Dumb inverter question

WMH Cheryl · March 2014

Since I am off grid I have found it easier and cheaper to have smaller solar systems in various
locations around the ranch. My current project is to install an ozone water purification system on my 2500 gal water tank.
Since it only draws 55 watts with no surge, etc (it runs 24\7 so 1320 watts per day), then should I get the smallest inverter possible?

Like a 80 or 100 watt inverter? My fear is that those tiny inverters don't seem beefy enough for 24 /7 use.

Bottom line, do I "waste" battery when I use a large inverter to run such a small load? Or should we size inverters close to max load? I am also setting up another system to run a refrigerator and even though it will only average out to about 800 watts per day, I know they have a big surge. Since it isn't purchased yet I can't measure start up requirements. Do you think 1000 watt inverter can handle it since they usually have about 1600 watt surge capability?
Thanks for your help and input. All my other systems I just turn on inverter when needed (no 24 hour\continuous use), or they operate on 12v DC with no inverter concerns.

jcheil · March 2014

Re: Dumb inverter question

WMH Cheryl wrote: »

Since I am off grid I have found it easier and cheaper to have smaller solar systems in various
locations around the ranch. My current project is to install an ozone water purification system on my 2500 gal water tank.
Since it only draws 55 watts with no surge, etc (it runs 24\7 so 1320 watts per day), then should I get the smallest inverter possible?

Like a 80 or 100 watt inverter? My fear is that those tiny inverters don't seem beefy enough for 24 /7 use.

Bottom line, do I "waste" battery when I use a large inverter to run such a small load? Or should we size inverters close to max load? I am also setting up another system to run a refrigerator and even though it will only average out to about 800 watts per day, I know they have a big surge. Since it isn't purchased yet I can't measure start up requirements. Do you think 1000 watt inverter can handle it since they usually have about 1600 watt surge capability?
Thanks for your help and input. All my other systems I just turn on inverter when needed (no 24 hour\continuous use), or they operate on 12v DC with no inverter concerns.

For the 55w 24/7 load, a small one would be fine. As long as you get a good quality one, like the 300w one the host sells you should be fine. You would only be running it at 1/6 of it's rating. Most devices don't like to be run at 100% all the time and will likely fail sooner. Plus you won't find a "quality" one smaller than 300w. Avoid cheap - china inverters.

For the fridge, a 1000w inverter is likely pushing it. Again, depends on the inverter. If it is a QUALITY inverter rated at 1000 and 2000 surge, it may be ok. However, I know for a fact that the cheap ones that claim 1000/2000 will be a problem. My neighbor had a cheap 2000/4000 12v inverter and THAT would not start the fridge. Just kept cycling as it tried to start it.

If the only thing on THAT inverter/system will be the fridge, then you would likely be better off with a DC fridge, even though they cost 2x as much. Because yes, running a big inverter consumes power even when it is not doing anything. Sometimes 20-30w+ which can add up to a 400-500w/h per day. In "search" mode they can drop down to 5 or 6w, and if the fridge is the only thing on it search will work well, but if you have ANY other loads on it, then search will not work.

qwertinsky · March 2014

Re: Dumb inverter question

It may be a little late but you might want to consider one of the old camper style refrigerators that can be run from AC/12VDC/Propane.

These do not have a compressor, and as you have probably found out you will have to dramatically oversize your inverter to handle the current demands needed to start a compressor.

The propane refrigerators work by evaporation you just heat (I think its Ammonia) that is sealed inside the system. It's evaporation cools the inside of the fridge. No compressor, now moving parts, and silent operation.

This also gives you the ability to run it off of Propane too.

Cariboocoot · March 2014

Re: Dumb inverter question

qwertinsky wrote: »

It may be a little late but you might want to consider one of the old camper style refrigerators that can be run from AC/12VDC/Propane.

These do not have a compressor, and as you have probably found out you will have to dramatically oversize your inverter to handle the current demands needed to start a compressor.

The propane refrigerators work by evaporation you just heat (I think its Ammonia) that is sealed inside the system. It's evaporation cools the inside of the fridge. No compressor, now moving parts, and silent operation.

This also gives you the ability to run it off of Propane too.

Unfortunately these refrigerators can be horrible in the Watt hours use because they use a heating element to replace the propane flame. So on the one hand you have less start-surge on account of not running a compressor but on the other hand you can have greater Watt hour demand from having to run a heating element. It can add up to more power used rather than less, particularly if weighed against the size of the refrigerator.

WMH Cheryl · March 2014

Re: Dumb inverter question

Thanks a bunch. Yes a DC frig would be ideal but I have"t found an affordable one that wasn't 3 to 4 or more times more pricey then the ac one I was considering.

zoneblue · March 2014

Re: Dumb inverter question

WMH Cheryl wrote: »

Thanks a bunch. Yes a DC frig would be ideal but I have"t found an affordable one that wasn't 3 to 4 or more times more pricey then the ac one I was considering.

Dont jump to conclusions... do the math.

DC fridge, 2X cost, fridge itself more effcient.
AC fridge, plus inverter cost, plus inverter inefficiency, plus tare loss

DC:
800Wh/day. Fullstop.

AC:
1000Wh/day * 1/0.85 + 20W*24 = 1660Wh/day

Assuming a system yeild of 2Wh/Wp, and a system cost of $5/W, that means you need to spend an extra:

(1660Wh-800Wh) *1/(2Wh/Wp) * $5/W = $2150

on your complete PV system, ie extra batterys, panels etc. (and this doesnt include the fact thay you save on the inverter).

How much more of a fridge does that buy?

Cariboocoot · March 2014

Re: Dumb inverter question

Well around here the price difference between a conventional refrigerator and a DC one was more than enough to provide the extra AC power as well as buy a larger capacity refrigerator.

Things don't cost the same everywhere.

WMH Cheryl · March 2014

Re: Dumb inverter question

Very true, but since I already own the panels, charge controller, and possibly a couple AGMs that might work and a temporary cheap inverter, then $200-300 is far easier to come up with then $1000+

Does anyone ever turn off their fridge and inverter at night? Would a chest freezer (used for refridge only) keep things cold over night until solar was back the next day? Guess I will have to experiment.

Cariboocoot · March 2014

Re: Dumb inverter question

WMH Cheryl wrote: »

Very true, but since I already own the panels, charge controller, and possibly a couple AGMs that might work and a temporary cheap inverter, then $200-300 is far easier to come up with then $1000+

Does anyone ever turn off their fridge and inverter at night? Would a chest freezer (used for refridge only) keep things cold over night until solar was back the next day? Guess I will have to experiment.

Depending on what inverter you have, some have a "standby" mode which powers down the actual inverting portion and checks for a load every so often. If one is found it powers up again to run it. In theory this saves Watt hours used by the inverter (may use 6 Watts when in standby, 20 Watts when running for example). For normal refrigerator cycling it doesn't make much difference because it will run about 1/3 of the time anyway.

Converted chest freezers use less energy, but can be annoying to use. Most refrigerators will keep things cool for several hours without running, providing they are not opened.

How 'minimalist' do you want to go?

WMH Cheryl · March 2014

Re: Dumb inverter question

I am super cheap and super frugal. When I had grid in town my gas bill was $10 per month and about $27 for electricity. That was with washer & dryer, refridge, TV, computer, shower,etc. Now I use solar flashlights and lantern and nothing else. Would love to have refrig. But expensive better batts & inverter are the problem.

I would love to find a small cold box that could run direct from panels. That is why I love my new solar well pump. No batts, just direct from pv panel.

waynefromnscanada · March 2014

Re: Dumb inverter question

I would not risk shutting down a fridge at night. It's not worth a good case of food poisoning. Nothing is more important than your health.
A basic chest freezer converted to fridge gives excellent cooling results for little energy use because freezers are insulated like fridges SHOULD be, but aren't. :grr

vtmaps · March 2014

Re: Dumb inverter question

WMH Cheryl wrote: »

I am super cheap and super frugal.
<snip>
I would love to find a small cold box that could run direct from panels. That is why I love my new solar well pump. No batts, just direct from pv panel.

There are several batteryless solar refrigerators available, Sundanzer makes one. But they are not for a super cheap type of person.

--vtMaps

zoneblue · March 2014

Re: Dumb inverter question

Aw Coot, thats a pretty tepid refutation for my decidedly pretty dodgy model ! If its even vaguely right then even a $3000 dollar DC fridge should still be cheaper from a capital cost point of view.

OP, just becasue you have gear, youll find out soon enough that you can never have enough. Hence what you have and how you use it ALWAYS matter.

WMH Cheryl · March 2014

Re: Dumb inverter question

Thanks everyone. I checked out the SunDanzer DDR165 and the BFR105. It will go on my fantasy list for when I win the lottery or $1400+ falls from the sky. My panels are 167 watt (about 22vdc and 7+ amps), so I think it will work.

Dumb inverter question

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