How Many Solar Panels Do I Need To Power A Refrigerator?

How Many Solar Panels Do I Need To Power A Refrigerator? featured image

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Refrigerators run over a 24 hour period, so any calculation of the power needed should reflect solar panel output over 24 hours and energy storage for the periods when the sun doesn’t shine.

An average fridge of 500 watts needs 2 solar panels of 200 watt each coupled to a DC/AC inverter and sufficient battery storage of 200Ah to supply power during night-time and cloudy days.

To answer the question properly you need to know:

  • The fridge’s rated watts
  • Peak power when the compressor motor starts
  • Power consumed when the compressor is running
  • Is it running over a 24 hour period?
  • Solar panel insolation (determines how much power solar panels will produce)

Contents

How Much Power Does A Refrigerator Need?

On the face of it this is a pretty easy calculation. The voltage across any appliance the current in amperes should equal the power in watts.

As you might have guessed, this doesn’t help much! We need to know how much power it takes across 24 hours of running.

For a simple resistive load, such as a water heater element, it’s easy to work out:

Voltage x current x hours = watt-hours

This works because once the heater is turned on it pulls a certain amount of current from the supply, which is more or less constant, until the water reaches the right temperature and shuts off the power.

A fridge doesn’t work like that. These factors must be taken into account:

  • When the compressor motor starts it takes several times more than the running current – called ‘motor surge current’.
  • A motor is an inductive load, which means the voltage and current curves are not in phase (see P.F. below).
  • The compressor turns off and on according to outside temperature variation, season and usage.

What Is Power Factor And Why Is It Important?

In a resistive load the voltage and current wave-forms are in phase, that’s to say, when the voltage is at a maximum so is the current. In this case volts x amps = watts.

When a circuit contains coils, as in motor windings, magnetic fields are induced which oppose the supply current. the current is said to lag the voltage.

The amount by which the current lags the voltage is called the Power Factor and is expressed as a decimal between 0 and 1.

Power factor for solar inductive loads

If the the Power Factor was 0.9 then the current measure by an ammeter at any time is multiplied by 0.9 before being used to calculate the watts being used. Imagine a 115 volt fridge pulling 5 amperes with a P.F of 0.9:

Voltage (volts) x current (amps) = watts

115 x 5 x 0.9 = 517.5 watts

To further complicate matters the power factor will be a different value when starting up, when running or when the compressor is idle.

Refrigerator duty-cycle refers to the fact that sometimes the fridge is off and sometimes it’s on. That’s why it’s difficult to give it an average power rating. it depends on how often the door is opened, how full it is, outside temperature fluctuation and the season.

This is why manufacturers give a rated wattage, current draw and also an average consumption figure over the year in kilowatt-hours.

An average value for a 15 cubic feet fridge is around 400 kWh per year. This is a useful figure based on actual test runs with loaded refrigerators, so it’s a great way to size a solar panel installation.

Wattmeter for checking refrigerator power consumption

Another way to determine exactly how much a refrigerator is consuming is to connect up an a.c. wattmeter that can also measure the Power Factor.

Such a meter will not only give you instantaneous readings of volts, amps and watts, but also show you the energy used over time in in kWh.

It’s just another way of verifying the conditions for your own refrigerator instead of taking the manufacturer’s annual energy assessment and dividing by 365 to give your daily consumption.

Your fridge may be an older model, or it may be difficult to get the annual energy consumption figures. these meters are quite cheap to buy and well worth the cost if you’re planning more solar projects.

How Much Power Will My Solar Panels Generate?

Moving forward with an annual energy consumption of 400 kilowatt-hours, the next step is to work out what solar panels will produce per m2 in your location – you need to figure the insolation level in your area.

What is insolation?

Insolation is the amount of sunshine it is a measure of the solar energy that shining on a specified area over a set period of time.

A useful measure is in KWh per square meter per day. Data can be found from government insolation databases  but this is a very rough guide and may not be really accurate for your particular location.

This insolation calculator at the NASA web site gives daily insolation averages for each month for a given year.

Solar power meter for testing insolation

An instant idea of how many watts per square meter can be found using a solar power meter (see left).

Solar panel power ratings are based on insolation of 1000 watts per square meter, but this is a perfect situation – yours may be much less.

The annual solar output depends on factors such as the changing angle of the sun due to seasonal changes, but a rough rule of thumb is to say that a panel will about 4 hours per day of it’s rated power over the year, which is 0.166 as a decimal.

That’s assuming the standard insolation measurement of 1000 W/m2.

If you use a solar power meter in high summer at mid-day you may well find that the insolation for your area is quite a bit less, but for most purposes it can be used.

One monocrystalline solar panel with an area of 1 m2 is rated at approximately 200 watts (1000W/m2 x panel efficiency of 20%), so annual energy generated per year would be:

200 x 0.166 x 365 x 24 = 280.3 KWh

The average refrigerator annual energy requirement is 400 KWh, so 2 panels at 200 watt each would certainly cover the overall energy requirements, but what about the duty cycle and peak load due to compressor motor surge current?

An average fridge rated at 500 watt may draw 750 watts during surge current – remember that solar panel current is at 12 volt while mains voltage is at 120 volt. This means that with unity power factor the current taken from the mains would be:

500/120 = 4.16 amps

However, this current delivered at 12 volt would be 10 times as much. 41.6 amps would require 3 x 200 panels, possible 4 to account for a little extra surge.

This could work, except that we need some kind of energy storage system such as lithium phosphate batteries to deliver power at night and to take up the slack on cloudy days.

An average fridge of 500 watt needs 2 solar panels of 200 watt each coupled to a DC/AC inverter and sufficient battery storage to supply power during night-time and cloudy days.

How Many Batteries Do I Need For My Solar Panels?

For every 24KWh of energy used per month you will need 1 KWh of battery capacity. 1 KWh of battery capacity equates to 83 Ah (amp-hours) or nearest size upwards.

How Many Solar Panels To Run A Refrigerator Calculator

Enter the annual energy usage in KWh for your fridge. This can be found in the manufacturer’s specs. The calculator will instantly show you how many solar panels you need to run a fridge, but also how much battery capacity you will need to run it 24 hours a day.

 

Standard insolation of 1000 watts/m2 is assumed so you should make an intelligent adjustment based on the insolation figures for your location.

Web Story – How Many Solar Panels To Run A Refrigerator?

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Related Questions

Can one solar panel run a refrigerator?

As a general rule one solar panel cannot run a refrigerator, but there is an exception. The biggest solar panel on the market if 415 watts. In some locations a combination of 415 watts of solar power and deep cycle batteries can run a refrigerator.

Can a house run on solar power alone?

Yes, solar panels can provide enough power for the average home. An design analysis must asses the energy needs of all the appliances and the irradiance for the location before sizing a solar system.

Do solar panels drain batteries at night?

Solar panels do not drain batteries at night because they are fitted with blocking diodes so that the DC current can only flow in one direction – from solar panels to the battery or load.

 


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