AC vs DC fridges for off-grid use

[JanC]

Hi,

I just bought a property in Portugal and I'm planning to buy a small off grid solar system to power a small fridge, about 2 LED lights and also charge a laptop, 2 phones, and a router for the internet. This is really all I need to power and with the fridge I think it's no problem to turn it off in winter time, on cloudy days for example. I've got an offer from a portuguese solar shop for 1685 €, which included 450w Solarday panel, Victron Smartsolar 100/50 charge controller, Victron Phoenix 800/12 inverter, 24v battery breaker, 2x NBA 6v 260Ah battery and all the cables, connectors, fuses with the shipping also included.

I just found out on this forum, that powering a fridge off grid might not be that simple. I've looked through several shops with electronics around me and made some calculations regarding the consumption of various types of fridges, but it seems to me that no matter how much I try to calculate, it's impossible to say exactly how much will a fridge consume before buying it. I have come to a conclusion, that I have these options:

1. Buy a basic AC vertical fridge with a size of 5-6 cubic feet (all of the fridges in these options are about the same size), and with a freezer on the top. My rough calculations showed, that these types of fridges should consume about 840w/24h and would cost me between 300-400 €.

2. Buy an AC chest fridge/freezer with the same size. The prices were pretty similar to the vertical fridge as well as the consumption stats, but they've been calculated with the thermostat set on freezing and not refrigeration. So that means it should be lower with chest fridge than the vertical one. It also makes sense because the chest fridge maintains cool air better.

In terms of practicality, the vertical one is more practical, but it's not such a big issue for me. The thing is, I'm a little worried about the inrush current of the AC fridges. It was discussed here already and some people seemed to have a problem with this. It was a surprise for me because you cannot really find out the exact power of the inrush current with most fridges. So how am I supposed to know for sure that my inverter will be big enough to handle it? The last issue is, that my inverters max efficiency is 90%, so let's say up to 100 watts would be wasted daily just because I bought an AC fridge, which needs to go through an inverter. That's about 6,5 % of the batteries whole capacity. Also as I mentioned, besides the fridge, I only want to charge a few devices occasionally and use a few led lights which has brought me to my other options.

3. Friobat DC chest freezer. It's a company from Spain I found online. They offer DC appliances in rather affordable price. I wasn't able to find out so far, if this freezer's thermostat can be set on temperatures above 0°C. I heard, that you can convert a freezer to a fridge, but I have no experience in this, and I wanted to avoid doing stuff like that. The consumption of this freezer should be around 480w/24h and the price is 823€.

4. Friobat DC vertical fridge with freezer on the top. The consumption should be around 840w/24h just like the AC fridges and the price is 747,35 €.

5. Sunstar DC chest fridge/freezer. The consumption should be about 260w/24h, and the price would be between 1300 and 1500€ with the shipping. These stats are probably very similar to any other of those high-end DC fridges, like Sundanzer, Steca etc.

I will say again, that I don't expect these consumption estimates to be a rule of thumb. With the AC fridges, I just used their annual consumption estimate and added extra 70 %, because that seemed like a more realistic consumption estimate. With the DC ones, I just added a little bit, because their consumption estimates seem to be usually closer to real life than AC ones.

So the thing is, if I buy a DC fridge, I would need to turn on the inverter just occasionally when I want to charge something. And that I think would make the system pretty efficient. The second advantage is, that a DC fridge would probably consume less, and that means more battery storage in days with bad weather. The last advantage is, that in this case, I might be able to go for a smaller inverter and save 100-200€. I'm not sure if it's a good idea though, because there might be a situation once, where I'd need to plug in something more powerful, and I won't be able to do it. But then again, let's say I would still take the same inverter and in this case, the 350-450 € that I would pay extra for a DC fridge could be used for a third battery, or a second panel.

I'd be grateful for any advice

Jan

 

[mikefitz]

Hi Jan, I spend some time in Portugal, what part of the country are you based?

I assume you have no AC power at the property is this the case? Have you investigated having AC power connected, if there are any overhead cables nearby it may be less expensive than you imagine.

Comment, you need to review the parts suggested.

Solar panel, 450 watt panel wont deliver much more than 30 amps, in theory when charging at 13.5 volts, it could via a MPPT controller deliver 33 amps but conversion and cable losses together with often less ideal solar suggests this is unlikely and the maximum current will be in the high 20s. Thus a Victron smart 100/30 could be used.
One advantage of the 100/50 is that at some future date or even at the start, an additional 450 panel could be added. Although there is some loss of panel power at the panels maximum possible output, 'over paneling' is accepted, the controller limits to 50 amps.

Batteries, NBA 6v 260Ah are GEL batteries, 260Ah at 12v for a pair of batteries, cost 600 euro
These give a cycle life with a moderate daily discharge of around 2 years. ( data sheet suggests 500 cycles but no depth of discharge is stated). By discharging to no less than 50% capacity these should give reasonable service life. Charge at no more than 30 amps in the bulk stage of charging. With just the two batteries additional solar power would exceed the recommended charge rate.
One issue with GEL batteries is the need of a long absorption period, no problem with AC powered chargers, but with the restricted solar day hours, getting a complete charge may be an issue causing reduced battery life.
If possible consider Lithium batteries, the supplier you are considering offers Victron 100Ah at 860 Euro, you may/will get a better deal elsewhere on alternative lithium. In practice you need about 60% of a lead battery capacity to get a similar performance. Due to fast charging and greater usable capacity, lithium is a better option for solar as it can accept all the solar energy. Once above around 70% SOC lead batteries accept less and less current, irrespective of the available solar.

Inverter, for small DC appliances, phone chargers, its possible to get power supplies ( effectively small Dc to DC converters) that will run from 12v, thus avoiding the need for an 230v AC inverter.
For the AC powered fridge, the Victron 800 inverter is a excellent product with good overload capability, but may not be able to start some AC fridges reliably. Starting current of the fridge compressor can be very high, perhaps 4 to 5 times the running current. This problem could be solved with a more powerful inverter or as you suggest a DC fridge.
Do you really need a large fridge, there are DC12/24 volt fridges on the market for marine and RV use, typically consuming 450 watts per day, often less. Don't consider turning the fridge off with food inside, over 5C the food will tend to 'go off' quite quickly.
With your limited solar and battery power I would suggest a small DC fridge would be the better option. You can get prolonged periods of poor weather, even in the south of Portugal, when the solar input is very low.

In summary I suggest:
double the panel array,
consider lithium battery,
smaller DC marine/RV fridge.

Mike

 

[SamDeleted]

When in doubt, more panels !

 

[boondox]

Just get the regular fridge. Remember that you only need to have batteries for the night time power consumption., During the day the solar will pay for it. So at night say 1/2 of your estimate. Just make sure the numbers all pencil out and you will be fine.

 

[JanC]

With just the two batteries additional solar power would exceed the recommended charge rate.Hi Jan, I spend some time in Portugal, what part of the country are you based?

I assume you have no AC power at the property is this the case? Have you investigated having AC power connected, if there are any overhead cables nearby it may be less expensive than you imagine.

Comment, you need to review the parts suggested.

Solar panel, 450 watt panel wont deliver much more than 30 amps, in theory when charging at 13.5 volts, it could via a MPPT controller deliver 33 amps but conversion and cable losses together with often less ideal solar suggests this is unlikely and the maximum current will be in the high 20s. Thus a Victron smart 100/30 could be used.
One advantage of the 100/50 is that at some future date or even at the start, an additional 450 panel could be added. Although there is some loss of panel power at the panels maximum possible output, 'over paneling' is accepted, the controller limits to 50 amps.

Batteries, NBA 6v 260Ah are GEL batteries, 260Ah at 12v for a pair of batteries, cost 600 euro
These give a cycle life with a moderate daily discharge of around 2 years. ( data sheet suggests 500 cycles but no depth of discharge is stated). By discharging to no less than 50% capacity these should give reasonable service life. Charge at no more than 30 amps in the bulk stage of charging. With just the two batteries additional solar power would exceed the recommended charge rate.
One issue with GEL batteries is the need of a long absorption period, no problem with AC powered chargers, but with the restricted solar day hours, getting a complete charge may be an issue causing reduced battery life.
If possible consider Lithium batteries, the supplier you are considering offers Victron 100Ah at 860 Euro, you may/will get a better deal elsewhere on alternative lithium. In practice you need about 60% of a lead battery capacity to get a similar performance. Due to fast charging and greater usable capacity, lithium is a better option for solar as it can accept all the solar energy. Once above around 70% SOC lead batteries accept less and less current, irrespective of the available solar.

Inverter, for small DC appliances, phone chargers, its possible to get power supplies ( effectively small Dc to DC converters) that will run from 12v, thus avoiding the need for an 230v AC inverter.
For the AC powered fridge, the Victron 800 inverter is a excellent product with good overload capability, but may not be able to start some AC fridges reliably. Starting current of the fridge compressor can be very high, perhaps 4 to 5 times the running current. This problem could be solved with a more powerful inverter or as you suggest a DC fridge.
Do you really need a large fridge, there are DC12/24 volt fridges on the market for marine and RV use, typically consuming 450 watts per day, often less. Don't consider turning the fridge off with food inside, over 5C the food will tend to 'go off' quite quickly.
With your limited solar and battery power I would suggest a small DC fridge would be the better option. You can get prolonged periods of poor weather, even in the south of Portugal, when the solar input is very low.

In summary I suggest:
double the panel array,
consider lithium battery,
smaller DC marine/RV fridge.

Mike

Hi Jan, I spend some time in Portugal, what part of the country are you based?

I assume you have no AC power at the property is this the case? Have you investigated having AC power connected, if there are any overhead cables nearby it may be less expensive than you imagine.

Comment, you need to review the parts suggested.

Solar panel, 450 watt panel wont deliver much more than 30 amps, in theory when charging at 13.5 volts, it could via a MPPT controller deliver 33 amps but conversion and cable losses together with often less ideal solar suggests this is unlikely and the maximum current will be in the high 20s. Thus a Victron smart 100/30 could be used.
One advantage of the 100/50 is that at some future date or even at the start, an additional 450 panel could be added. Although there is some loss of panel power at the panels maximum possible output, 'over paneling' is accepted, the controller limits to 50 amps.

Batteries, NBA 6v 260Ah are GEL batteries, 260Ah at 12v for a pair of batteries, cost 600 euro
These give a cycle life with a moderate daily discharge of around 2 years. ( data sheet suggests 500 cycles but no depth of discharge is stated). By discharging to no less than 50% capacity these should give reasonable service life. Charge at no more than 30 amps in the bulk stage of charging. With just the two batteries additional solar power would exceed the recommended charge rate.
One issue with GEL batteries is the need of a long absorption period, no problem with AC powered chargers, but with the restricted solar day hours, getting a complete charge may be an issue causing reduced battery life.
If possible consider Lithium batteries, the supplier you are considering offers Victron 100Ah at 860 Euro, you may/will get a better deal elsewhere on alternative lithium. In practice you need about 60% of a lead battery capacity to get a similar performance. Due to fast charging and greater usable capacity, lithium is a better option for solar as it can accept all the solar energy. Once above around 70% SOC lead batteries accept less and less current, irrespective of the available solar.

Inverter, for small DC appliances, phone chargers, its possible to get power supplies ( effectively small Dc to DC converters) that will run from 12v, thus avoiding the need for an 230v AC inverter.
For the AC powered fridge, the Victron 800 inverter is a excellent product with good overload capability, but may not be able to start some AC fridges reliably. Starting current of the fridge compressor can be very high, perhaps 4 to 5 times the running current. This problem could be solved with a more powerful inverter or as you suggest a DC fridge.
Do you really need a large fridge, there are DC12/24 volt fridges on the market for marine and RV use, typically consuming 450 watts per day, often less. Don't consider turning the fridge off with food inside, over 5C the food will tend to 'go off' quite quickly.
With your limited solar and battery power I would suggest a small DC fridge would be the better option. You can get prolonged periods of poor weather, even in the south of Portugal, when the solar input is very low.

In summary I suggest:
double the panel array,
consider lithium battery,
smaller DC marine/RV fridge.

Mike

Thanks for the reply.

I'm located near Castelo Branco. There is no possibility of connecting to the grid on this property. The size of the fridge I'm looking for is about 5-6 cubic feet (130-160 litres). That's a pretty small size in my opinion. We cannot really go any lower with the size.

To explain my situation better let me say it like this. I have about 2000€ for the whole solar system including the fridge. I don't want to build the solar system gradually or anything like that. I want to spend money on it just this one time, and in future only change the batteries when they finish their lifespan.

You mentioned, that the panel won't deliver more than 30amps. What does this mean for my situation? Does it mean, that the batteries will charge too slow and I might run out of the battery? So in your opinion this one panel would not be enough for my intended use?

Also what is the actual difference between lithium and lead acid batteries in terms of charging speed? How much faster are lithium batteries approximately? Do you think this difference would be critical for my system and my intended use?

I was also thinking, that 100/30 controller should be enough, but they offered me this one.

I'm not sure if I understood this correctly- "With just the two batteries additional solar power would exceed the recommended charge rate." This means, that you need to buy bigger batteries or more of them, if you're adding solar panels, because of the charge rate? What happens if you exceed it?

You mentioned, that it's possible to get a some power supplies (DC-DC convertor). That is very interesting for me, because as I said, besides the fridge, I'm only planning to charge a laptop, 2 phones, a mobile router and the led lights. You can get a DC LED lights, right? So if I could somehow charge the remaining appliances without the invertor, I should be able to manage completely without it. But I'm not sure if it's such a good idea. I couldn't power any AC appliance this way.

How does it even work, when you power things without an inverter? Do you just connect the cables straight on the battery. I'm a beginner in this and I can't really imagine it. For example the DC fridge- does it come from the shop prepared to be connected straight to the battery? Do you actually connect it on the battery terminals? But what if you want to connect more things like this? There is not that much space on the terminals, or is there? Can you perhaps explain?

 

[Shimmy]

The size of the fridge I'm looking for is about 5-6 cubic feet (130-160 litres). That's a pretty small size in my opinion. We cannot really go any lower with the size.

Generally speaking, volume of the refrigerator doesn't really have a huge impact on energy consumption. Many top-freezer 18cu-ft units will use about the same amount of power based on standard ratings.

Do keep in mind though, if you load up the fridge with stuff that needs to be cooled that will add energy use.

Personally, I would buy a big AC fridge, freeze a 2L bottle of water during the day, stick it in the fridge at night, and shut off the inverter. I would power everything else from DC converters.

Gel batteries are really a pain; if you can spring for an equivalent LFP battery you will be much happier in the end. Same goes for extra PV panels.

 

[SamDeleted]

Think about insulating the fridge with kingspan or similar

Top, bottom & 3 sides ( obviously not the workings at the back) . The better the insulation is, the less power it will use !

Also plumb for an external digital thermostat, they are much more accurate & there for your fridge will use less power. Try running the fridge at 8c not 5c

And try to open it as little as possible, every time you open the fridge it will have to run twice as hard to get back down to temperature

 

[eXodus]

Generally speaking, volume of the refrigerator doesn't really have a huge impact on energy consumption. Many top-freezer 18cu-ft units will use about the same amount of power based on standard ratings

Only when you don't open them as people frequently do. The rating doesn't tell you anything.

With regular daily use and staring into the fridge to find food my 18 cubic AC fridge needs about 2-3kwh every day vs the 8 cubic AC variety only needs 1kwh.

Chest freezer are much better since the cold air doesn't fall out when opening.
I would go DC fridge or freezer.

A 800w inverter is a joke and will not start anything. Even a small 2 cubic dorm fridge had trouble starting on a 1000w inverter. I needed to up size to a 1500w for my van build. Lots of standby consumption.


I

 

[eXodus]

So the thing is, if I buy a DC fridge, I would need to turn on the inverter just occasionally when I want to charge somethin

You can get 12v chargers for almost everything laptops, phones, cameras etc. Don't know what else you need. Chargers are generally producing DC no need to go up to AC and then back down to DC

 

[Shimmy]

With regular daily use and staring into the fridge to find food my 18 cubic AC fridge needs about 2-3kwh every day vs the 8 cubic AC variety only needs 1kwh.

We have a stupid number of refrigerators (as well as a chest freezer, and they are each hooked up to our energy monitoring system. Aside from the energy hog SubZero dual-compressor unit, size does not matter. The Chest freezer uses about as much energy as the wine fridge (set at 8C) and as the beer fridge (set at 1C).

(The crazy subzero has a 500W(!!) heating element that runs continuously in the ice chute, only way to disable it is to keep the ice drawer partly open.)

 

[OzSolar]

One more vote for getting a regular fridge. The money saved on buying something standard local and the convenience of having a "real" fridge will more than pay back.

We also have more fridges and upright deep freezers than we ought to but...."happy wife, happy life" and since we're net zero with kWh to burn I don't care.

FWIW, I've tried most of the DC fridge/freezers and to honest I found them to be overpriced pieces of crap that failed within 5 years. Very disappointing.

I just checked the energy monitoring for the 10 year old 25 cu ft (700 liters?) ft side by side by side that now lives in the garage. It's been averaging ~ 2kWh per day. A bit more in the summer and a bit less in the winter.

edit to fix typo

 

[SamDeleted]

I just checked the energy monitoring for the 10 year 25 cu ft (700 liters?) ft side by side by side that now lives in the garage. It's been averaging ~ 2kWh per day. A bit more in the summer and a bit less in the winter.

I have a similar size chest freezer , I was thinking about running it off grid ... At 2kw a day maybe not possible right now !

 

[OzSolar]

I have a similar size chest freezer , I was thinking about running it off grid ... At 2kw a day maybe not possible right now !

I totally get it.

My system sizing was dominated by what it was going to take to start my well pump. For most, including me, that involves a painfully expensive 48v inverter. ~23 years ago when I did it the only choice was a pair of Trace SW048 inverters. By the way those inverters are still given-er every day with no complaints.

 

[SamDeleted]

I totally get it.

My system sizing was dominated by what it was going to take to start my well pump. For most, including me, that involves a painfully expensive 48v inverter. ~23 years ago when I did it the only choice was a pair of Trace SW048 inverters. By the way those inverters are still given-er every day with no complaints.

I've just picked up this trace 2624 (2.6kw 24v) , inverter + 60amp charger combo , it does the job. Big old buzzing thing.

 

[OzSolar]

View attachment 121622

I've just picked up this trace 2624 (2.6kw 24v) , inverter + 60amp charger combo , it does the job. Big old buzzing thing.

Yeah baby! I haven't seen one of those for years. About 8 years ago I got called in to decommission a system that a eccentric oil tycoon had built in the 90's. Three Bergey 10kW wind turbines on site built towers which turned out to be disastrous and the old Arco Carrizo pv modules. I bet it had 20 of those inverters. They all still worked when I got them back to shop! I would test and then list them on eBay and I was stunned how well they sold. People still love those tanks!

 

[SamDeleted]

Yeah baby! I haven't seen one of those for years. About 8 years ago I got called in to decommission a system that a eccentric oil tycoon had built in the 90's. Three Bergey 10kW wind turbines on site built towers which turned out to be disastrous and the old Arco Carrizo pv modules. I bet it had 20 of those inverters. They all still worked when I got them back to shop! I would test and then list them on eBay and I was stunned how well they sold. People still love those tanks!

I would've kept them all lol

 

[OzSolar]

I would've kept them all lol

I sat on them for a year or so. If you only knew how much 15+ yr old stuff I still have laying around. My solar and wind boneyard looks like something out of MadMax.

 

[SamDeleted]

I sat on them for a year or so. If you only knew how much 15+ yr old stuff I still have laying around. My solar and wind boneyard looks like something out of MadMax.

Do you have the strappy leather bondage outfit ?

 

[OzSolar]

Do you have the strappy leather bondage outfit ?

Several. I outgrew the first two.

 

[eXodus]

have a stupid number of refrigerators (as well as a chest freezer, and they are each hooked up to our energy monitoring system. Aside from the energy hog SubZero dual-compressor unit, size does not matter

Since you have a large amount of refrigerators you probably average the human use over them.

I got the comparison of 2 people opening the same fridge multiple times every day. Guess that fridge is open at least 30 times every day.

When I leave our 18 cubic at home alone and never open it. It needs about half the energy then when we use it regularly. While the smaller one is only 25% more between daily uses.

I agree when you just plug them into a power meter. It hardly maters what size. The difference start to appear when you use them frequently.