Microwaves And Inverters

[Ray]

I'm not good with the technicalities of electricity, so answers if possible in simple terms please.

We have one leasure battery however it gets very light use, charging a phone or two while under way, a reversing camera only ever used for a minute or two sometimes when mooring and cabin lights. When out and about we never cruise less than 3 hours and often more.

My better half thinks a microwave cooker would be a good addition and I agree as I don't like using the small gas oven to the point where it's never used and a bit more variety for onboard meals would be nice, even if simple ready meals.

I'm really hoping to avoid fitting another leasure battery, so in finally coming to the point, will the one leasure battery cope with occasional microwave use using an inverter. I want to get one of the simple ones that plug into a 12v accessory socket (cigarette lighter type).

Bearing in mind it won't be the end of the world if the battery gets very low at times and that usagae will not be heavy does this sound doable?

Is there any point in looking at 12v microwaves, if available?

Thanks in advance for any tips.

[Guest]

A cig lighter inverter wont run a microwave, the watts figure on the front is cooking output, so you need to check the actual consumption which will be much higher. I would think you will need a good quality inverter rated at least 1500w continuous output, properly installed.

It will work on one battery, but you will have to be very careful about usage, not ideal really and a bigger battery bank would be preferable

[Ray]

Thank you, it never pays to cut corners in the end does it? I'll do it properly but maybe next year now!

[Meantime]

Hi Ray,

The simple answer is that you will not be able to run a microwave oven off an invertor plugged into a 12V cigarette lighter socket. You will need an appropriate invertor properly wired into the battery. Ideally I would also install a second battery otherwise your one battery will be dead in no time at all.

Some basics, I'll try and keep it as simple as I can. Microwaves are typically rated at anything from 500 watts to 1000 watts. There is a simple formula that works out from the power of an item, how much voltage and current is needed. So Power=voltage x current As long as you know at least two of the values you can work out the missing one.

So a 600 watt microwave, not the most powerful on 240V is worked out as follows 600 =  240V x 2.5A So needs to draw 2.5A from your mains supply. No problem when at home.

On 12V the maths is as follows; 600 = 12V x 50A so needs to draw 50amps. This is way more than you could ever draw from a cigarette lighter, which are normally rated at 10, or maybe 20 amps.

So assuming a 600 watt microwave you need to draw at least 50 amps. In reality due to inefficiencies in the invertor and microwave you will need more like 70 amp. A typical 110 amp hour leisure battery is considered discharged when it has given up half its capacity and should never be discharged below this. So you have 55amp hours to play with. That means that your microwave drawing 70 amps would probably run for no more than 45mins and would give the battery a real good hammering. To many times and the battery would give up the ghost. That is why I think you need a minimum of two batteries.

I think Samsung do a 12V microwave and it would save a little as you wouldn't need an invertor and there would be less conversion losses, but remember that power formula, you are still going to draw some big current.

[Ray]

Excellent explanation! Many thanks 

[Cockatoo]

5 minutes ago, EastCoastIPA said:

Hi Ray,

The simple answer is that you will not be able to run a microwave oven off an invertor plugged into a 12V cigarette lighter socket. You will need an appropriate invertor properly wired into the battery. Ideally I would also install a second battery otherwise your one battery will be dead in no time at all.

Some basics, I'll try and keep it as simple as I can. Microwaves are typically rated at anything from 500 watts to 1000 watts. There is a simple formula that works out from the power of an item, how much voltage and current is needed. So Power=voltage x current As long as you know at least two of the values you can work out the missing one.

So a 600 watt microwave, not the most powerful on 240V is worked out as follows 600 =  240V x 2.5A So needs to draw 2.5A from your mains supply. No problem when at home.

On 12V the maths is as follows; 600 = 12V x 50A so needs to draw 50amps. This is way more than you could ever draw from a cigarette lighter, which are normally rated at 10, or maybe 20 amps.

So assuming a 600 watt microwave you need to draw at least 50 amps. In reality due to inefficiencies in the invertor and microwave you will need more like 70 amp. A typical 110 amp hour leisure battery is considered discharged when it has given up half its capacity and should never be discharged below this. So you have 55amp hours to play with. That means that your microwave drawing 70 amps would probably run for no more than 45mins and would give the battery a real good hammering. To many times and the battery would give up the ghost. That is why I think you need a minimum of two batteries.

I think Samsung do a 12V microwave and it would save a little as you wouldn't need an invertor and there would be less conversion losses, but remember that power formula, you are still going to draw some big current.

Remember that the advertised wattage of a microwave is the output power of the magnetron. An 800 watt Microwave typically uses about 1200 watts

[Guest]

3 minutes ago, SteveDuk said:

Remember that the advertised wattage of a microwave is the output power of the magnetron. An 800 watt Microwave typically uses about 1200 watts

Indeed, hence my suggestion for at least a 1500w continuous rated inverter (another thing to watch for, some cheap inverters quote the peak output to grab your attention)

[Meantime]

7 minutes ago, SteveDuk said:

Remember that the advertised wattage of a microwave is the output power of the magnetron. An 800 watt Microwave typically uses about 1200 watts

A very good point I had forgotten to add. In addition remember that any sort of grill or combo microwave will draw even more current. All microwaves have different efficiency levels therefore you really need to look at the specifications to find out how much current is drawn at 240V. Using the formula above you can then work out the power needed, and then use the formula again to work out how much current is needed at 12V.

Edited to add:

If considering a grill or combo microwave then because the grill is an inductive load, you would need a more expensive sine wave invertor, rather than a quasi sine wave invertor.

 

[Ray]

I'm seriously glad I asked for advice, once again NBN member's help and advice is invaluable!

[Jonathan]

Ray,

I bet the gas oven is looking more appealing now!

Another thought:  It is possible to buy 12v "slow cookers".  They will be no use for "ping food" instant meals, but you could prepare a meal, cruise for a few hours (the alternator would feed the 7 or 8 amps used by the cooker whilst the engine is on), and the food would be cooked when you moor up.  With the engine off, this style of cooker would be using less than a tenth of the power consumed by a microwave.

[KaptinKev]

21 minutes ago, EastCoastIPA said:

 then because the grill is an inductive load, you would need a more expensive sine wave invertor, rather than a quasi sine wave invertor.

 

I think you will find a grill or other style of heating element is resistive, inductive normally boils down to motors, old style ballasts and anything else that uses coiled wire in its operation.

[Ray]

Jonathon, that slow cooker sounds good! The rest is beginning to make my head swim lol

[scaniaman]

Hi Ray,  although you look as though you have the answers you need for now. You may be interested in some  light youtube vlogs to give a better understanding of boat electrics

"cruising the cut" this is journalist living on a narrowboat, a good starter for electrics.

"journey with jono" still a narrowboat but more techi.

But the best vlog for me is a guy called "greg virgoe" he`s converting a merc sprinter van into a motorhome.It gives advice on planning,mains,12 volts,power audits,solar panel,cable sizing and lots more.Although its got a bilge pump and nav light or two missing its still relevant for boat use. Enjoy if you can stay awake

paul

[Ray]

Thanks Paul, I do like to understand how things work and go together, the electrical side of things being a weak spot for me so I'll be busy youtubing if anyone wants me! 

[StillCruising]

We have an 800watt microwave that is normally used when we are on shore power but we do have a 2Kw Quasi sine wave inverter that is permanently wired in via a switch and fuse to the two domestic batteries. It is not connected to the 240 volt system but on the odd occasion when we want to use it we have a cable that plugs into the socket on the front of the inverter and the other end plugs into the shore power socket, this ensures that you can't have the shore power and inverter on at the same time. Someone once told me that as a rule of thumb you can expect to draw 10amps from the battery for each 100watts pull on the inverter which means that our microwave/ inverter set up draws around 80 amps which is probably about right and does clobber the batteries. You should also remember that the amount of charge that you can take out of a battery is dependent not only on its rated capacity but the age and state of charge of it at the time. The hire boats that have no gas have a huge number of batteries and big alternators to charge them.

[oldgregg]

4 hours ago, StillCruising said:

The hire boats that have no gas have a huge number of batteries and big alternators to charge them.

Indeed they do - I believe Barnes fit twin alternators as standard.

Swings and roundabouts really. I kind of prefer gas in that I don't have to run the engine much in order to make a cuppa, but equally a nice electric combi oven is better to cook with than an LPG stove.

[ranworthbreeze]

When go on narrow boat holidays we normally remove the microwave and save on the wasted space, it is an ideal space for storing the cereal boxes in the galley.

I think that most of the boatyards suggest that you run your engine whilst operating the microwave.

The fridge, and bow thruster take their toll on the battery banks, more so a bow thruster that will draw 300 to 400 amps dependent on its size

All this power has to be produced by the alternators to charge those battery banks, the more electrical items used the more the draw on the power. On narrow-boats they recommend 5 hours of cruising, it would be interesting to know how many hours of cruising is recommended by the Broads operators.

Regards

Alan 

[oldgregg]

32 minutes ago, ranworthbreeze said:

On narrow-boats they recommend 5 hours of cruising, it would be interesting to know how many hours of cruising is recommended by the Broads operators.

I think it's usually "at least 4 hours". Not something I usually have a problem with - But I've taken over hire boats where the batteries needed a couple of days to get up to normal as the previous hirers had not done enough running.

[Matt]

Got to say I’m a big fan of Thunder’s new battery monitor (might need to send some instructions around!).

In the morning the lowest I saw the batteries was 85% granted it was summer but that fridge was working hard :-)




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[imtamping2]

21 hours ago, Matt said:

Got to say I’m a big fan of Thunder’s new battery monitor (might need to send some instructions around!).

In the morning the lowest I saw the batteries was 85% granted it was summer but that fridge was working hard :-)




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Oooo do share 

[Starbrite]

My 1500 watt inverter just manages to work the microwave but it turns slowly and takes longer to warm the food

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