Power Usage for Refrigerator

[Freddmc]

I am contemplating replacing my propane fridge in my motorhome with an electric one and want to make sure my math for power usage is correct.
Using a hypotherticas fridge that is rated at 297 KWH/year(365 days) would this work out to 6.78 Ah per 24 hour period? Im not taking into account any of the variables. Just want to make sure my basic calculations are correct.

Thanks

 

[SCClockDr]

297000 / 365 = 813.69 wh per day. This is usually an optimistic prediction accepted by some federal bureaucrat or committee.
67.8 ah for 12v, 33.9 ah for 24v, & 16.95 ah for 48v by my calculator.

 

[Freddmc]

Isn't 813wh per day 6.78 ah per day?

 

[SCClockDr]

Isn't 813wh per day 6.78 ah per day?

Watts to amps requires the application of the appropriate voltage.

 

[SCClockDr]

Please allow me to clarify, watts are watts regardless of the voltage being discussed. When looking at a 120 VAC appliance its watthour rating is the same regardless of the voltage supplying the power BUT the 12 VDC to 120 VAC amp difference is 10 to 1, for every amp of AC current needed there will be a 10 amp DC draw. So 6.78 AC amp hours @ 120 volts per day is equal to 67.8 DC amp hours @ 12 volts per day.

 

[Rider]

Too much math, not enough practical.

I have a small top-door freezer, 3.5cf. Put an external thermostat on it to regulate to fridge temps. On surge, it draws 1200W, running is 70W. Over a 24 hours period, the average power usage is 10 watts per hour during summer heat. I have one 270W panel feeding a 100Ah AGM battery and a 1000/2000w full wave inverter. Without sun, it will run for at least 36 hours. If just used overnight, the battery will be fully charged in 3 sun hours the next day while the fridge is still running.

A regular small dorm fridge will use a bit more, but very easy to keep powered. Switch out the AGM battery for a LiFePO4 and it will run even longer. With about 200Ah of battery, I can run a full size 22cf kitchen fridge .

 

[atatistcheff]

IMHO the best way to do this is using a Kill-a-watt meter. I did that on my upright freezer and kitchen fridge. Just run it for 24 hours (assuming it's a typical day) and you will get a good idea of the daily KWh usage. You don't need to do any math, the Kill-a-watt will tell you how many watt-hours have been used.

As a side note, be sure and check that your inverter will run the fridge while it's within the return window. I have a 1500W pure sine wave inverter that will not work with my 12 year old kitchen refrigerator. Yet it works with the 30 year old upright freezer. This is even though the actual running wattage is way below the inverter's max capability. Apparently the startup current is so crazy high that the inverter kicks off as soon as the compressor kicks in.

 

[Newenough]

Too much math, not enough practical.

I have a small top-door freezer, 3.5cf. Put an external thermostat on it to regulate to fridge temps. On surge, it draws 1200W, running is 70W. Over a 24 hours period, the average power usage is 10 watts per hour during summer heat. I have one 270W panel feeding a 100Ah AGM battery and a 1000/2000w full wave inverter. Without sun, it will run for at least 36 hours. If just used overnight, the battery will be fully charged in 3 sun hours the next day while the fridge is still running.

A regular small dorm fridge will use a bit more, but very easy to keep powered. Switch out the AGM battery for a LiFePO4 and it will run even longer. With about 200Ah of battery, I can run a full size 22cf kitchen fridge .

How long?

 

[Newenough]

Been looking for an inverter type fridge. Not easy to find.

 

[morth]

I ran my 17CF top door freezer/fridge (no ice maker or water dispenser) for 16 hours on battery alone. Used 1383wh. Could have gone a bit longer, battery was at 3%.

 

[Rider]

How long?

If you mean, how long can I run the 22cf fridge, at least overnight before it starts recharging. Didn't run it longer than that.
But I also turned off the auto-defrost and ice maker.

 

[Freddmc]

So, if Im looking at a fridge that is rated at 150w,9a starting and 1.5 amps running does that mean in 24 hours it wll use 36AH? What is the correlation between 150 watts and 1.5amps?

 

[Rider]

...does that mean in 24 hours it wll use 36AH?

No. It means you need to put a watt meter on the fridge and measure it's actual usage to see how long your fridge with your battery and your inverter will run. My fridge uses 70watts while running, but only runs for 3-4 minutes each cycle for an average usage of 10w/24hours, so my battery run-time is very good. The more often a fridge runs, the more Kwh it will use, so a meter is needed to measure it's actual usage over time.

The correlation between watts and amps and volts is described in the the formula W (watts) = V (volts) times A (Amps),
W=V*A or
V=W/A or
A=W/V

A KW (killowatt) is watts time 1000. A KWH (killowatt hour) is how many total kilowatts are used in one hour.

EDIT: "for an average usage of 10w/24hours". I meant, an average of 10 watts per hour over a 24 hour period.

 

[morth]

As Rider mentions, you need to measure actual usage. The rating numbers on the sticker in the fridge tell you max current it will draw, not duty cycle (how often it runs). The duty cycle changes depending on how hot is it in the location, how much stuff is in the fridge, how often the door is opened, ect.

 

[Freddmc]

?"No. It means you need to put a watt meter on the fridge and measure it's actual usage to see how long your fridge with your battery and your inverter will run."
I understand that and do have a killowatt meter.But I don't think the store will allow me to do that and its even more difficult if you buy it from Amazon.. So without putting a meter on it what figure do you use to try and establish battery sizing for a given time frame.

 

[Rider]

So without putting a meter on it what figure do you use to try and establish battery sizing for a given time frame.

The only thing you have to work with is the annual KWH predicted usage on the EnergyGuide label. Start dividing that number to get KWH per day, then figure out how much battery you need to cover it for as many hours as you need. Not just overnight or 24, but add in extra for 2-3 days of rainy weather.

 

[Delta-V]

Example, my used 18.6 Cu. Ft. top freezer regular kitchen refrigerator in my house runs at around 185 Watts, but averages about 125 watts per hour or about 3,000 watts a day, according to my Kill-A-Watt meter.

 

[Will Prowse]

Yeah fridges are a pain to predict in an energy audit. Really depends on how much work they require in a given environment. As others have stated, watt meter is ideal method. Must measure consumption for full 24 hours at different times of year. The hotter the environment, the more energy they consume.

In a RV or badly insulated cabin, the consumption of a fridge at different times of year can be extreme. During summer, a fridge can use a ton of power! During winter, not much at all. If you live in a hot, but cloudy place, it can throw off your equations a lot. Always oversize your system when powering a fridge, they can be tough.

 

[morth]

Given the scenario where the fridge isn't owned yet, I think Riders suggestion to use the estimated annual KWH usage on the Entry Guide label to start the calculations from makes the most sense. It's really the only option unless you can find someone with the same fridge that can give you real numbers. I would also think those tests are done under ideal conditions, so I would consider those numbers an optimistic minimum. Much like EPA numbers for cars.... So, run the numbers and see what they say. The extra capacity for rainy days really snaps you into reality. Going 18-24 hours is pretty easy.

 

[Rider]

Must measure consumption for full 24 hours at different times of year. The hotter the environment, the more energy they consume.

I did all my measurements in the heat of July/August here on the East Coast. It was so warm the inverter fans kicked on with no load. For any other time of the year, I'm covered.