How to calculate needed battery size (for only 1 fridge and 1 freezer) to run indefinitely?

[ck42]

Complete newb here. I know enough to understand the concepts and be dangerous.

Here's my goal: I want to setup enough solar to be able to both run the fridge/freezer during the day from solar... while simultaneously charging the batteries enough so that the fridge/freezer can then run from the batteries after the day's solar charging time until the next day when the fridge/freezer can run from solar again. If I could get this type of setup, I could power the fridge/freezer indefinitely. (obviously there's variables at play here, but for hypothetical discussion, let's just assume perfect sunny days every day - once I understand better how to calculate, then I'll start factoring in no-solar charge days into the equation)
- First question: Is this too lofty of a goal? (i.e, yes, it's possible but would cost a fortune)
- If too ambitious, would it help to scale down the requirements such that this cycling would only last me 1, 2, or only 3 weeks before the batteries wouldn't last overnight (or next 1 or 2 cloudy days)?

What I know: The fridge has a 24hr measured average consumption of 51W/hr. The freezer is 61W/hr. So I'll just round this up to 120W/hr. There's also the inrush current for both of these to be considered (which should be approx 1.5 x the running wattage of the compressor).

Are there any other values I need to figure this out?

With this just being my first solar setup, I want to keep this simple and just protect cold/frozen foods for now. I plan on expanding in the future, but am waiting until I move to a another home before going deeper and building a bigger system.

 

[Mattb4]

If you are going to move to another home I would put off your solar plans until then. Perhaps some simple system that you can get familiarity with and power a few things without achieving your goal of indefinite power to fridge and freezers.

I have tried working out a simple equation for load to inverter to battery to charge controller to solar panels but have ran into problems due to variables. I will say you always need to start with load and work back to supply.

 

[ck42]

I would normally wait, but moving to the new home won't be for another 5-7 years. (My point being originally that I don't want to install anything significant right now that I can't easily take with me when I move....and don't want to invest in system components that would no longer be used once I move and build a bigger system - the one exception being the battery itself. I'm okay with even NOW investing in a good (oversized) battery since I can easily move it and also add onto it later).

Knowing my load requirements, is it as simple as this? (Before accounting for any component inefficiencies or dark days, etc)
- Fridge/Freezer load: Roughly 120W/h
- Hypothetical Battery: Say 48V battery @100Ah => 4800Wh capacity
- Running off battery for 16hrs/day => 16*120Wh= 1920Wh of battery power used per day

Then I would just need to configure enough panel power to be able to replenish that 1920Wh to the battery the next day before the battery starts being used again?

 

[Mattb4]

Sadly it is not that simple. Conversion losses and inverter power requirements change the totals. And the big question once you need to recharge the batteries is how much wattage your solar panels will make during usable sunlight hours. You likely will not get rated watt capacity from them. And they only reach maximum wattage at maximum solar peak for the day. So it is a curve from dawn to dusk.

Charging batteries also has to allow for how much additional wattage is needed to charge over how much capacity you gain. In other words it may take 100 watts in to get 90 watts worth of charge. Further complications are batteries have charging requirements. It is not a direct 1-1 ratio.

As you look more into solar power you will see about the concepts of over paneling as well as too much battery for the panels.

 

[ck42]

Sadly it is not that simple. Conversion losses and inverter power requirements change the totals. And the big question once you need to recharge the batteries is how much wattage your solar panels will make during usable sunlight hours. You likely will not get rated watt capacity from them. And they only reach maximum wattage at maximum solar peak for the day. So it is a curve from dawn to dusk.

Charging batteries also has to allow for how much additional wattage is needed to charge over how much capacity you gain. In other words it may take 100 watts in to get 90 watts worth of charge. Further complications are batteries have charging requirements. It is not a direct 1-1 ratio.

As you look more into solar power you will see about the concepts of over paneling as well as too much battery for the panels.

Yep, understood. This is why I am trying to talk in hypotheticals for now. Given these various efficiency and conversion losses, and variable panel output power and such, is the basic premise of my calculation correct? If yes, then I would then just need to dive a little deeper and start to take into consideration these various variables to make the calculation more accurate, yes?

 

[Mattb4]

Yes. Now getting that down to a size that is portable that you can move to your next home may not be possible.

 

[ck42]

Agreed. But understanding now how to start calculating things (even ballparking it) will hopefully tell me what sort of battery requirements I'd be looking at....whether I can proceed with the infinite power solution or need to scale back to 1-3 weeks before depletion.

Appreciate your feedback!

 

[Bvillebob]

There is no "answer", it's a matter of probabilities since it ultimately depends on weather. Where I live my winter production is at best 10 percent of my summer production. If I want 100 percent guarantees I need an absolutely enormous system.

To give you some idea, my first system was 3.5 kw of panels, 24v at 400 amp hours and a 2.2 kw inverter. It ran 3 freezers just fine about 50 weeks a year, in January or February there were a couple of times when the batteries got a top up from outside power. Eight months of the year it produces 10 times the amount of power needed.

You need to look at your winter production, day length, cloud cover, etc., then decide how many days of backup you want to provide. Then you can start designing a system.

 

[efficientPV]

I mention this only to illustrate the vast difference between an intelligent refrigerator control and what everyone else does. My fridge only operates in daylight hours and from an old car battery. The control will only start when battery is over 13.5V indicating there is sufficient sun. Timers prevent hot restarts and checks on battery condition prevent battery from being discharged more than 10%. Running power is almost totally supplied by panel production. Battery is only there for start surge.

That battery is pretty old and last year I sensed that its capacity was dropping. This year in the early morning the fridge has been running for a minute and turning off. By 7:30 with more sun it had been running fine. Two weeks later I was getting these short starts mid day when there were clouds. This had been the battery that was keeping me alive at night. So, I finally broke down and got a 50AH lithium for the house battery. I had been using a 2011 date code lawnmower battery I got from town recycling and had restored somewhat. I had gotten into binge watching DVD movies and that battery wasn't enough for 4 hours of that. I took that old battery and paralleled it with the car battery and the fridge is back to normal. Between the to of them there can't be 10 amp hours. And everyone else has 200AH and is failing.

 

[ck42]

There is no "answer", it's a matter of probabilities since it ultimately depends on weather. Where I live my winter production is at best 10 percent of my summer production. If I want 100 percent guarantees I need an absolutely enormous system.

To give you some idea, my first system was 3.5 kw of panels, 24v at 400 amp hours and a 2.2 kw inverter. It ran 3 freezers just fine about 50 weeks a year, in January or February there were a couple of times when the batteries got a top up from outside power. Eight months of the year it produces 10 times the amount of power needed.

You need to look at your winter production, day length, cloud cover, etc., then decide how many days of backup you want to provide. Then you can start designing a system.

Ugh...should've been obvious but totally forgot about winter months having much lower solar delivery. ?

 

[Bud Martin]

OP: What I know: The fridge has a 24hr measured average consumption of 51W/hr. The freezer is 61W/hr. So I'll just round this up to 120W/hr.
What kind of fridge and freezer that consume such low power for the whole 24Hr period? It does not sound right?
Are they DC or AC?

 

[ck42]

Both are 120V AC. The freezer is a small model (only about 5ft tall). The fridge is a counter-depth model so much less cubic footage. Both energy star rated.
And just to make sure we're on the same page, those 51/61W/h numbers are....PER hour.
I used my Kill A Watt meter and had left them plugged into it and waited 24hrs. Took the KWh final reading and divided that by 24.

[EDIT]
Kill A Watt has now been plugged into the freezer for exactly 48hrs. 2.850KW. Comes out to 59.4W/hr.

 

[Bud Martin]

Watt and Watt hour
59.4W x 24h = 1425Wh or 1.425kWh power consumption per day, that is how utility will charge you per day as shown in your utility bill.
61W x 24h = 1464Wh or 1.464kWh power consumption per day.
So total usage in one day is 1425Wh + 1464Wh = 2889Wh or 2.889kWh
The power the inverter will consume from batteries due to conversion loss, 85% is typical, will be about 2889Wh/0.85 = 3399Wh or 3.399kWh that your batteries will have to supply for 1 day if you have no power from Solar in worst case.

 

[12VoltInstalls]

Sadly it is not that simple.

Look at prior bud martin post
It’s quite simple.

The power the inverter will consume from batteries due to conversion loss, 85% is typical, will be about 2889Wh/0.85 = 3399Wh or 3.399kWh that your batteries will have to supply for 1 day if you have no power from Solar in worst case.

need

You need to look at your winter production, day length, cloud cover, etc., then decide how many days of backup you want to provide

Which is good advice.
A wildhat guess? To be way sufficient and not minimalistic? I’d just use bud martin numbers, triple them for battery and use at least 65% in watts of panels of the watts you’d need in winter to recharge overnight use in 3 hours tops.
With an aio or whatever- on some days in winter you can probably cover all your daytime electrical with a system that size.

Having said that doing the actual work with the math will be way more accurate.

 

[Leeds]

Where are you located? Are these to be indoors or out? What is your precise goal?

Someone in Texas probably only needs to cool their fridge and freezer year round. Someone in the North woods might also need to heat them at times during the winter months...