Inverter sizing question

[Part 77]

I have 2-12 volt Marathon 12V70F station batteries rated at 72 amp hours each (144 total) that I plan to use for a backup residential power supply. They will be wired in parallel. This would power general lighting during short term power outages of 6 hours or less. What wattage rating would be recommended for the inverter? I have a portable gas generator for longer term outages.

 

[Part 77]

This link shows the batteries specs. Thanks in advance. http://www.ecovantageenergy.com/catalog/items/item462.htm

 

[Besoeker]

I have 2-12 volt Marathon 12V70F station batteries rated at 72 amp hours each (144 total) that I plan to use for a backup residential power supply. They will be wired in parallel. This would power general lighting during short term power outages of 6 hours or less.

If you have a portable generator, I think the question might be is it worth bothering with the batteries and an inverter?
With 144Ah you could get 24A for 6 hours. That is continuous 288W for that period. How much power would you need in the event of an outage?

 

[ty]

I don't think there is nearly enough info about what you are trying to do, but to simply answer the question, try Pep Boys. They have simple 12v dc to 120v ac inverters.

 

[ron]

As pointed out you have about 275w worth of source for 6hours guessing that your batteries are in perfect condition.
So, depending on the inverter efficiency, you might get enough output from the inverter to power two 100w bulbs.

 

[LarryFine]

What wattage rating would be recommended for the inverter?

Enough to power the intended lighting load. The source AH capacity only determines run time at a given load.

 

[Part 77]

My guesstimate must have been low. I envisioned the batteries supplying more than 2-100 watt bulbs. As far as more info and using the gas gen, my plan was to have an inverter set up that my wife or teen-age daughter could safely transfer to for short term lighting when I'm not home. If needed, I could then later set up the generator for longer term. My wife and daughter can not physically handle the generator.

As far as Pep Boys, I needed to now what size inverter to shop for. Automotive type inverters tend to auto shutoff at 12.5volts+/-to protect an automotive type battery. The batteries I have are deep cycle and can run down to 10.8 volts and be within the design specs.

 

[Part 77]

Enough to power the intended lighting load. The source AH capacity only determines run time at a given load.

My math is off today. I hoped to run 4-5 60 watt bulbs so the house was not a cave. I have a average size house with 1 floor and 300 watts would be great. I had thought the batteries would supply about double that. How long would they serve at 600 watts usage?

 

[Part 77]

If you have a portable generator, I think the question might be is it worth bothering with the batteries and an inverter?
With 144Ah you could get 24A for 6 hours. That is continuous 288W for that period. How much power would you need in the event of an outage?

I'm getting the batteries free, or else this would not be economically feasible They are new, but were damaged in shipment. They were dropped and have "cosmetic" damage to the cases and have been condemned by the engineer involved. Being AGM lead batteries, they are spill and leakproof if the case was ruptered, which they weren't. I enjoy DIY projects and I found this interesting.

 

[Besoeker]

How long would they serve at 600 watts usage?

About 2 hours and 50 minutes from fully charged. About because the Ah capacity depends on discharge rate and temperature.
Note that incandescent lamps (regular bulbs) take an inrush current that is many times their steady state current. You need to ensure that the inverter can handle this.

 

[Part 77]

Double tap

 

[Part 77]

Thanks Besoeker.
I am considering the 1000 watt unit in the link below. It is >90% efficient and basically double of what I will draw at max. It has a 3000 watt surge rating, cooling fans and some extra protective features that some cheaper units lack.

http://www.theinverterstore.com/the-inverter-store-product.php?model=pwrb1000#

 

[LarryFine]

I hoped to run 4-5 60 watt bulbs so the house was not a cave. I have a average size house with 1 floor and 300 watts would be great.

If you need to power 300w, then you need a 300w inverter. The inverter must be sized to supply the maximum load, even if it's needed for only 10% of the time.

I had thought the batteries would supply about double that. How long would they serve at 600 watts usage?

To start, you'd need a 600w inverter, but they're efficient enough to figure that a 600w inverter supplying 300w of load will use about the same amount of current at 12v as a 300w inverter supplying the same 300w.

Presuming 100% efficiency, 300w at 120v required 300w at 12v, so each amp at 120v requires 10a at 12v. 300w at 120v is 2.5a, and at 12v is 25a. So, 300w will require 25a times the number of hours desired.

Again, with 100% efficiency, 144ah will supply 25a for just under 6 hours. In real-world use, you'd probably receive about 4 hours at 300w before the voltage drops too low, and about half that at 600w.

 

[Part 77]

Thank you Larry.

 

[ty]

As far as more info and using the gas gen, my plan was to have an inverter set up that my wife or teen-age daughter could safely transfer to for short term lighting when I'm not home.

And how is this 'transfer' going to take place??

As far as Pep Boys, I needed to now what size inverter to shop for. Automotive type inverters tend to auto shutoff at 12.5volts+/-to protect an automotive type battery. The batteries I have are deep cycle and can run down to 10.8 volts and be within the design specs.

They would work for a couple 100 watt light bulbs.

I enjoy DIY projects and I found this interesting.

I still think there is not enough info, and much speculation.
I am much more concerned with how you are going to transfer power at the moment, than I am about your inverter size.
But,
to simply run a couple light bulbs, you don't need much of an inverter.

You could run a drill off of your deep cycles hooked to an inexpensive automotive inverter to wire a house. Have seen it done.

 

[gar]

100227-2227 EST

Part 77:

I think for your purpose that it is a mistake to consider incandescent bulbs.

See if there are any fluorescent fixtures designed for 12 or 24 V operation in trailers.

There are some 6" fluorescent battery powered lights found in flashlight areas. The one I have uses 4 AA batteries. Thus, 6 V. You could get or make a moderately efficient stepdown supply to power these. There may be larger battery powered fluorescent lights available, and maybe at 12 or 24 V.

Note: a fluorescent that produces the same light output as an incandescent will do so with about 1/3 the input power.

Then you could also consider LED lights that are designed for 12 or 24 V operation. This cost is considerably higher. Any LEDs that you might consider should include a high efficiency converter from the input voltage you desire to the current required by the LEDs.

Converting to a 12 or 24 VDC system for you purpose makes a lot of sense. The 24 is better because of the reduced distribution wiring losses. When going to fluorescent or LEDs it is most efficient to use the DC directly rather than including an additional inverter with its losses. If you continue with the incandescent approach put both batteries in series, use large distribution wire, and use 24 V bulbs.

.

 

[dbuckley]

I haven't pulled the specs on the battery, but generally, if you try and discharge them in less than 10 hours you'll get less power out than you thing. So you can probably get 14.4A at 12V for 10 hours, which is about 150W. Go faster and you'll loose capacity.

 

[Part 77]

ty,
I am going to install a GenTran 300660 30-Amp 6-circuit manual transfer switch for my generator supply. This will feed 6-general lighting circuits. It is a 30 amp-120 volt switch, which matches the generator capacity. It has a flanged inlet to receive power from a cord.
The inverter supply will feed into the same flanged inlet. The only exception for the inverter system will be that the appliance circuit in the kitchen that powers the refrigerator will be turned off and of course the limited use (a max of 4-5, 60 or 75 watt lamps, with an average of 2-3 in use). This transfer switch will be a permitted and inspected installation. In characterizing this as a DIY project, I'm sure I raised some red flags. Poor choice of words. Besides making a living as an electrician since 1977, it is also my hobby at times.

gar, While those are reasonable suggestions, I don't want to install any new fixtures and associated conductors.

 

[ttokash]

Part 77,

You will need to know what load you are going to power. The batteries you've described can put out more than 1000 watts, just not for very long. 72 AH tells the actual energy stored in the batteries. The rate (watts) at which you use it is determined by the load.

Note that you should never discharge a lead acid battery to less than half of its total capacity. Any time you discharge an AGM or Gel battery, you shorten the number of charge discharge cycles that you can get from the battery. The deeper you discharge the battery, the shorter its life. There's a lot of info on the web on battery capacity and charge/discharge cycles. Look around and read.

Having said that, you will want to reduce the load that you want to power from these batteries to get the longest usage from them. Fluorescent lights or LED lights are an excellent suggestion. 72 AH batteries are not very big and will not last long under heavy loads. 72 AH (because you don't want to discharge them to less than have their rated capacity of 144 AH) at 12 volts gives you 864 WH or 0.864 KWH. Look at your electric bill to see how many KWH you use in a month. It's a little frightening isn't it.

Inverters are inefficient. You lose 10-20% of your power simply due to their lack of efficiency. If you can use 12 volt lamps, similar to camping lamps and the like, you will skip this loss of power. Your lamps may last all the longer. Then, simply find a battery charger that you can permanently attach to the batteries. I've read articles where people wired their house with automotive style (cigarette lighter) 12 volt outlets so they could avoid using an inverter. That seems a little extreme for me, but it's not a bad idea.

Lastly, keep an eye on wire size. To get 300 watts out of a 12 volt battery you will draw 25 amps. This means the wire on your batteries must be at least 10 AWG. Larger if your inverter is very far from your batteries. Be careful.

Good luck with your project. I had a similar need. I live in a rural area and supply my water through a well. During power outages, we were without water. I set up what amounts to a UPS for my water pump. It uses four 92AH batteries and an inverter/charger that automatically keeps the batteries charged when power is on and switches over when we lose power.

 

[techntrek]

I was about to jump in, but ttokash covered what I was going to say. Reduce your load, you really only have 72 AH available to you (up to 80 or so occasionally), you loose the 20% or so due to inefficiency.

I'll add: most small inverters use 1 to 2 amps just to run, so add that to your figures. Also, they tend to be most efficient at 75% load, so find out what your actual load is and that will determine what size inverter to buy.