Inverter question - power saving mode

tomba · April 2010

Hello, first post. Building a backyard system with a Morningstar Sunsaver 10 Amp 24V, and 2 Suntech 180W 24V panels in parallel. I have yet to pick an inverter. I have somewhat committed to 24 Volt land, want to run an outdoor frig (300-400 kWh/year), weed whip, "saws-all", etc. So I am aiming for at least a 600 Watt, possibly a 1000 Watt 24 volt true sine inverter. For starters I have 250 A*H of cheap Costco marine battery. I realize I may want to double that, but we are just getting started.

So picking the inverter: The Exeltech brand looks great, made in USA, recommended by the hosts of this site - super. But it doesn't have a power saver mode that I can tell. The no-load consumption on the 600 W is 10W, 20W on the 1 kW unit. This seems significant, and in the range of 10 percent of my production at least.

The Samlex units (and the similar Go Power units on Amazon.com) have a 3W power saver mode. I presume the unit can sample the resistance on the 120 V end, and powers up when that drops low. This would be great if the frig is compatible with this. But does the refrigerator thermostat need to be drawing small current to operate anyway? Because if it does, it obviates the benefit of purchasing an inverter with a sleep mode, as the inverter will always have to be 'up', powering the thermostat.

Yes, I know get a Kill-A-Watt. I have one, but I don't have the refrigerator yet! Costco Sanyo 3.7 L unit is what I am aiming for now. But likely this is a general frig question that others have faced. Thanks for any help. I have benefited from hours of lurking here and reading many posts, but I haven't seen a thread along these lines.

The basic question is does a power saver mode on an inverter actually work with a refrigerator as the sole load.

niel · April 2010

Re: Inverter question - power saving mode

i don't think this will quite work for you. the pv's vmp are rated too low for single use on a 24v battery system without putting another in series with it and in doing that you'd want an mppt controller to minimize the losses from the higher voltages. 24v battery banks need to have pvs with a vmp of at least 34v either by itself or with 2 in series. i think you may have confused the voc with the vmp. the 34-35v vmp area would be good for pwm controllers, but when it starts to look like 25.8v vmp x2 =51.6v vmp total the excess is wasted without mppt.
now your choice of 10a for the cc would work with those 2 180w suntechs in series, but with the loss of at least (51.6v-35v=16.6v x 6.98a imp

115.37w. i don't know if you already purchased those stated items yet, and i'm assuming by your talk you did, but you need to make some changes for it to work well. please tell me i'm wrong and you didn't do a ready, fire, aim.

tomba · April 2010

Re: Inverter question - power saving mode

Thanks for the reply. The Vmp on the panel is 35.6 V, with Voc of 44V (Suntech STP180S-24). I think this is suitable for the little Morningstar CC, right? If not, I can't think of a panel setup that would be(??). I realize a series setup would be a huge waste without MPPT. I did get the charge controller already but it was only 50 dollars. I was eying the Rogue, but knew I wanted to reserve money now for a good sine inverter and thought I could get by for now with this basic charger.

The story is I knew I wanted to get started and bought the solar panel first to put some fire under me to get it done. But before I commit to a 500 dollar 24V inverter, I figured I better run it by the folks who know. And the truth is, I have one PV panel now, but plan to expand with an identical panel, have the combiner box on the roof ready to accept it, rack is ready, etc. So it was a fire, ready, aim situation that I put myself in, on purpose, but the damage is not yet complete.

BTW I put the Kill-A-Watt on our frig here at work and it's reading 0.00 Amps, so at least for this frig, the thermostat is simply mechanical and requires no standby power. Generalizing, an inverter with standby power mode would be well suited for a refrigeration-only application, so unfortunately I am shying away from the Exeltech inverter. Too bad cause it seems an otherwise great product.

tomba · April 2010

Re: Inverter question - power saving mode

The charge controller is actually the 20 Amp unit : Morningstar SS-20L

Cariboocoot · April 2010

Re: Inverter question - power saving mode

All inverters have a "power save" mode. It's called "OFF". If you want on-demand power, there's a price to pay.
But I agree with you about those units' consumption: 20 Watts is what my 3.5 kW Outback uses when it's twiddling its electronic thumbs.

To actually answer your question, yes a refrigerator should draw enough to bring an inverter out of hibernation. But there could be issues with it too; the 'frige will have a whopping great start-up surge which could trigger the inverter to start and then fault immediately even though it is perfectly capable of running the unit otherwise.

I have to agree with Neil that your proposed set-up will prove disappointing. Not sure why you chose 24V if your only going to have loads under 1 kW. Also, you have 360 Watts of panel with a potential of 15 Amps output @ 24VDC, so a 10 Amp charge controller isn't suitable.

It's good to ask the questions before you spend the money!

niel · April 2010

Re: Inverter question - power saving mode

tomba,
if those are the actual specs for the pv then you should be ok with paralleling 2 of them to charge a 24v battery. i was seeing specs on it like this and may be different model,
STP180-18/Ub-1
WATTS-180 Watts
Open Circuit Voltage (V)-32.70 Voc
Short Circuit Current (A)-7.65 Isc
Maximum Power Voltage (V)-25.80 Vmp
Maximum Power Current (A)-6.98 Imp

the ss20 should work also, but you did specify the ss10 in your original post.

tomba · April 2010

Re: Inverter question - power saving mode

Yes, that was my dumb mistake on the 10A vs 20A controller.

I settled on a 24V battery wiring mainly because the charge controller is cheap, given these panels. Granted, a 12V inverter is usually cheaper. In the 1kW range of inverter where I should probably be looking for an on-demand draw from a refrigerator, however, the prices tend to converge.

Watching the Kill A Watt go from 0 to 700 Watts on this little frig, then settle at 75 W, is a surprise for me. I can believe a 600W inverter might freak out when asked to wake up and provide 700W like now, even if it is spec-ed to handle that surge. I just hope a Samlex or better would be up for it.

The other option now is to shoot for efficiency, retire the PWM charger in favor of a Rogue or Morningstar MPPT, wire for 12 Volts, get a hibernate-mode sine inverter @ 600W which is much cheaper at 12V (ebay is loaded with samlex 12V inverters, not one single 24V) and power up a frig with one 180W panel and hope the inverter can handle the surge. I get 5 sun-hours per day here.

It's a fun project and I appreciate the input.

niel · April 2010

Re: Inverter question - power saving mode

tomba,
as to this,
"I have somewhat committed to 24 Volt land, want to run an outdoor frig (300-400 kWh/year), weed whip, "saws-all"."

these items are heavy duty users of power and with these types of loads the inverter should without a doubt be a sine wave. most refrigs need at least a 1kw inverter and if you power more than one item on it at a time your wattage ability from the inverter needs to be upped. same is true of the batteries, but it all depends on how much you use in a day to determine what you need in batteries. keep in mind you also don't want to drain the batteries beyond halfway or 50% dod. if you do opt for more batteries, and i think you may need to, it should be done soon so as to keep them at relatively the same age and condition and by all means get the same exact batteries to avoid inequalities. more batteries also means a larger need charge wise so more pvs may be needed. if you do goof then these are the type of batteries to goof on rather than the more pricey batteries.

RandomJoe · April 2010

Re: Inverter question - power saving mode

If you saw 700W on a Kill-A-Watt, odds are the startup surge is actually even higher. Not sure if other models are more responsive, but mine is rather slow to update and seeing the real startup surge on it is pure luck.

My standard-size fridge runs about 110W once running, startup surge runs in the neighborhood of 1100W! And it isn't a half-cycle or even 3-cycle surge, it holds that power level for several seconds. Many of the cheaper inverters claim "double rating" surge capacity, but when you look at the specs that's only for a half-cycle or maybe a few cycles which is worthless in this case. So you will need an inverter that can handle the full surge rating within its normal specs, or be sure the one you choose actually has a surge rating of many seconds to maybe a few minutes.

Unless your fridge has some sort of electronic gizmo in it, the thermostat won't use any power in and of itself - it is just a mechanical switch. When the temp warms up inside, it switches on. So the inverter won't see a load until that switch makes. Some inverters will allow you to set the "trip" current for the sleep mode so you could keep it from coming on for small parasitic loads.

Cariboocoot · April 2010

Re: Inverter question - power saving mode

Here's some Fun 'Frige Facts:

A small refrigerator may use as much electricity as a full-size unit, as they tend to be cheap and not well-designed with no thought given to conservation. This is especially true if you measure energy usage compared to size; there's a sort of 'minimum power requirement' just to get the process going.

A refrigerator's power consumption, including start-up surge, will vary depending on a number of conditions including ambient temperature.

Keeping a refrigerator cold can use less over-all energy than allowing it to warm up and then re-cool. More power is used to 'cycle down' the unit than would be needed to just leave it running. This is a time/use trade off which will vary between one model and another. Sometimes greatly.

AC induction motors such as used on 'friges will consume more power when run by MSW than PSW. They will not operate as efficiently, and will have a shorter motor life.

You might be better off leaving it run 24/7, and designing your system accordingly. Especially if you need the kind of power required for AC tools.

tomba · April 2010

Re: Inverter question - power saving mode

These little 4 cubic foot bar frigs use more energy than an energy star chest freezer.

I agree that one would definitely want to size the inverter to handle the surge within its normal spec. I still think a GOOD 1KW inverter will hang in there. I see another pair of batteries and the second panel in my near future. A 2 kW inverter @ 24V is becoming prohibitive. The 2kW Xantrex sine wave inverters @ 12V are still reasonably priced. There's some difference between the ProWatt and ProSine versions they sell, but I haven't figured out what it is yet.

If I switch to a Morningstar 15A MPPT so I can run 12V batteries and a somewhat cheaper sine inverter, as far as I can tell I can only use a single 180W panel, as the 15 Amp limit * ~15V charging voltage maxes me out at 225W. I'd have to get a Rogue (clearly there are worse fates in life). This is one reason I was happy with a 24V bank and a 48 dollar charge controller, and I have to keep reminding myself of this! I don't think I'm wrong on that limitation.

Thanks all.

Inverter question - power saving mode

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