Help sizing an inverter - downgrading from 2000W to 1000W with an AC fridge

[Doug864]

Will be used for the same items as tomthumb listed above so I didn’t want to start a new thread.

 

[Tomthumb62]

've been using a Samlex EVO-1212 powering mainly our AC compressor fridge. It is rated for up to 3x surge current so 3600 watts for this model. It works great and is a step up from using an inverter only.

Thanks for the review on this unit. Looks nice. It might be in line for when we upgrade to an inverter charger.

For the time being though, we want to try to use the generator only for emergencies, not daily charging. The 20W idle draw of the Samlex isn’t much better than our current renogy. I think separate units of inverter and charger are going to help us more in our current use case.

Someone else pointed out that the power saving mode is pointless with a fridge, due to the thermostat on the fridge won’t have any power and therefore won’t signal the compressor to start running. So in my case, a low idle draw is essential. I’m looking for 5-10 watts.

Your comment about the idle draw only happening when the compressor isn’t running is a good one. I had come to believe that the idle draw is the self consumption draw of the inverter itself. So if you have a 12W idle draw plus a 100W load, your total energy use is 112W. When no load is present, then you only use 12W. Do you understand it different than that?

 

[time2roll]

Yes the idle draw is no load. However if there is a 20w load I don't think it will pull 32w. The idle does not fully add to the load.

 

[Tomthumb62]

Just a thought. If you are running it using a transfer switch to the circuit marked "fridge" be careful. What I found is I have other outlets on the fridge circuit. I run a 1500w psw inverter from Windy Nation. Works well. .5a draw at idle.

Jim h

Thanks, Jim. That’s the lowest idle draw I’ve seen yet. And I’ve appreciated the excellent customer service we’ve gotten from Windy Nation. How are the fans on it? I mean are they noisy or come on too often? That’s one thing I’ve liked on or renogy- the fan rarely comes on and when it does, isn’t too noisy in our tiny trailer and shuts off fairly quickly.

 

[Jims94vmx]

Thanks, Jim. That’s the lowest idle draw I’ve seen yet. And I’ve appreciated the excellent customer service we’ve gotten from Windy Nation. How are the fans on it? I mean are they noisy or come on too often? That’s one thing I’ve liked on or renogy- the fan rarely comes on and when it does, isn’t too noisy in our tiny trailer and shuts off fairly quickly.

They do cycle when under load. I sometimes run a tower heater on low and the fans will cycle. I have a 5th wheel and it is only background noise.

 

[50ShadesOfDirt]

A possibility here ... if I understood the (mostly dedicated) loads correctly in OP (usage patterns).

Rather than leaving inverter on all the time, costing X watts per hour per day, have you considered reconfiguring a bit such that the inverter is powered on automatically about 4 to 6 times per day (for an hour or so each time), to match fridge getting to operating temp, and then inverter cycles off other times. Laptop charging or whatnot might also occur during the fridge "on" time.

Anything else runs directly off of batteries (depending on what requirements are). Possibly saves 50% to 75% of inverter idle draw. Gives you a chance to play with "home automation".

I've rec'd in a 24v MPP, so I'll be facing the same idle draw problem (but in a tiny home, truck-topper set of projects); not all parts are in yet, but this was my proposed answer to that problem.

In the main house, we actually run our fridge and freezer units 16 hours per day, and they are "off" during the overnight period of 8 hours, when everyone is sleeping. Stays within temps for us, and saves on (mostly) useless cycling at night, when nobody is using it. Another friend runs his fridge on 4 to 6 on/off cycles per day.

Hope this helps ...

 

[Doug864]

A possibility here ... if I understood the (mostly dedicated) loads correctly in OP (usage patterns).

Rather than leaving inverter on all the time, costing X watts per hour per day, have you considered reconfiguring a bit such that the inverter is powered on automatically about 4 to 6 times per day (for an hour or so each time), to match fridge getting to operating temp, and then inverter cycles off other times. Laptop charging or whatnot might also occur during the fridge "on" time.

Anything else runs directly off of batteries (depending on what requirements are). Possibly saves 50% to 75% of inverter idle draw. Gives you a chance to play with "home automation".

I've rec'd in a 24v MPP, so I'll be facing the same idle draw problem (but in a tiny home, truck-topper set of projects); not all parts are in yet, but this was my proposed answer to that problem.

In the main house, we actually run our fridge and freezer units 16 hours per day, and they are "off" during the overnight period of 8 hours, when everyone is sleeping. Stays within temps for us, and saves on (mostly) useless cycling at night, when nobody is using it. Another friend runs his fridge on 4 to 6 on/off cycles per day.

Hope this helps ...

Pretty interesting info. Thanks for sharing.

 

[RF_Burns]

Thanks for the review on this unit. Looks nice. It might be in line for when we upgrade to an inverter charger.

For the time being though, we want to try to use the generator only for emergencies, not daily charging. The 20W idle draw of the Samlex isn’t much better than our current renogy. I think separate units of inverter and charger are going to help us more in our current use case.

Someone else pointed out that the power saving mode is pointless with a fridge, due to the thermostat on the fridge won’t have any power and therefore won’t signal the compressor to start running. So in my case, a low idle draw is essential. I’m looking for 5-10 watts.

Your comment about the idle draw only happening when the compressor isn’t running is a good one. I had come to believe that the idle draw is the self consumption draw of the inverter itself. So if you have a 12W idle draw plus a 100W load, your total energy use is 112W. When no load is present, then you only use 12W. Do you understand it different than that?

My electric fridge has a simple mechanical thermostat so actually takes no current when the compressor is OFF (well except for the internal lamp when you open the door!). Its the microwave and other devices that I have powered by the inverter that do not like to be power-cycled every few seconds. I remove them from the inverter circuit and operate it in Power-save mode, but its not an issue for our use.

Inverters have circuitry to generate and regulate the 120VAC output. This and other power losses contribute to the less than 100% efficiency (typically about 85-low 90% efficiency). The higher the power rating of the inverter, the higher the current the inverter's circuitry consumes. So using a 2Kw inverter to operate an LCD TV is going to be less efficient than using a 200 watt inverter. But having a number of small inverters running can negate that as well.

So if your electric fridge is consuming 100W DC measured at the input of the inverter, and with no-load the inverter is consuming 25W, most of that 25W is also being consumed by the inverter when powering the fridge giving only about a 75% efficiency. If you increased your load to 1000 watts, that 25W is still there but your DC input power would more than 1025W. The increased current will increase IR losses (current x resistance) within the inverter, plus increased power consumption by the output circuitry may add another 75w of loss for a total of 100w consumed by the inverter. But now your efficiency has increased to 1000/1100 = 91%.

With a simple mechanical fridge thermostat like mine, you could dedicate an inverter of the appropriate size to run your fridge using the thermostat to power up the inverter only when required. You would also need a delay timer between the inverter output and the fridge to allow the inverter to start with no-load. The timer could also be set for a longer time delay (5-10 minutes) to allow pressures to equalize so the compressor would not stall if the inverter were to power cycle (this could happen if someone was adjusting the thermostat setting). This would allow higher efficiency in the inverter and no standby power consumption.

 

[Tomthumb62]

A possibility here ... if I understood the (mostly dedicated) loads correctly in OP (usage patterns).

Rather than leaving inverter on all the time, costing X watts per hour per day, have you considered reconfiguring a bit such that the inverter is powered on automatically about 4 to 6 times per day (for an hour or so each time), to match fridge getting to operating temp, and then inverter cycles off other times. Laptop charging or whatnot might also occur during the fridge "on" time.

Anything else runs directly off of batteries (depending on what requirements are). Possibly saves 50% to 75% of inverter idle draw. Gives you a chance to play with "home automation".

I've rec'd in a 24v MPP, so I'll be facing the same idle draw problem (but in a tiny home, truck-topper set of projects); not all parts are in yet, but this was my proposed answer to that problem.

In the main house, we actually run our fridge and freezer units 16 hours per day, and they are "off" during the overnight period of 8 hours, when everyone is sleeping. Stays within temps for us, and saves on (mostly) useless cycling at night, when nobody is using it. Another friend runs his fridge on 4 to 6 on/off cycles per day.

Hope this helps ...

Those are some good ideas. I’ll have to test the fridge in summer to see how well it holds temp overnight without power.

 

[rustythorn]

My electric fridge has a simple mechanical thermostat so actually takes no current when the compressor is OFF (well except for the internal lamp when you open the door!). Its the microwave and other devices that I have powered by the inverter that do not like to be power-cycled every few seconds. I remove them from the inverter circuit and operate it in Power-save mode, but its not an issue for our use.

Inverters have circuitry to generate and regulate the 120VAC output. This and other power losses contribute to the less than 100% efficiency (typically about 85-low 90% efficiency). The higher the power rating of the inverter, the higher the current the inverter's circuitry consumes. So using a 2Kw inverter to operate an LCD TV is going to be less efficient than using a 200 watt inverter. But having a number of small inverters running can negate that as well.

So if your electric fridge is consuming 100W DC measured at the input of the inverter, and with no-load the inverter is consuming 25W, most of that 25W is also being consumed by the inverter when powering the fridge giving only about a 75% efficiency. If you increased your load to 1000 watts, that 25W is still there but your DC input power would more than 1025W. The increased current will increase IR losses (current x resistance) within the inverter, plus increased power consumption by the output circuitry may add another 75w of loss for a total of 100w consumed by the inverter. But now your efficiency has increased to 1000/1100 = 91%.

With a simple mechanical fridge thermostat like mine, you could dedicate an inverter of the appropriate size to run your fridge using the thermostat to power up the inverter only when required. You would also need a delay timer between the inverter output and the fridge to allow the inverter to start with no-load. The timer could also be set for a longer time delay (5-10 minutes) to allow pressures to equalize so the compressor would not stall if the inverter were to power cycle (this could happen if someone was adjusting the thermostat setting). This would allow higher efficiency in the inverter and no standby power consumption.

i assume your thermostat is part of the fridge, however, do you know if they make mechanical independent thermostats that also use no or little power? i did a quick search and i'm wondering about this one, or this one

 

[RF_Burns]

i assume your thermostat is part of the fridge, however, do you know if they make mechanical independent thermostats that also use no or little power? i did a quick search and i'm wondering about this one, or this one

Sorry for the delay in replying. Either of those should work, however the first one has it's own enclosure and plug'n play wiring (note your internal lamp won't light when you open the door if the compressor is OFF. You would need to cut a rather large hole to get the cord back outside since the unit needs to be mounted inside. Nice that it has its own temperature scale rather than numbers that only give a relative meaning.

 

[Tomthumb62]

Update to say that I sold our Renogy 2000W inverter and settled on a Victron 12/1200. It's 1200VA, which roughly equates to 1000W. What a beast! It powers our 7.2CF fridge just fine, including on ECO mode. It's nearly as large as the Renogy was, even though it produces only half the watts. Got the bluetooth adapter as well so I can adjust the settings. Still experimenting with the ECO settings, currently it's set to cycle every 15 seconds and provide power for 5 seconds. The default settings were far too conservative (would cycle every couple seconds and power for a fraction of a second) for the fridge to work. The fridge needs at least 2 seconds of power before the compressor kicks on and provides enough load to keep the inverter running instead of going back into an ECO 'sleep' cycle.

Not quite sure how this equates into reducing idle draw, but I'm guessing about 1/3 of non-ECO idle draw (9W), so about 3W sounds right. Time will tell but I think this was really the right move to maximize our battery power especially in hot weather. It powers mostly the fridge and a few low-wattage items (100W or less, laptop chargers, etc). We also added two more panels to our severely underpaneled system (400W total now) and also have 200W more in portable suitcase panels when needed. Fingers crossed that our system will power our fridge totally on solar, reducing or eliminating the need to ever plug into shore power except during extended cloudy days.