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Old 28-10-2015, 05:08   #1
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Inverter Musings

OK,
This is what I'm bumping into, it seems logical that inverter size should be according to load, that is for example that if you have a 200W load, you should have say a 300W inverter, that having a larger inverter will work of course, just it's inefficient, there is of course power used just with the thing turned on and doing nothing. But that's not what I'm finding? This 2000W for instance has a no load power consumption of .7A and at low power a 95% efficiency, efficiency drops as power output climbs to 90%
2000 Watt Pure Sine Power Inverter - AIMS Power

While this 300W also has a no load power consumption of .7A and 95% efficient at low power, dropping to 90% at high power.
300 Watt Pure Sine Power Inverter 12 Volt - AimsPower

Now what I'm getting out of this is the big inverter uses slightly less power to make 200W than the little one does, just exactly opposite of what I've read. Now the big one does cost a LOT more, but if it cost no more power to use it, actually a little less, then what's the logic of sizing an inverter to the average load? Wouldn't it make more sense to size it to the highest possible load? That way you don't have to remember not to use the toaster.
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Old 28-10-2015, 06:11   #2
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Re: inverter musings

I've got a 1000W Xantrex that outputs a modified sine wave. I've never wanted a smaller one and often wish I had a larger one. Other than the purchase price, I don't see the downside of a larger unit. In my case, the biggest power savings are a result of a conveniently located remote control panel that encourages me to turn it off when I'm not using it. It probably uses a little over half an Amp when it's not powering a load.
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Old 28-10-2015, 06:52   #3
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Re: inverter musings

I would think that a newer design, larger capacity inverter would have an advantage over a smaller design older inverter. Many of the components have been improved over the years. More efficiency less heat ect.
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Old 28-10-2015, 07:17   #4
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Re: inverter musings

I think a lot boils down to cost, the stirling power inverter/chargers have a stby mode for the inverter and the 2.5kw one uses less than 0.5 amps in this mode, but these large combi's are not cheap. The only downside is you cannot couple with another or generator like some of the Victron products.
The flexibility of a large inverter is redundancy, ( and as a fellow aviator I like this) if your generator or normal AC power supply fails then hopefully the inverter will still run.
The added advantage of the inbuilt charger is again redundancy and more rapid battery charging if you have a second separate battery charger, if you have this setup just make sure you don't use the inverter to run the separate battery charger, it's definitely not very efficient!

I have stirling power 2500w inverter charger and a 60amp battery charger, and all work well together, and so far with the 6 stirling power items installed, no problems.

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Old 28-10-2015, 07:28   #5
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Re: inverter musings

I've got one of these:

Go Power! GP-SW300 Power Inverter

While the no-load power consumption is very low (.2 amp), the efficiency is not as good as some at 80-90 percent.

So, for a user who needs very low power draw (charging phones etc.) with only intermittent need for higher power, this particular small inverter might consume less power than a larger model.

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Old 28-10-2015, 08:12   #6
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Re: inverter musings

Quote:
Originally Posted by a64pilot View Post
OK,
This is what I'm bumping into, it seems logical that inverter size should be according to load, that is for example that if you have a 200W load, you should have say a 300W inverter, that having a larger inverter will work of course, just it's inefficient, there is of course power used just with the thing turned on and doing nothing. But that's not what I'm finding? This 2000W for instance has a no load power consumption of .7A and at low power a 95% efficiency, efficiency drops as power output climbs to 90%
2000 Watt Pure Sine Power Inverter - AIMS Power

While this 300W also has a no load power consumption of .7A and 95% efficient at low power, dropping to 90% at high power.
300 Watt Pure Sine Power Inverter 12 Volt - AimsPower

Now what I'm getting out of this is the big inverter uses slightly less power to make 200W than the little one does, just exactly opposite of what I've read. Now the big one does cost a LOT more, but if it cost no more power to use it, actually a little less, then what's the logic of sizing an inverter to the average load? Wouldn't it make more sense to size it to the highest possible load? That way you don't have to remember not to use the toaster.
The duty cycle of the load is also a variable.

Its the startup or spike loads that will trip out your inverter. This will determine whether a load can even be run.

Sizing to the average load is more useful in sizing the house bank used by the inverter.

You will always lose energy across any ac to dc or dc to dc conversions.

We use our 2000W inverter very little. Really only for intermittent loads. Ironing a shirt on the hook for example just last weekend.

We have moved all our loads to direct 12V. Including NAS, router, modem, etc. The TV and laptop charging we run from little 300W portable inverters.

This move dropped our house bank consumption by 50% per day. The actual delta is probably closer to 20%. We have a few unnecessary parasitic loads on our 110V circuits. The bigger inverter is more efficient under load. With no load the losses may be more than good quality smaller inverters.

To avoid analysis paralysis the following rule of thumb applies:

1) size your house bank so you only consume 50 to 80% of your battery capacity. Having 2 - 3 days capacity without needing to recharge is ideal.
2) size your inverter to your lifestyle. A 2000W inverter will suit most cruisers. For condo cruising you'll need a much bigger inverter. Think ice maker, domestic fridge, etc. Up to 5000W.
3) size your solar pessimistically to recover your daily consumption.
4) log your consumption over a month or more to accurately determine your daily energy needs.

The above is ideal but somewhat expensive.

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Old 28-10-2015, 09:47   #7
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Re: inverter musings

I got all the Solar, bank size etc. I'm 660 AH of AGM and 750W Solar.
I have an on board Diesel Generator so any real AC consumption I'll crank it, but say for instance if I want toasted bread for my peanut butter sandwich, the toaster is a lot easier than the oven, it's a big load, but for maybe 1 min. I mean .7A If I leave the thing on, that is less than what one light used to be.

But assuming the AIMS 300W is a good quality converter, it's less efficient to pull 200W from it than it is from the 2,000W, and of course your not running the toaster on 300W either.

Multiple 300W inverters would be even less efficient as you have .7A Dark current from each one.
Inverters, at least these are NOT more efficient under load, they are more efficient lightly loaded, which is not what I expected.

It seems the most efficient is one big inverter, lightly loaded. Not having a couple of small ones. ref the .7A Dark current and the 95% efficiency vs 90%

So now I'm wondering do I pop another $100 and get a 3KW inverter, knowing that gives me a reserve for a surge, start up current etc., instead of the 2KW ? I started out thinking a couple of 300W was the way to go and move them around as needed, but now it seems as one big one wired directly in is the way to go.
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Old 28-10-2015, 11:19   #8
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Re: inverter musings

Your inverter must be sized to the max load you expect to use. The average load has no use in sizing an inverter, although any inverter has to be able to sustain all the longer loads on it. Some inverters can put out more power intermittently than they can over a longer period. It is not healthy for most inverters to be over-stressed most of the time. The inverter manufacturers should give you the info you need.
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