Is it OK to under-power an inverter?

So I am facing a temporary situation where I need to run a computer off a car battery.  The current cigarette lighter inverter I have won't cut it so I need something bigger.

I was wondering about skipping the intermediate step and getting something large that I could later reuse for future alternative energy.  The thing is, how do they take to being under-powered?  Like, say, it was a 10kw inverter, obviously one tiny battery can't deliver that, but my load won't need that much either.  Can it cause problems to under-utilize an inverter like this?

Not that i know. As long as the voltage of the batteries stay into the tolerance range of the inverter, the low load will not be a problem and the inverter should work okay.

K Eilander wrote:So I am facing a temporary situation where I need to run a computer off a car battery.  The current cigarette lighter inverter I have won't cut it so I need something bigger.

I was wondering about skipping the intermediate step and getting something large that I could later reuse for future alternative energy.  The thing is, how do they take to being under-powered?  Like, say, it was a 10kw inverter, obviously one tiny battery can't deliver that, but my load won't need that much either.  Can it cause problems to under-utilize an inverter like this?

realistically don't go too big on a 12 volt inverter. Anything above 3000 watts gets to a size where the wires can't deliver that kind of amperage anyways. Also for a larger system you don't want to lock yourself into a 12 volt infrastructurevas it's very limiting.  So my advice would be to get something bigger but not the eventual home inverter.

I think David B. gives sound advice. 12VDC is not great for a larger, fixed solar system.

In addition, I would be concerned that a really smart "big capacity" inverter would sense that it is under-supplied and give you trouble.

I say get yourself a reasonably sized pure-sine-wave inverter for now. The cost has come down an unbelievable amount.

Short answer is no.  12V is a poor choice for large systems.  You can NOT buy a 10kW 12V inverter.  They simply do not exist.  Realisticly, the best you can hope for is 1500W at 12V.  Sure there are 12V inverters that SAY 3000W.  Check though to see if they are UL-listed.  They are scam products in name only.  

Keep in mind that a car battery is even a poor choice at 12V.  Car batteries are designed to release a large amount of current quickly to start the engine, and then be IMMEDIATELY recharged.  That is not what happens when you use a battery for off-grid.  It will work, sort of, for a couple of days, but the quality of the battery will start to deterate almost immediately.  Once the battery can no longer supply what the inverter wants, it will simply shut off.

You haven't mentioned anything about how the battery will get recharged?  Please don't say "I'm going to hook up a 100W solar panel to it"!  A 300W high-voltage residential grid-tie panel is a good choice, if you have an MPPT controller.  A PWM controller looks cheaper, but in the real-world, the extra panels you need to buy to keep up with a MPPT controller actually makes the more expensive MPPT cheaper.

Could you explain the current application for the inverter?

Any inverter that can continuously do 10kw will cost over $3k and the wiring would be expensive at $2-$4/ft. You'd also need to times this times two to account for both polarities.

If you reply back with what you plan to run, I can give better advice on the car battery situation. Because so far your plan will require you to run the car which in turn would make it cheaper just to buy a gas generator or solar generator(jakery/renogy), which would cost $800-$2k and leave you with $1k in saving.

K Eilander wrote:So I am facing a temporary situation where I need to run a computer off a car battery.  The current cigarette lighter inverter I have won't cut it so I need something bigger.

Can you go into a bit more detail as to what is actually happening here?  Are you between jobs right now?  Do you have to spend some time living out of your van for a few weeks because you've gotten laid off?  How long might you expect to stay in this situation?

If you're between jobs, and just need to sleep in your truck for a few days/weeks, most likely you'll want to run things off your truck battery, but keep in mind that it's long-term health will be effected.  I suppose though if you have to shave a year or two off your battery's life right now just to get by, then maybe an inverter in the 500-600W range will do for now.  Don't however think that any of this would be saveable for constructing a whole-house system later in life.  12V is NOT the best choice for a large whole-home system.  If you select the right solar panels though, they at least might be useable in a system upgrade.  Everything else, just maybe.  Here is what I would put together if I was staying in an RV for a few weeks.....

1-2 large high-voltage residential grid-tie panels:  Look on Craigslist in your local area and get panels cheap with cash and carry purchasing.  75$ seems like the going rate for a 250W panel
modest sized MPPT controller: An Epever Tracer 3210AN or 4210AN
500-600W inverter: Remember that the inverter consumes power just being left on.  Maybe at least 300Wh over a 24hr period.  Turn the inverter off whenever it is not needed.
DC only appliances like lights, phone chargers, and maybe a laptop charger will reduce or eliminate the need for the inverter.

Once you are in a better position financially, we can further discuss what you can salvage for a bigger whole-house system.

Keep in mind that if you are laying panels flat on a RV roof, your solar output will only be ~60% of what nameplate output would be, because the panels are not at an optimal angle to the sun.  So, that means you need about 40% more watts of solar to make up for that inefficiency.  So, with the 30A controller charging the battery at ~12.5V you could have a max of
(30A X 12.5V)/60% = 625W of panels.

K Eilander,

Going back to your original question, I think that if you have a good inverter, you won't be able to underpower it.  I am tinkering with an inverter project myself, and I have a 1500 watt inverter.  However, my 20 ah LiFePo4 battery will only put out 20 watts at best.  This means that the best my battery can hope to put out is 240 amps (20 amps x 12 volts = 240 watts).  I tried wiring the inverter in and powering it on, just to see if I could power a minimal load like a USB wall charger.  As soon as I turned the power to the inverter on, the power undervolt protection tripped and it immediately squealed at me and put out no power.  I can only assume that the inverter will successfully work if there is sufficient power to the device.

Just to add into the fun, LiFePo4 batteries are somewhat limited in their power output (amps).  Basically, a LiFePo4 will yield up a maximum of 1 C, or one multiple of the amp-hour rating.  In this case, the amp hour rating is the same as the amp output.  But lead batteries can put out 2-6 (or even more!) C.  A car battery might well be able to put out a very high C rating so that battery might support your inverter.  But as has already been mentioned, car batteries are designed to put out a whole lotta amps for a short period of time, after which they need a recharge.  

Try it!  Maybe it will work.  At the worst, you will find what works and what doesn't.

Good luck,

Eric

yes, you can use an inverter at below its rated capacity. it's good to do so as that inverter is built for way more amps than you're putting thru so will run cooler and more efficiently than an inverter that's only just sized for your appliance.

get a pure sine wave inverter. these are MUCH more efficient than square/modified inverters.

be careful of ruining your car batt. if you run it below 12.4V and/or leave it discharged for any period of time then it will quickly deteriorate.

Steve Farmer wrote:yes, you can use an inverter at below its rated capacity. it's good to do so as that inverter is built for way more amps than you're putting thru so will run cooler and more efficiently than an inverter that's only just sized for your appliance.

get a pure sine wave inverter. these are MUCH more efficient than square/modified inverters.

be careful of ruining your car batt. if you run it below 12.4V and/or leave it discharged for any period of time then it will quickly deteriorate.

Hi Steve what you mentioned is generally true to a point. A large inverter running a medium sized load is at it's most efficient. A large inverter running a small load would not be efficient as the stand by losses of the large inverter are a constant and the first 100 watts it generates are very inefficient compared to running a medium sized load. Every inverter has an efficiency curve.
Cheers, David Baillie

It's generally not recommended to under-power an inverter, mate. It could lead to some performance issues and might not handle the load you're trying to put on it. It's better to go for an inverter that matches or exceeds your power needs to keep things running smooth and steady.

David Baillie wrote:...the stand by losses of the large inverter are a constant...

agreed esp as the OP here mentioned a 10kW inverter. this is likely to run an operating system, wifi & bluetooth modems, temperature monitor, lcd,/led panel....

A 2000W brick inverter is likely to have none of this, just maybe a green light and a fan that shouldn't have to come on at low load.