1.5 kw invertor : How to calculate current load requirement

[HarisHashim]

Hello,

I am going to buy pure sine wave inverter from China called Bert. You can have a look here:

http://www.aliexpress.com/store/802124/209715798-296192002/1500w-Pure-sine-wave-power-inverter.html

I know that it is a risk but I would like to try it anyway and perhaps post a review here after I complete all my test.

But here is where I am very rusty. I thought I only need to estimate my power requirement (which is 1kw). But then I ask the seller is there other requirement related to the current output (Which is 7A for 220v system).

Since my purpose is to connect fluorescent lamps (let say 10 piece), a fridge and 2 tv. I wonder if 7A current output is enough. I have no idea whether

1. 7A is like a limit. As long as I don't have appliance that consume more than 7A it is ok!

OR

2. Or it is like lamp (1 Amp) + tv (2 Amp) + fridge (3 Amp) = 6Amp. In which if I add another fridge it will exceed 7 Amp and I will see some magic smoke.

I know this sound silly, but really appreciate feedback from everyone.

And any suggestion on how to test the inverter is also appreciated.

Best Regards!

[WindJuggler]

Use the second one. If you want to know more about it you can take a look at Ohm's law: U=R*I.
U = constant = 220V
R = variable. Connections on the grid are parallel so that every application works independantly => more applications is less resistance.
I => if the resistance drops I will increase to maintain the same voltage

[hydrosun]

The inverter needs to be big enough for the total of all loads likely to be on at one time. And loads like the refrigerator with a motor will have a starting surge 3 times it nameplate rating. The inverter should have some surge rating (usually 2 times it steady rating for these high frequency light weight inverters).
 Most inverters won't let out the magic smoke if you overload it a few times, it will shut down to protect itself. But do it too many times and it might fail early. Or the motor will lock up and not start if the inverter can't supply the surge needed.
 An AC voltmeter would tell you if the inverter is keeping the voltage steady as the loads are increased. Most inexpensive inverters can do that up to it's rating.  You would need an oscilloscope to tell if the inverter is really a pure sine wave inverter. If it runs all the loads you put on it without overheating or causing the loads to hum loudly and lasts several years it is a keeper. That is really the question about unknown inverters is how good the quality is and how long it will last. If you buy a known brand you can perhaps rely on other peoples experience over time. The only inexpensive inverter that my wholesaler carries is the Samlex brand because of it's reliability over time.  Here you are the first tester. Good luck.
Chris

[HarisHashim]

The inverter needs to be big enough for the total of all loads likely to be on at one time. And loads like the refrigerator with a motor will have a starting surge 3 times it nameplate rating. The inverter should have some surge rating (usually 2 times it steady rating for these high frequency light weight inverters).

Does this means that once I do the watt sizing correctly I don't have to worry about the amp? In this case I believe my sizing is correct. A big 2 door fridge that I have is rated 300w at defrost (worst case scenario during runtime). Multiply it by 3 and we get 900w. 1.5k inverter leave about 600w room for 2 TV and 10 lamp. Which I doubt all of the lamp will be all in operation at the same time when fridge motor start surge is happening. (Probably I will cut down to 1 TV and 7 lamp).

I do some more checking today. It looks like some appliance have watt rating and some have amp rating. But both can be convert to each other.

I begin to think that doing correct sizing for watt means at the same time correct sizing for amperage. Two side of the same coins. Appreciate if someone can confirm this though.

Thanks in advance!

[birdhouse]

haris-
i think what you alloking for is watts = volts x amps, or amps = watts / volts.
i'm assuming your power requirements are 220v.  if so the 300 watt fridge would draw 1.36 amps running, and around 4 amps at start up.

so basically gather up all the info on all the stuff you wanna run and convert those numbers to all amps, or all watts and you'll get your total power needed.  sounds like this inverter is on the brink of being able to run all of your loads. 

let us know how it works!

adam

[HarisHashim]

I already go ahead. Whether is ok or a mistake ...  ... well what I will learn out of this experiment I will post it here.

Thanks for all the replies!

[wooferhound]

You have been talking about your working loads on the output of the inverter, but have you considered what your inverter will pull from your batteries ?
Is your system voltage 12 or 24 volts ?

If the output is 220 volts AC like you describe . . .
Each 1 amp you use at 220 volts will cause the inverter to pull 20 amps from a 12 volt battery, or 10 amps from a 24 volt battery.
If you max out the inverter output and use all 7 amps, then you would be pulling 140 amps from a 12 volt battery, or 70 amps from a 24 volt battery.

So the next question would be, do you have enough battery and charging to keep up with your inverter ?

[HarisHashim]

wooferhound,

At the moment I use Yuasa 100AH lead acid battery. Actually I have not done battery sizing yet. I think I will use 600w solar panel with 400AH battery. And try to tune number of panel and battery base on usage later.

Just so happen I was thinking about battery too. In the morning, surprisingly the inverter arrived (was in heated discussion with the seller because he give me wrong tracking number).  It caught me off guard because basically I have nothing in hand. Not even a battery. So it is just luck when I went out, stumbled into mom & pop battery shop and they sell N100 battery for abour US$100.

So with the inverter and the battery, with no solar panel and charge controller yet. I just do short simple tests.

1. Connect inverter to battery and an AVR to inverter. The AVR have volt meter and the volt is arround 220v and stable. [PASS]

2. Main concern is that it is not pure sine wave. Heard that power tools will not function if not pure sine wave. So I connect  Black & Decker 500w drill and play with it. Volt meter indicator move a little bit. But it function as connected to the main, [PASS]

3. Connect only 36" TV (around 200w) to the AVR. Just in case there is noise in sound and visual. It works fine. [PASS]

4. Connect only fridge (between 200w - 300w) to the AVR. I think it start since I can hear normal in operation sound. And the freezer does felt cold after some time. [PASS]

5. By this time I wonder how to check the 1500w continuous power rating.  So I test the fridge + black & decker drill plus a 350w blender connected to the AVR, Estimated that should be around 1kw. (Unfortunately TV is no where nearby the fridge). All three work fine. [PASS]

6. I have a microwave with rating 900w at the back plate. Connect only the microwave to AVR. Set power to high and try to microwave some nut for 1 minutes. I think it does it for about 5 second before I heard warning buzzer from the inverter. It cut off the microwave and then turn it back on. Puzzling, I don't know much about microwave but since it is rated 900w it shouldn't do that right? [FAIL]

7. I have a 1500w vacuum cleaner. But since the microwave fail, I stop testing [TBA]

Here is link to picture of the inverter. http://picasaweb.google.com/harishashim/Inverter

Pictures with inverter on top green table is from the seller. The other two is from me. You can see that the seller give me 2 white socket but the picture he show has 2 black socket that looks nicer. I'm going to complain about this.

My method might not be correct. So really appreciate suggestion. Unfortunately I don't have scope to really see whether it is pure sine wave or some kind of very good modified wave.