Inverter Power Draw

[solar_jeff]

Hi everyone. I'm pretty new with solar power and all so I apologize if I get something wrong in this post. I'm interested in setting up a small off grid solar power system to use to run some things in my apartment. I want to use two 100 watt 12V panels, a 30 amp charge controller, a 2000 watt inverter, and two 125AH batteries wired in parallel . I've been doing a lot of research on solar energy and was able to answer most of my questions. The problem I'm having is using the inverter. I'm building this system to mostly power an air conditioner. The air conditioners power needs are 115 Volts, 8 Amps, and 950 watts. I originally had thought that the batteries with a combined 250AH would power the unit for approximately 30 Hours, in reality it will never run for that long anyway. I was going off of that until a friend of mine said that the inverter would draw 166 Amps. Does that mean I need to add the 166 Amps plus the 8 Amps which is 174 Amps? If that is true it would mean that instead of lasting 30 hours it would only last a little over one?


Thanks,
Jeff

[lkruper]

Hi everyone. I'm pretty new with solar power and all so I apologize if I get something wrong in this post. I'm interested in setting up a small off grid solar power system to use to run some things in my apartment. I want to use two 100 watt 12V panels, a 30 amp charge controller, a 2000 watt inverter, and two 125AH batteries wired in parallel . I've been doing a lot of research on solar energy and was able to answer most of my questions. The problem I'm having is using the inverter. I'm building this system to mostly power an air conditioner. The air conditioners power needs are 115 Volts, 8 Amps, and 950 watts. I originally had thought that the batteries with a combined 250AH would power the unit for approximately 30 Hours, in reality it will never run for that long anyway. I was going off of that until a friend of mine said that the inverter would draw 166 Amps. Does that mean I need to add the 166 Amps plus the 8 Amps which is 174 Amps? If that is true it would mean that instead of lasting 30 hours it would only last a little over one?


Thanks,
Jeff

Inverters are not 100% efficient and most manufacturers will list their efficiency. If it is 90% then that means if your 2000w inverter was loaded to 50% (1000w) it would draw 1100w (110%). Also, there is a small "self-use" amperage that the user manual may list which might be less than 1 amp to 1 or 2 amps at 12v. However for your particular inverter, check the documentation.

Your two 125AH batteries in parallel will give you 250AH at 12v. You should only use half of that on a routine basis, so that would be 125AH. 125AH X 12v = 1500 Wh capacity. This amount of power will run 1500Wh / 950w = 1.6 hours. But your inverter is not 100% efficient, so it would be 90% of that at best.

I think where your calculation went wrong is that you are taking the entire 250AH and dividing by 8 amps. However that is 8 amps at 110v, not 12v.

You will find it easier (and it took me a long time to learn this the hard way) to make your calculations in watts, and not AH.

Your 2 100w panels will probably give something like 200w / 18v = 11 amps in full sun. 125 AH / 11 amps = 11.36 hours to bulk charge plus a couple extra for absorb. You won't have 13 hours of sunshine in one day, so your panels are underpowered for filing 1/2 of your 250AH battery in one day.

BTW, check my calcs. I have found that when I am in a hurry I make mistakes. Hopefully others will chime in with some real experience to assist you further.

[LETitROLL]

Hi everyone. I'm pretty new with solar power and all so I apologize if I get something wrong in this post. I'm interested in setting up a small off grid solar power system to use to run some things in my apartment. I want to use two 100 watt 12V panels, a 30 amp charge controller, a 2000 watt inverter, and two 125AH batteries wired in parallel . I've been doing a lot of research on solar energy and was able to answer most of my questions. The problem I'm having is using the inverter. I'm building this system to mostly power an air conditioner. The air conditioners power needs are 115 Volts, 8 Amps, and 950 watts. I originally had thought that the batteries with a combined 250AH would power the unit for approximately 30 Hours, in reality it will never run for that long anyway. I was going off of that until a friend of mine said that the inverter would draw 166 Amps. Does that mean I need to add the 166 Amps plus the 8 Amps which is 174 Amps? If that is true it would mean that instead of lasting 30 hours it would only last a little over one?


Thanks,
Jeff

If the inverter will handle the startup surge and even get the ac running then it will work, but not for long, you will be pulling much more from your batteries than the 2 panels will be able to replace. The 200w rating (combined) on the panels is perfect world maximum, in reality most of a day they will put out much less, your controller is probably PWM and overall after all system losses you can expect to average more like 100-140 Watts input to your battery under normal sunny conditions. If you do try this, be very careful with your battery to inverter connections, they need to be very robust, when you get close to 900-1000W load(s) you are looking at 80 amps from the batteries to the inverter (on a 12v system), that is getting in the range of stuff non-experts should not be playing with especially if you plan on letting it run for anything more than a few minutes, that is also a very high discharge rate for that battery bank (they will rebel) see peukert's law.

[Mike90250]

in an apartment, where are you going to place the panels ? Indoors behind window glass, will give you about 30% of the full sun rating, if there is full sun indoors. If your panels are in shade (no direct sun) you get nothing out of them.

[solar_jeff]

in an apartment, where are you going to place the panels ? Indoors behind window glass, will give you about 30% of the full sun rating, if there is full sun indoors. If your panels are in shade (no direct sun) you get nothing out of them.

They will be outside on my second balcony. There is no obstructions or any shade, which is why I'd like to start a solar project.

[solar_jeff]

If the inverter will handle the startup surge and even get the ac running then it will work, but not for long, you will be pulling much more from your batteries than the 2 panels will be able to replace. The 200w rating (combined) on the panels is perfect world maximum, in reality most of a day they will put out much less, your controller is probably PWM and overall after all system losses you can expect to average more like 100-140 Watts input to your battery under normal sunny conditions. If you do try this, be very careful with your battery to inverter connections, they need to be very robust, when you get close to 900-1000W load(s) you are looking at 80 amps from the batteries to the inverter (on a 12v system), that is getting in the range of stuff non-experts should not be playing with especially if you plan on letting it run for anything more than a few minutes, that is also a very high discharge rate for that battery bank (they will rebel) see peukert's law.

So what your saying is the inverter needs 80 amps from the battery to make 1000 watts? Also does the 8 amps that the unit draws fit in anywhere in that calculation? I've decided to put this project on hold and start out smaller then get back to it since its such a high amp draw.

[lkruper]

So what your saying is the inverter needs 80 amps from the battery to make 1000 watts? Also does the 8 amps that the unit draws fit in anywhere in that calculation? I've decided to put this project on hold and start out smaller then get back to it since its such a high amp draw.

Amps X Volts = Watts

Watts / Volts = Amps

1000w / 80 Amps = 12.5 volts

8 Amps X 120 Volts = 960 watts

If you convert to Watts the number will be the same no matter what the voltage.

http://www.solarpaneltalk.com/showthread.php?23882-Inverter-Power-Draw&p=181533&viewfull=1#post181533

[Sunking]

Will you please name me as your beneficiary. If not me make sure your Last Will & Testament are up to date because you are going to die soon if you do this.

Secondly why do you want to pay 10 times more for electricity than buying it from the electric company? You do know that is what you have decided to do right?

[LETitROLL]

So what your saying is the inverter needs 80 amps from the battery to make 1000 watts? Also does the 8 amps that the unit draws fit in anywhere in that calculation? I've decided to put this project on hold and start out smaller then get back to it since its such a high amp draw.

Yes, usually when you get up to running bigger loads (>500W) you would start looking to use 24v or 48v system, at 12v the current draw is very high and can become a safety conern (not to mention efficiency). Any unit draw just adds to the total the inverter is drawing from the battery bank. One of the hardest things you can do with a simple solar system is to try and run an air conditioner, Physical motors and compressors, are all very high draw plus difficult start up surges, etc. on the other hand electronics and L.E.D. lighting are usually very low and you can be amazed at how much you can run from a 200w system, if you are interested in solar it is fun to experiment with and you shouldn't be discouraged, just stay away from appliances and you can do quite a bit and learn some valuable stuff along the way.

[solar_jeff]

Will you please name me as your beneficiary. If not me make sure your Last Will & Testament are up to date because you are going to die soon if you do this.

Secondly why do you want to pay 10 times more for electricity than buying it from the electric company? You do know that is what you have decided to do right?

I never planned on hooking it up myself. I have a brother in law up north who is a master electrician. I just wanted to see if it was possible before I bring him into it. I don't mind the cost because it's an interesting project and I like doing new things.

[Sunking]

I never planned on hooking it up myself. I have a brother in law up north who is a master electrician. .

Does not matter who hooks it up or not, you have planned a death trap using a 12 volt high power inverter. It is your plan and design that will kill you. Makes no difference if the person hooking it up knows what they are doing or not. If that person cared about you and know what they were doing would refuse to help you. I would do it for you for any amount of money. Well I might if you are a... Then I would do it for free.