Power requirements of an off grid freezer
When you need to decide on the required power supply of an appliance, like for instance your off grid freezer or fridge, the math can get rather complicated.
Watts / Amps/ Volts …confusion 🙁
For the purpose of this discussion, we look at powering up a 12-volt freezer as an example.
Typically, these fridge / freezers are used in areas that don’t have grid power supply readily available.
The reason that we use a 12 volt unit, is because it could be directly powered from a 12 volt DC battery bank without the need for converting the power to AC, or the installation or use of an invertor.
The batteries will be recharged by solar panels.
It is getting a bit complicated when we try and determine the size and specifications of the off grid electrical components we will be needing.
First, we need to establish the power consumption
(Power Hunger) of our freezer.
The specs of our freezer example are as follows:
This freezer is ideal for off grid cabin use.
280Lt chest freezer (9.8 cu ft.)
12volt Danfoss compressor. (These direct current compressors are widely used as many manufacturers build their fridge and freezer cabinets around such.)
6 Amps running current.
Basic Electrical Terms:
We need to understand a few basic terms.
Electricity is basically the flow of electrons through a wire. It is often referred to as current.
Ampere is the description of how many electrons per second are going through the wire. It measures the flow of current.
Volt is the pressure the electrons are under as they flow through the wire. The power source (battery) feeds electrons out into a wire at a certain pressure (volts), for instance 12 volt or 24 volt.
Amp-hours is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour.
In reality, a deep cycle battery will not render 100% capacity. It will need to be recharged when it is down to about 80%, but it is best to keep them in a cycle of 50% to 100% charge.
A typical Amp Hour specification might be: “100 AH @ 20HR”. This relates to the ability of the battery to render 5 Ampere for 20 Hours (100Amp Hours divided by 20 Hours = 5 Ampere per hour.)
The curve ball is:
It is not a simple calculation to decide on the power needs of our freezer.
There are many factors to consider, like for instance:
– Ambient temperature
– Load inside the freezer
– Running period of the compressor per day (thermostatically controlled)
– Recharge source of the batteries
– Sunlight hours per day available for solar charging
We will therefore need to get a specification chart from the supplier of the freezer indicating the power needs of the unit.
For instance, a freezer with the specs as discussed, will have the following approximate requirements (as per the supplier)
Summer: 60 amp-hours/day. = 2,5 Amps per hour
Winter: 48 amp-hours/ day. = 3 Amps per hour
Battery Capacity Needed:
Normal car batteries cannot withstand the charge cycles required for an appliance using a component like a 12-volt compressor freezer.
Deep cycle batteries, like the ones normally used in the leisure market, are designed to be able to handle the charge and cycle rates required and need to be used.
Say for instance, we need to provide for 3 cloudy days with basically no charge from the solar panels.
We will be using 48 amp hours per day, and we will need:
3 X 48 = 144 Amp-hours.
If we then further allow for maximum of 80% battery use:
(144 / 80 X 100) = 160 Amp-hours requirement.
Our battery need will therefore be 2 X 80 Amp-hour batteries in series.
Solar Panel Requirement:
In our example, we use 12 Volt batteries. We also calculated that we will need 3 Amps per hour supply. If we put our faith in 8 hours of sunlight per day, we therefore need to generate (3 X 24 /8) = 9 Amps per hour from our solar panels.
Put all this into our formula of :
Power = Voltage(12) X Amps(9) , then it relates to a solar power requirement of 108 Watts panels.
But take heed, Solar panels are rated at their peak power output in perfect conditions. …Nature does not work that way.
It is therefore best to over design with about 50% of the basic solar panels requirements.
For the purposes of our example, we would therefore opt for 2 X 75 Watt solar panels = 150 Watt in total.
Our panels my deliver power at a higher voltage than what our batteries are designed for. As our power production will fluctuate and we will be producing power of a different voltage and occasionally also more power than we drain from our batteries, we will have to install a charge controller in our system.
The function of the charge controller is simply to prevent damage to our batteries from being over charged.
We can get a lot more technical with all of this, but for the purposes of this discussion, this should be enough information to allow anyone to hook up that fridge / freezer to a basic off grid power system.
Thanks for reading. Your comments will be appreciated 😊
Cobus vdM / https://offgridbasics.com