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Celestialsailor · 04-09-2012, 00:10

I hear size is everything. This should apply to inverters I suppose. Today I installed the LED 22" flat screen (claimed 45 watt) and a DVD player (claimed 8 watt). The only other it that might be on the inverter system would be my laptop(s). Correct me if I'm wrong but I think my Dell 1505 draws 200 watts. Does that sound right?
So total would be just over 200 watts. I'm assuming a safe factor of 50%, so a 400 watt inverter should do it. I am also assuming a Pure Sine Wave is needed as to not damage any microprocessors and alleviate any RF noise.
Your input is most welcome.
Also...Can I run the output parallel with the shore power? Or do I need to use a 3 way switch?

**noelex 77** · 04-09-2012, 00:47

The quoted figures on the back of appliances are usually the very worse case, with a bit of leeway. You wil find your consumption is much lower than the numbers you are quoting.

I have a 21 inch LED flat screen that runs directly off 12v (which is more effecient), but about 13 watts is typical.
Likewise a laptop like your 15 inch widscreen core duo model is an average of about 40w. With the laptop you will want to allow for running the computer while charging the internal laptop battery in wich case you might peak at 120w or so.

Inverters are most efficient near the top of power band. The standby current also rises significantly. So going smaller has advantages other than just initial purchase cost.

Pure sine wave is not strictly needed for the appliances you mention, but the extra cost will ensure it runs everything and I think a pure sine wave inverter is sensible.

Having said all of the above I would still go for about a 300w inverter. There will be other appliances that you will add later.
An alternative which has a lot of merit for a crusing boat is two (or more) smaller inverters. This gives you redundancy, and is often slightly more efficient.

**Andina Marie** · 04-09-2012, 08:37

I would go with the pure sine wave. Although a computer may be OK, you may want to use a UPS on one or more items to protect against inverter shut-down. UPSes monitor the power line and typically detect a power failure on non-sinusoidal power so they won't work.

**David M** · 04-09-2012, 08:49

Most AC loads will work with an inverter that is not a pure sine wave but not all loads. Get a pure sine wave and you will be covered.

I would build in some overhead so your inverter is never working close to 100% of its capacity. You will also have some room for future AC loads.

onestepcsy37 · 04-09-2012, 09:00

sort of what noelex77 says.

i have two iinverters. one is about 100 watt and plugs into a cigar lighter at the nav station. this one charges things like my cell phone or electric razor and will run the led tv.

for running the vacuum cleaner - a mini shop vac that the admiral loves - i need about 500 watts, so i have a hard wired 700 watt inverter. it is not connected to the 120 system on the boat. it just has two outlets on it and i run an extension cord to whatever i need. it will also run power tools. i only turn it on when i need it and always turn it off when i'm done. it also runs the computer although it's probably too big for that.

both inverters are plain and simple (and cheap) and have been working for over four years now....

Florida Mike · 04-09-2012, 11:13

Do not run your inverter in in parallel with your shore power unless the inverter is designed that way. Either run dedicated plugs ( most probably the best option ) or put some sort of double Pole single throw or double Pole single throw center off between the shore power and inverter. A small boat I once had, had 120 V. double pole dubble throw relay hook up so that when shore power was available it would automatically switch. Just my two cents, Mike.

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**David M** · 04-09-2012, 11:38

Do not run your inverter in parallel with the shore power regardless. It is illegal. Doing this has the potential to electrocute someone who has to service the marinas shore power. Even if it was legal it would make no sense to do this. It only makes sense to get a shore power connection with more current if this is what you need, for example from 30 amp to 50 amp service.

There are inverters that are designed to be paralleled with each other....they are able to match phases without increasing the amplitude (voltage) of the sine wave. They effectively increase the current that is available to the loads.

Celestialsailor · 04-09-2012, 11:45

All great information. So here is what I am taking home from this help...
1) 600 watt pure sine wave unit for TV, DVD player and maybe my mini shop vac.
2) Wire the inverter to a DC breaker and then hard wire the output to a designated AC outlet separate of the shore power.
3) Maybe use a ground fault interrupter type outlet also.

How did I do?...

belizesailor · 04-09-2012, 16:56

Quote:

Originally Posted by noelex 77

The quoted figures on the back of appliances are usually the very worse case, with a bit of leeway. You wil find your consumption is much lower than the numbers you are quoting.

I have a 21 inch LED flat screen that runs directly off 12v (which is more effecient), but about 13 watts is typical.
Likewise a laptop like your 15 inch widscreen core duo model is an average of about 40w. With the laptop you will want to allow for running the computer while charging the internal laptop battery in wich case you might peak at 120w or so.

Inverters are most efficient near the top of power band. The standby current also rises significantly. So going smaller has advantages other than just initial purchase cost.

Pure sine wave is not strictly needed for the appliances you mention, but the extra cost will ensure it runs everything and I think a pure sine wave inverter is sensible.

Having said all of the above I would still go for about a 300w inverter. There will be other appliances that you will add later.
An alternative which has a lot of merit for a cruising boat is two (or more) smaller inverters. This gives you redundancy, and is often slightly more efficient.

Good points.

I have also found a few appliances, off brands, where the numbers on the label are pure fiction. I think they just plagiarized the label from something else and stuck it on the device. A good multimeter is a big help in determining what loads you are really running.

Using devices which are directly 12V, or at least use a DC-DC converter, makes a significant difference in energy usage. Even larger, more sophisticated, inverters are typically only around 90% efficient. Most cheap power supplies are even less efficient. So, if you go from inverter (DC-AC), then power supply (AC-DC typically for laptops etc), then you can end up with a total loss of around 30% or more!

Given that pure sine wave inverters are becoming more common and the price has come down, it is probably a good way to go, but, at least in my experience, I've only ever encountered one device that simply would not run on a modified sine wave inverter.

Having a second inverter aboard is a good idea. You could carry a smaller one just in case the primary inverter craps out. This is what we do, but my good old big clunky Freedom 2000 inverter has not missed a beat in over 10 years. Although, I do quite often encounter other folks with inverter problems -- so we loan out the small unit.

belizesailor · 05-09-2012, 06:32

Quote:

Originally Posted by Florida Mike

Do not run your inverter in in parallel with your shore power unless the inverter is designed that way. Either run dedicated plugs ( most probably the best option ) or put some sort of double Pole single throw or double Pole single throw center off between the shore power and inverter. A small boat I once had, had 120 V. double pole dubble throw relay hook up so that when shore power was available it would automatically switch. Just my two cents, Mike.

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Also, a rotary switch is a good way to do this, especially if you have or plan to install a gen set. I have one with "off, inverter, generator, shore" positions.

Celestialsailor · 05-09-2012, 07:26

Quote:

Originally Posted by belizesailor

Also, a rotary switch is a good way to do this, especially if you have or plan to install a gen set. I have one with "off, inverter, generator, shore" positions.

Where can I find these switches?

belizesailor · 05-09-2012, 08:10

Blue Sea Systems makes a variety of rotary switches. Most any major marine retailer can get them for you.

See Defender.com Search Results: rotary switch for some examples.

Captain Bill · 05-09-2012, 09:26

I have a modified sine wave inverter in my boat and it runs most things quite well. I actually use a cigarette lighter plug in model when I am only doing light loads as the overhead power consumption of th unit itself is less. I have found two limitations with the modified sine wave. They are absolutely incompatible with solid state temperature control modules on electric blankets. They overheated and started smoking. Fortunately I noticed this and discnnected them before actual flames erupted but that was the the end of the electric blanket. Actually the blanket was fine, it just didn't work without the temperature controller. The other limitation is with electric motors. They simply do not have the same output as they do with a pure sine wave. On most items this is not a huge difference but some appliances may not work well. I had a drink refrigerator/ice maker that would keep thngs cool but would not make ice on the inverter. It worked fine on shore power or the genset, but not the inverter.

laforge24 · 05-09-2012, 10:00

Electrical motors have an inner resistance that needs to be fed before running !!
A 1000w motor will need a 1500w inverter to get going !!!!
inverters change DC into AC an will try to convert ANY wattage you want to use except that the electronics will not be able to handle that High Amps(if not adapt) and thus will burn/short out, or if decent inverter; blow fuse !!! Always use a bigger inverter compared to what you think you will be using HEAT=LOSS