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Which fridge rating should we use to choose a more efficient fridge?
August 12, 2011 1:26 PM   Subscribe

On a small solar system that trips out a lot, what's the most important rating on a replacement fridge to indicate efficiency: full-load amps, or kWh/yr?

We've got a small solar system (panels and batteries) in our off-grid weekend place, and we struggle with maintaining adequate charge on the batteries. I'd say about once every two months it trips out due to low battery charge. Batteries test okay with a hydrometer. This started up a few months after the system started up and has been an issue for years.

The weird thing is, we've barely got any load and our system should be fine to cover us. Several people have pointed at the fridge as the likely culprit, since our water pump is small, only runs in short bursts, and our septic pump hardly ever runs (and there's no TVs or computers). The builder bought the fridge and I'm sure it was cheap and way inefficient. It runs constantly, even when the house is below freezing.

We shut it off for a week and saw drastic improvement. Normally our batteries are drawn way down by night time and wouldn't reach full charge until close to 10am after they've been in full sun for several hours, but when the fridge was off they were fine at night and completely charged up by 7:30 am (on a cloudy day).

So we're shopping for a new fridge of a similar size (appx 18cu.ft.).

The complication is that nobody can really tell us what rating we should be focusing on. The two main ones are full-load amps, and kilowatt-hours per year.

It seems like FLA is an important rating but only with a fridge that runs a lot. Low is still good, but ideally we want a fridge that over the course of a year uses less total energy, because then the average impact on our solar system should be less. Which means I'd think we would want to focus on kWh/yr. And kWh/yr is basically just another measure of amps, but with a time component added. Also, all the FLA rating tells us is about the motor, but the kWh/yr rating seems like it also relates to how well the fridge is insulated.

So to reduce the impact on a smallish-solar system that gets drawn down at night, should we buy a fridge with the lowest FLA rating we can find? Or the lowest kWh/yr?
posted by TheManChild2000 to Home & Garden (8 answers total) 3 users marked this as a favorite
Sunfrost fridges may be a good website to start with. They offer fridges specifically designed for solar power.
posted by odinsdream at 1:29 PM on August 12, 2011

I'm not a solar power systems expert, but I do know a bit about motors and electricity. First, "full load amps" needs to be distinguished from "running load amps". When the compressor on the fridge kicks in, the motor initially draws a large amount of current (the "full load"); as the motor speeds up, the amount of current drawn goes down and levels off at some steady rate (the "running load"). This happens fairly quickly—you're basically at running load current by the time the motor is fully up to speed—so most of the time the fridge is running, the current it's consuming is the running load current. The overall energy consumed by the fridge is therefore going to be something like (running load current)*(amount of time the fridge runs)*(120 volts or 240 volts, or whatever the supply voltage is).

I'm not an electrical engineer, but my basic understanding of batteries is that they also have a certain fixed voltage and can supply a certain number of amp-hours at that voltage (once fully charged). This basically means that there's a fixed amount of energy stored in the battery, and so you want a fridge that consumes that energy as slowly as possible. Insulation helps the fridge consume energy less quickly, since less heat gets into the food compartment and so the motor doesn't have to do as much work cooling it back down.

The main use of the "full load amps" figure is not for energy conservation, but rather to be sure that your circuits can handle that amount of current flowing through them. If your fridge "wants" to draw 20 amperes of current when it's starting up but your home wiring is only rated for 15, you're going to trip your circuit breaker or blow a fuse (or, less ideally, start an electrical fire) every time the compressor cycles on.

TL;DR summary: I'm pretty sure it's the kWh/year rating that you want to look at. The motor only draws "full load" for a brief amount of time during its cycle, so that number is less important to the rate at which energy is consumed.
posted by Johnny Assay at 2:03 PM on August 12, 2011

You could convert a chest freezer to a refrigerator and really cut down on your power consumption.
posted by Floydd at 2:21 PM on August 12, 2011

I don't think either can be safely ignored.

It's worth noting that kWh/year is only an estimate based on more or less realistic assumptions about operating conditions, whereas full load amps can be a fairly hard/accurate number. With that in mind, kWh/year tells you roughly how much energy is consumed over time, but FLA tells you how much current must be available to get the thing started at all. You should choose a fridge with a low estimated kWh/year, but also be sure that your solar setup can consistently provide the momentary oomph needed to start it.
posted by jon1270 at 2:34 PM on August 12, 2011

Floydd made my suggestion. Some of the conversion instructions/information I have seen cite shockingly low power consumption. The energy star ratings are also good for up to a 10% decrease in consumption.

The builder bought the fridge and I'm sure it was cheap and way inefficient. It runs constantly, even when the house is below freezing.

That's a big what-the-heck. My stand up fridge doesn't run constantly if it's not opened. I doubt there's that much difference in consumption between them. I'd be more concerned that what's happening is your system isn't providing enough juice during the initial (and high power consumption) kick-on for the compressor, resulting in it never actually working correctly.

So that's a big vote for FLA, though even that probably doesn't tell the whole picture; I'd imagine the FLA rating doesn't measure an initial instantaneous peak. Your problem might be more one of inverter (this is an AC fridge, yes?) than consumption.
posted by phearlez at 3:31 PM on August 12, 2011

Overall efficiency is of importance of course, but your fast-drain problem may be due to your TLA. Take a look at this graph (from here). Your expected amp-hour capacity is rated for a particular current draw, usually C/20, that is the current draw that would exhaust the capacity (C) in 20 hours. Drawing more than that affects the capacity of the battery as described by Peukert's law. The different lines in this graph give different values for n, a property of the particular battery in consideration. But the main thing I'm pointing out is the steep drop in all the curves as you increase load, particularly beyond C/20. Thus, the time that your fridge is running may be drawing more than C/20 amps and draining the battery quicker than you'd expect.
posted by buzzv at 7:13 PM on August 12, 2011

Oh hey, my industry! (I make solar charge controllers for a living. :) Disclaimer: though I am an electrical engineer, this is not legal/wiring/etc. advice, I am not representing my company, blah blah blah.

You're having throughput problems: in basic terms, you're occasionally needing more energy (kWh) than your sun input system can provide in a given day. The way out of this is either 1. more energy in over the course of the day (bigger panels, better tilt, whatever) or 2. use less energy. Since 1 is usually expensive, that means the biggest thing you're concerned about is kWh/yr. That's literally "how much energy will it take to handle a typical cooling load in a year", which roughly translates to efficiency. Be more efficient and the limited solar input that you get will go farther.

This is all assuming your inverter (this sounds like a standard plug-in fridge? or is it DC?) is big enough to handle the "full load amps" for when the refrigerant compressor is actually running. If your fridge tries to draw 10A and your inverter is only capable of supplying 8A, then the inverter will either fault (if it is a nice one) or cook itself to death (if it is like 90% of them on the market).
posted by introp at 7:16 PM on August 12, 2011

(I should add: another issue might be total battery capacity. If you get blackouts only at the end of cloudy weeks then you just don't have enough storage capacity to carry you through the lean solar times. All of this should've been sized correctly for typical local weather when the system was installed but oftentimes folks skimp on panels or batteries.)
posted by introp at 7:18 PM on August 12, 2011

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