an inverter anyway, an Argos el cheapo 700 watt toaster is only a fiver which even on my pension is affordable - just - but even better is that it can do it's own thing without having to be tended in case of incineration if left a bit too long on heat, so seeing as the power is free it still seems like a reasonable idea to me! The stated power consumption on a friends £35 basic Tesco value type 700 watt output microwave is 820 watts input which is nothing like 1100 watts - or am I missing something? Link to comment Share on other sites More sharing options...
Frankkia Posted July 27, 2012 Share Posted July 27, 2012 Hi Rich I am very surprised at that consumption for a 700w, even if it is wh rather than max watts. I have seen some experiments done which showed that even for a 700w output microwave the consumption was over 1100w and on start up and regularly during cooking would surge over 1250w. The magnetron takes an initial surge to warm itself up and then during cooking regularly shuts down and has to warm up again. Even using a microwave at say 3/4 power has the same effect as the magnetron only runs at full power but shuts down more often. As for the toaster - it wasn't the initial outlay that concerned me more than the large bites it would take out of your battery - but that depends on how hungry you get but I haven't come across any experiments of toast to battery consumption. Saying that you use a 700w toaster (pretty slow) to make 1 slice of toast say 5 mins. Adding warm up time and inverter inefficiency I would guess that 1 slice would cost you approx 6Ah from your battery or 72w of your 300w per day from your solar panel. As you said there is also the problem that some heating devices don't like modified sine wave and the toaster may not work at all. We had a travel iron of very conservative wattage which wouldn't do a thing on a 1500w modified sine wave inverter. Oddly though I did connect the Carver water heater to it and it did work - the battery went flat rather quickly though! You may ask why I would attach the water heater to it - ok the answer is: when on a site with only a 6 amp supply I was able to heat the water and the battery was replenished by the site hook up without tripping the darn thing. Link to comment Share on other sites More sharing options...
Syd Posted July 27, 2012 Share Posted July 27, 2012 Hi Rich We have a 1000w modified inverter and run our microwave absolutely no problem and have been doing so for about six years. The microwave and hair dryer are the only things in our caravan that are 240v. I believe that you have to move onto more sensitive equipment before you need Pure wave inverters. Hi Again Forgot to mention that we have a cut out break before make relay fitted to the 240 supply to the inverter on both the Neg and Pos wires Link to comment Share on other sites More sharing options...
w1ntersun Posted July 28, 2012 Share Posted July 28, 2012 Hi we have a cheap ebay invertor we bought for £98. It runs a 650W microwave no problem and we bought a cheap 12v £70 led TV from Comet and it too runs with no wavy lines or inteference Richard Link to comment Share on other sites More sharing options...
Melvin Posted July 28, 2012 Share Posted July 28, 2012 We run our cheap 800 watt microwave of a cheap £25.00 invertor, no problem. Link to comment Share on other sites More sharing options...
hallii Posted July 28, 2012 Share Posted July 28, 2012 A 1000w inverter isn't man enough for a 700w microwave. It MIGHT work but it would be near it's limits, not a good thing with inverters. Better to buy a 1500w one (£99 pure sine wave from Amazon). and have a bit of reserve. That's 140 amps approx at 12v, the cable and connections will have to be 101% right or things will get hot and smokey! Electronics do not like rough sawtooth waves (modified sine wave) up 'em. They go pop. The fact that some devices actually work on sawtooth waves simply shows how good the Chinese components are getting now. Your batteries had better be good, and the connections between them. Leisure batteries are not really made for such heavy currents and at these sorts of currents even short connecting cables can result in an unbalanced load on each individual battery. (Cable resitance becomes important). H Link to comment Share on other sites More sharing options...
Tracker Posted July 28, 2012 Author Share Posted July 28, 2012 Thanks folks - some really sound and yet conflicting views there! I plan to take the + feed to the inverter from one leisure battery and the - from the other, having linked the two with starter motor grade cable. That ought to equalise the power drain as I figured that with 90 or so amps the existing 30 amp cable won't be adequate and might indeed fry? I will do the 'mark one finger' hot connection test with the microwave running to check for resistive connections and cables Such an amazing variation of cost but am I right to be wary of cheap, especially when they claim to be pure sine wave, as there seems no way for the layman to check wave forms - unless things don't work? Link to comment Share on other sites More sharing options...
hallii Posted July 28, 2012 Share Posted July 28, 2012 Yes you need at least 90amp cable, Clean the conections to the batteries and make sure they are tight. Soldered joints are OK but Silver soldered are better. Here is an far better explanation of how to do it than I could manage http://www.smartgauge.co.uk/batt_con.html H EDIT Actually the amps is over 100, depending on the efficiency of the microwave. Link to comment Share on other sites More sharing options...
Tracker Posted July 28, 2012 Author Share Posted July 28, 2012 Thanks Geoff - fascinating article for those of us who are fascinated by such things and I am!! I didn't realise that the diagonally opposed connection for two batteries makes such a difference and it makes me wonder whether the Eberspacher heating which takes a hefty 20 amp fire up burst would work longer between battery charging with the modified wiring. I wonder how many vans or boats with two leisure batteries use the correct method for both input charging and output consumption - mine soon will! I reckoned on using shop made battery cables intended for starter motor use as they come premade with connections on each end and in various lengths. Microwave wattage - now there is a vexed issue with differing views! Our modern Panasonic has a power output of 1000 watts and a stated power input of 1135 watts, whereas our old one (1985 Sharp and still working) states 600 watts output and 1200 watts input! So I guess progress does have some benefits! Link to comment Share on other sites More sharing options...
Clive Posted July 28, 2012 Share Posted July 28, 2012 A cheap toaster (one with no electronics) will run of any type of inverter of suitable power capability. SOME microwaves and TVs and Laptops will run of any type of inverter of suitable power capability. ALL microwaves and all TV and all laptops will run of a PURE SINE WAVE inverter of suitable power capability. C. Link to comment Share on other sites More sharing options...
T8LEY Posted July 29, 2012 Share Posted July 29, 2012 Be careful you don't overestimate the capability of your solar PV panel. At best, depending on your particular unit, a fixed SPV panel will have an efficiency of around 18%. So, you would be lucky to pull 10Ah on a good day. Link to comment Share on other sites More sharing options...
Tracker Posted July 29, 2012 Author Share Posted July 29, 2012 Clive - 2012-07-28 10:16 PM A cheap toaster (one with no electronics) will run of any type of inverter of suitable power capability. SOME microwaves and TVs and Laptops will run of any type of inverter of suitable power capability. ALL microwaves and all TV and all laptops will run of a PURE SINE WAVE inverter of suitable power capability. Thanks Clive. I am prepared to buy a pure if needs be and it might well be the best solution to remove as many variables as possible in the name of reliability and hassle free travel. However with such a huge price variation is there a way to ensure that it is worth paying top dollar for a branded unit rather than one for half that price off Fleabay? How does one tell the difference? Link to comment Share on other sites More sharing options...
Clive Posted July 29, 2012 Share Posted July 29, 2012 If you are attending one of the motorhome shows it should be possible to try your microwave with a modified sine wave inverter on a sellers stand. We have had both types and one particular Laptop caused a modified sine wave inverter of a considerable power rating to start smoking and expire. Pure sine wave jobs are not as expensive as they once were but they do run anything within their power rating. Peace of mind does not come cheap, that's why our motorhome is a Merc!! but we keep them about 10 years at a time. Take care C. Link to comment Share on other sites More sharing options...
Tracker Posted July 29, 2012 Author Share Posted July 29, 2012 Is this, I wonder, the answer to a motorhomer's microwave prayer!! http://www.kitchenwareonline.com/acatalog/info_1010.html A 500 watt microwave modified to reduce power surges - not particularly inexpensive but if it does the job, who cares! Cooking times are not an issue when on holiday - so what if it takes 4 minutes instead of 2 to cook me porridge in the morning! I've asked them to advise on pure or modified sine wave inverters. Link to comment Share on other sites More sharing options...
Frankkia Posted July 29, 2012 Share Posted July 29, 2012 Hmmm - 500 watt microwave that requires a 1500 w inverter - interesting! Link to comment Share on other sites More sharing options...
Tracker Posted July 29, 2012 Author Share Posted July 29, 2012 Frankkia - 2012-07-29 8:35 PM Hmmm - 500 watt microwave that requires a 1500 w inverter - interesting! Yeah - I thought so too!! Maybe the 1500 watt 'requirement' is erring on the side of hassle free and dependable as it also says limited to 5 amp consumption to remain within the 6 amp limitations of some site ehus to avoid trip outs - but it does tend to support your view that the initial power surge on start up is considerably more than the stated input wattage on the appliance would suggest? Link to comment Share on other sites More sharing options...
Tracker Posted July 30, 2012 Author Share Posted July 30, 2012 This is the reply from Kitchenwaresonline, which is a tad confusing re inverter types - Thank you for your enquiry. 2 fully charged batteries, especially if connected together in parallel would produce enough power to run the microwave or toaster. The inverter you would need would have to have a continuous output of at least 1200 watts to run the microwave. The specifications can be a bit tricky as they often mention 2 outputs. The continuous and the surge output. Most appliances surge as they start up before settling down to a continuous drain as they run. It is the continuous output that you will need 1200 watts at least. Our ovens don't surge like a normal domestic microwave so that wouldn't be an issue. Our ovens don't use any electronic circuitry so the type of inverter is not an issue. If the oven had complex circuits you would need to consider a modified sine wave inverter. With our ovens you can get away with using the cheapest type. We don't sell them but you can I'm sure find a range of specifications and prices on ebay. If the inverter has a normal plug socket on it the oven will plug in nice and easy. Link to comment Share on other sites More sharing options...
Clive Posted July 30, 2012 Share Posted July 30, 2012 I smell cow poo! C. Link to comment Share on other sites More sharing options...
Tracker Posted July 30, 2012 Author Share Posted July 30, 2012 Clive - 2012-07-30 5:09 PM I smell cow poo! C. Interesting observation Clive - I did wonder myself being an equally cynical old codger!! What say you to the thought that we buy a cheapy £35. 700 watt microwave, perhaps avoiding touch screen electronic gizmos in favour of a good old fashioned basic mechanical timer and power level switchs, and run it off a cheaper modified sine wave inverter? What wattage inverter would you use on a 700 watt microwave? Spending less to get the same job done always sounds good to me!! I'm struggling to get to grips with this technology and every time I think I have it sussed somebody tells me that I haven't. I don't mind that as it's better than buying the wrong kit but it is a bit confusing! Link to comment Share on other sites More sharing options...
Clive Posted August 5, 2012 Share Posted August 5, 2012 The 700 watts is the heat output. Microwave ovens have those noisy fans because they chuck out lots of heat as losses. They are not that efficient, just quick if the food is a small amount. I would chose a 1500 watt inverter to run a 700/800 watt microwave oven. But there is no guarantee that the one you chose will be OK with "Modified sine wave" until you have tried it. Thankfully I did not have to make a decision myself as our tardis has a 1500 watt pure sine wave inverter fitted as standard. Its your money. C. Link to comment Share on other sites More sharing options...
Tracker Posted August 5, 2012 Author Share Posted August 5, 2012 Thanks Clive. We now have the 1500 watt pure sine wave inverter as below. It seemed reasonable for the price and we have dealt with Midsummer Energy before and found them to be helpful and reliable. http://midsummerenergy.co.uk/buy/sine_wave_inverters/Fountainhead1500WPS.html As soon as we get the van next week we will get a cheap microwave with mechanical controls locally and it will probably be the one that SWMBO likes the best and is under forty quid! We already have a 'spare' 800 watt two slice toaster which should be OK and we will see how it all works out and if anyone else is interested I can report the results on here in due course? Meanwhile my thanks to all for their various inputs. Link to comment Share on other sites More sharing options...
spospe Posted August 5, 2012 Share Posted August 5, 2012 Rich A small point. The inverter you mention from Midsummer has a, "Current consumption when idle is less than 10W". Over a 24 hour period, that will add up to about 20 a/h, which is a fair drain and one which the solar panel will struggle to make good on some days. Do remember to switch the inverter off after use, not just the output, but the input. i.e. make sure that it is disconnected from the batteries! Link to comment Share on other sites More sharing options...
Tracker Posted August 5, 2012 Author Share Posted August 5, 2012 Thanks Michael - I had noticed the 10w drain and I intend mounting the inverter on the end of the bed locker so that I can easily turn it off, it has plenty of ventilation, and I can use the output socket for other things when needed! 10 watts does not sound like a lot but that is 240 watts a day that does not need to be replenished by the 60 watt solar panel (best part of half a day's charging depending on the weather!) or the engine (2 hours at 10 amps from the alternator!) Bearing mind that 10 minutes of microwaving will only consume a similar amount of battery capacity that 10 watts an hour is well worth avoiding! Always assuming both my maths and my understanding of watts = volts x amps is correct!! Link to comment Share on other sites More sharing options...
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