Selecting a ~1500W inverter

[jons]

I have a 20W solar panel with a charge controller charging a 12V 115AH deep cycle battery.  This system is working very well for weekend use at a remote cabin for lighting and water pumping.  It gets to charge all week, so I have plenty to use for the weekends.

I'm doing some minor renovation work to the place from time to time, and I have a gasoline generator there to run my table saw, circular saw, etc.  Very light use of the tools overall, probably cumulative less than 15 minutes of the tools actually running per weekend.  I'm not trying to run an air compressor or anything for a long time.

I'm tired of the noise from the generator and having to constantly start and stop it and gas it up and change the oil, so now I am looking into inverter options for this kind of usage pattern.

I would add a second 12V 115AH deep cycle battery in parallel to get more capacity (though I'm not sure I really need it), and I'm trying to find the best deal on an inverter that will meet my needs.

Here are my questions:

1. Inverter sizing - the table saw is the biggest possible load, with a 15A maximum draw.  I haven't measured it, but I doubt that it draws 15A unless it is under heavy load.  So, do I get a 15A * 120V = 1800W minimum inverter size, or will a 1400W unit with surge capacity (or even smaller) work?  When inverters say they can handle 2x surge, how long can they handle it?
   
2. Sine or Modified Sine - I have read that most tools run fine on the cheaper modified sine inverters, but variable speed tools and other items can have problems? What experiences have you had?
   
3. 12V @ 115AH = 120V @ 11.5A for 60 minutes (fully charged, assuming no loss).  My cumulative use is only 15 minutes or so at an average of 10A, so do I need another battery?
   
4. Suggestions for the best place to buy an inverter?
   
5. There are lots of brands of inverters that look pretty similar (some have nicer meters than others, some with or without cables).  Are there reviews of PowerBright vs Cobra vs AIMS vs  Are some of the inverters really the same with just different branding?
   
   

Here are the inverters I have looked at so far (all modified sine) - any comments/suggestions?

http://www.invertersrus.com/

AIMS 1250W, $129

AIMS 2500W, $289

http://www.amazon.com

Xantrex X-Power 1750 Plus, $249

Coleman Powermate 2000W, $249

http://www.voltageconverters.com/inverters_110v.html

PowerBright 1400W, $144

PowerBright 2300W, $279

http://www.powersystemsdirect.com/Cobra/2500_Watt_Power_Inverter_CPI-2550_329.php

Cobra 2500W, $219

I could go on and on - how to choose?

[maker of toys]

Sorry, can't recommend an inverter.  Others have recommended a Kill-o-watt (r) meter to see what you really need.

BUT: I can say these things:

 induction motors will be happier on a sine inverter.  and they want a BUNCH of surge capacity to get up to running speed.  (figure 5 seconds?  That's beyond the limit of any inverter I've ever seen.  Remember the inverter isn't going to pull up a motor as fast as a genset or the grid.)

  Universal (brush) motors (Circular saws, drills, routers, dremel tools, etc.) don't care what sort of inverter you're using; they'll even run on 120Vdc just fine.  the variable speeds will be touchy but otherwise fine on anything that crosses zero regularly; if you use them on DC, the variable speed won't work, but the motor will happily run at 100% RPM.

1200 watts out of the inverter is 100 amps at 12 volts, with no loss.  That's a bit much for a single deep-cycle battery.  By the time you figure copper loss, conversion loss, etc, at 1800 watts you're going to be pulling close to 200 amps. you're in starter-battery or electric vehicle territory at that point; most import cars need about 200 amps for 2 seconds or so to get started.

Also, that 115 Ah rating is assuming you take 20 hours to drain EVERY last joule out of the thing.  for durability, figure on drawing no more that 1/2 that in 24 hours. . . the EV guys figure no more than about 20% of your capacity per hour for the sustained (more than 30 sec) discharge rate.  True, you can get more, faster, but you're going to pay for it in physical battery deterioration.

better get SEVERAL more batteries. . .

Run the big stuff on the genset. Drills and the like will be happy on an inverter.

Get a big bottle of CO2 for your air tools, if you have any.  A 20lb bottle and 150psi preset regulator ran me about $200; fills are about $2/a pound. (a 20 lb bottle lasts me a LONG time. . .  I use it for airbrushing and tire filling, and get about 6 months of occasional use per fill.)  Your tools will last longer, too. . .  no water = no rust.  just remember to oil them!  I figure a pound of CO2 will give the same service as 1/2 gallon of gasoline in a construction compressor, assuming mostly idle time.

[Volvo farmer]

You can get them cheaper than that. I've got an Xantrex Xpower 1500 in my garage. They're regularly on Ebay for less than $100 and they come with cables, though the cables should be upgraded for extended periods near the 1500W limit. I've put mine through some serious abuse but not over an extended period of time. So far, no problems.

Maker of toys gives good advice. My little system is 160W panels, 4xgolf cart batteries and this inverter. If you add tool use to the water pumping and lighting loads you already have you might want more panels and more batteries.

I've got this Craftman circular saw that's rated at 13A, For a while I thought the surge current was too big because it would cycle on and off without starting. The inverter never went off on overload though. Yesterday I was out there and really didn't want to start the generator for 20 minutes to make about a dozen cuts on 2x4s. So I held the trigger for about 30 seconds and it eventually ramped up and ran. After that it seemed to start within 7-10 seconds for all the rest of the cuts. Once it was running, it would cut a 2x4 no problem. I don't know if it was the saw or the inverter that "learned" to start but I was happy about it.  

[RobC]

One thing I can say is that Cobra will stand behind their warrantys. I had a friend who accidently connected his to the grid not only once but several times. Finally it quite working. Then he calls them and up and told them what he had done. They fixed it sent it back free of charge postage paid. Nobody else is going to do that! RobC

[Ungrounded Lightning Rod]

You're going to want more batteries if you are going to run a serious motor even intermittently.  You can suck down a single battery into taking-damage level with a motor load in an amazingly short time - and (as was pointed out above) you're pulling 'way more current than the battery was intended to source.  And you can run out of power very quickly if you're not paying attention, leaving yourself with no lights and water at night for the rest of the weekend.  (Pulling it into the BOTTOM 20% of charge will damage it right away, and a good inverter will shut down if you're approaching that.  With a bad inverter you'll go ahead and pull more.  Do that just a couple times and you'll find your lights don't work any more.)

You're probably cyclng your battery significantly already.  The reason for the TOP-20%-only discharge limit is not just to have emergency capacity, but to avoid deep-cycling the battery.  You only get a couple hundred of those.  With weekend use you can deep-cycle it and only replace it ever few years.  Someone deep-cycling daily might be replacing batteries more than once a year.

Once you've added more batteries you'll want to add more panels - at least one more if you're going to use the genny when you have to make more than a couple cuts per weekend.  With a 20 watt panel and a 115 AH battery you're putting under half-charge into your single battery now over the week, so it will take you two weeks to make up a deep-cycle.  Multiply by the number of batteries to get the number of weeks to get back to full charge if you actually make use of their deep-cycle capacity.  Then divide by the number of panels to see how many to add.

Even without additional power usage the added batteries also mean added leakage losses.  Those have to be made up and so they come out of your charging rate before you can accumulate anything in the batteries.  Like the charge, the leakage adds up over time.  So you'll want at least one more panel if you add batteries even if your usage doesn't change.

No answers for your inverter questions.  But I thought I should mention this other issue.

[jons]

Thanks - this is a lot of good info I hadn't fully considered.

Where am I off in my math here: 10A @ 120V = 1200W = 100A @ 12V * 15 min = ~20% of a single 115AH battery.  As long as I don't exceed 15 minutes of inverter use (with little or no other use) then I am staying roughly within the top 20%?

Also, how does the math work for figuring out the amount of charge I put into my battery over a week?  I probably have this wrong, but I was figuring that 20W * 4 hours/day peak panel output * 7 days = 33600 watt minutes.

1200 watts for 15 minutes = 18000 watt minutes, so I may have excess charging capacity to cover a second battery?

Of course I am not figuring any loss, and every day is a sunny day ;-)

[jons]

Thanks - I am tempted by the Cobra, it looks like a good price for the size, but I like the digital readouts on the Xantrex and the PowerBright.  Cobra does have a very nice support page with all the manuals though.

Interesting terminals on the Cobra - it looks like they grip the wire directly rather than a post/lug attachment.

[jons]

You say your 1500W inverter doesn't start your 13A circular saw normally, like it was running off a generator or the grid?  Is this typical for other inverters too or do you think it was a cable capacity problem?

[Volvo farmer]

I think it has something to do with the "soft starting" circuit built into these inverters nowadays. I think the inverter pulses the power on and off to try to get these huge inductive loads started without burning itself up or going into fault mode. I upgraded my cables to the next size up so I don't think that is the problem.

[ghurd]

I think that's probably right.

Some items will start easier if you give them a turn by hand when throwing the switch. Use some common sense doing that.

[Ungrounded Lightning Rod]

Where am I off in my math here: 10A @ 120V = 1200W = 100A @ 12V * 15 min = ~20% of a single 115AH battery.  As long as I don't exceed 15 minutes of inverter use (with little or no other use) then I am staying roughly within the top 20%?

If you use watts you miss several things:

1. Solar panel wattage is rated at a higher voltage than you get when you're charging.  The difference is pure loss.  (It's not deliberate deception:  Some governments mandate that they rate them that way.)  They're deliberately somewhat overvoltage so they can still charge when they get hot and their voltage drops as a result.
   
2. Batteries are far from 100% efficient.  Most of the loss is from charging at a higher voltage than they discharge due to internal resistance and the potential necessary to drive the reactions.  (Some is from leakage currents.)
   
3. Batteries don't charge at max rate as they approach full charge.
   
4. Inverters (and the wiring to them) are far from 100% efficient, too.  Most of the loss is voltage drop due to current, so the PERCENTAGE of loss goes up with the power drain.  (The inverter pulls even more current to compensate.)
   
   

You can avoid most of the pathology of 1) and 2) by using amphours rather than watts.  The panel's output current, is proportional to the amount of input light, regardless of the output voltage (as long as the panel is loaded adequately).  Except for leakage (which is a small part of the losses) and electrolysis losses (which happen mainly when you're fully charged), amphours out of a battery are about the same as amphours in.

3. isn't really a problem because you're not interested in getting the batteries to absolute tip-top - just back to where they were the week before.
   
4. is significant and you just have to take it into account when you compute the amphours you're draining.
   
   

Also, how does the math work for figuring out the amount of charge I put into my battery over a week?  I probably have this wrong, but I was figuring that 20W * 4 hours/day peak panel output * 7 days = 33600 watt minutes.

4 solar hours per day is probably a good rule-of-thumb number to use if you're in the temperate zone.  (Probably a tad low so use that to let you ignore leakage.)  Figure you're getting maybe 1 1/3 amp out of your 20 watt panel during charging at noon-light levels.  (I'm not sure if they're rated at 14.5 volts or what so assume 15 volts.)  That's 5 1/3 amphours / day, or 37 1/3 amphours per week.  So you can recover from about a 32.5% (call it 1/3) discharge of a single battery in a week of good weather.

With more batteries you can pull more power without destroying them.  But you can't pull any more power if you want the panel to replace it.

Let's be conservative (and use round numbers) by assuming the losses in the inverter are equivalent to discharging the battery at 10 volts.  You get 325 watthours at full load.  That comes out to 16 minutes of operation at 1200 watts - to eat a whole week's charging, with NOTHING left over for lights, radio, water pumping...

How's that?

[Jeff7]

I've read that it's not a good idea to parallel batteries. Maybe for discharging, ok, but NOT for charging. To clarify this, I'll also use proper terms.

Cell - In the case of lead-acid, one single container that produces 2VDC

Battery - a group of cells hooked in series

Notice how cells are always in series everywhere? If you put 4 AA's into a device, they are connected in series. A 9V battery - 6 1.5v alkaline cells in series.

Now, translate this to charging. Two cells in parallel. One sucks up ALL the charge really quickly, either due to slightly different chemistry, or beginning charge state. The charger will keep applying a good bit of current to charge the other, dead cell. This will overcharge the charged cell, damaging it.

Same thing with paralleling two 12V batteries. One gets charged first, while the other is still not quite as charged, but the charger keeps pushing current into both of them. Soon, one of your expensive batteries is dead.

Series hookup helps to avoid this problem.

How do they make high amperage ratings without parallel hookups? Each cell is simply made bigger. A AAA cell is tiny, and is 1.5V (alkaline). A D cell is larger - much more amperage, same voltage.

So figure out if you want to go through with the project - then figure out how many 12V batteries you're going to want, multiply that number by 12, and that's what the input rating of your inverter will need to be.

[jons]

Update - After searching for as much info as I could on various inverters and weighing the price, features, and reputation, I settled on the Xantrex XPower 1750 - and since my first posting the price went down too, now its $199 with free shipping from Amazon!

Very nice documentation, very good, quick technical support and customer service to answer my questions.

http://www.xantrex.com/web/id/24/p/1/pt/8/product.asp

Just got it the other day, picked up some 20" #2 battery cables from the local auto parts store, hooked it up to my battery at the camp, and plugged in the circular saw to cut some plywood and....it works great!  All of my tools start instantly, just like it was regular grid power.  None of the delay or slow start I was worried about.

I checked the voltage drop on the supply wires and I am in good shape, probably because of the very short 20" run.  Only 0.1 volt drop over the wire at full load.

The voltage meter and load meters on this inverter are very nice, and the unit appears to be well made.

I will probably add a second deep cycle battery at some point to give more capacity, and maybe more panels, but my light use of the tools this last weekend hardly drained the battery at all - resting voltage after the day was 12.4V

I really should have a fuse in the supply line, but this is for intermittent use, so I'm considering a battery disconnect switch rather than connecting and disconnecting the cables all the time (the inverter has internal fuses, although they are not user replaceable).  Can anyone recommend a reasonably priced battery switch that can handle the amperage?

[rcmron]

For Jons, and anyone who is wondering about Powerbright vs cobra vs aims. I am no expert but my powerbright 6000w inverter failed to provide capcity and equipment such as TV's etc were tripping at very light loads. This happened 9 months after I had it, 3 months before the end of the 1 year warranty,  and power bright stopped at nothing to come up with excuses why not to provide warranty. I was amazed at the excuses but in the end they just said no. Fun company to deal with.  This inverter is definitely a buyer beware item. I also found out that powerbright refurbish inverters and retailers sell them as new. I believe that they make the inverters for energy plus. The meter that shows volts and power draw never worked well. In the end, they out right refused to provide warranty. I now have a cobra 2500 watt and it seems to work fine but i do not know what inverter to reccomend or which is the best. I do on the other hand know which one does not work well and which company does not provide good customer service, from my experience, and would sugggest that if you do not want to have hassels with your inverter that you not buy a power bright inverter.

Does anyone have experience with cobra inverters in cold climate?

Does anyone have expereinece with other inveres that work well in cold climate,  ie -20 degrees C?