A reader recently asked "what size solar panel do I need to buy to run my fridge". He has a small campervan with a small Engel fridge.
Whether you have this same fridge or any other you can use the same equations to work out your own needs.
To answer this question you need to ask yourself a few other questions. How many amps does the fridge draw and what is its duty cycle? The amps drawn can usually be found on the fridge itself or in the paperwork. The duty cycle (or percent of time the compressor is on) is dependant on a number of factors. These factors include how well the fridge is insulated, the gauge and type of wire used, its ventilation, its position in relation to direct sunlight, and ambient temperature.
To work out the panel
size required you need to take a number of steps
1. Work out the current draw for a day. The fridge in question is rated
as drawing 2.5 amps (found on the compliance plate on the fridge). This
quoted draw is at an ambient temperature of 25 degrees. This 2.5 amps
will increase by 5% for every degree warmer so if you are in Queensland
in summer averaging 30 degrees the draw will be 3.13 amps (2.5 amps +
(2.5 x 5% x 5)). It cycles about 50% during the day and 25% during the
night so average it at about 33% or 8 hours/day. Therefore the fridge
draws 25amp.hrs/day (3.13amps x 8hrs).
2. To this you need to add a safety factor and add in an account for losses
from the system. A figure of 20% is standard so multiply your 25 amp.hrs
from step 1 by 1.2 (30amp.hrs day)
3. Average sun hours where maximum charging can occur varies throughout
Australia but maps are readily available that show averages. Large sections
of the north of Australia achieve 5-6 sun hour day with most of the south
receiving 4-5 hours. Tasmania is only 2-3. Assuming an average maximum
charge of 4.5 hours a day from your panel you need a charge of 6.67 amps
from you panel (30amp.hrs divided by 4.5 hrs).
4. A panel that is rated at 80 watts only delivers this wattage at 17
volts. Therefore you need to divide the panel's quoted output by 17 to
obtain the output in amps. In this case you would therefore multiply the
6.67amps required by 17volts (volts x amps = watts). This equals a panel
of at least 113.3 watts or one 120 watt panel.
In summary
| Fridge draw | 2.5 | amps | ||
| Ambient Temperature | plus 5% for each degree over 25 | 3.13 | (a) | assuming 30 degrees |
| Duty Cycle | 8 | (b) | hours on | |
| Amp.hrs/day | (a)x(b) | 25 | (c) | |
| Safety Factor | (c)*1.2 | 30 | (d) | amp.hrs/day |
| Maximun Sun hrs/day | Brisbane | 4.5 | (e) | hours |
| Amps required | (d) /(e) | 6.67 | (f) | amps/day |
| Panel size | (f) x 17 | 113.3 | watt (minimum) panel | |
| = | 1 | 120 | watt panel |
We calculated our GMC would need 500 watts - therefore we installed 4 x 123 watt panels. Note we run a 12 volt electric fridge. If you have a gas fridge you need less panels
To have an effective
solar system you need to work out your total load of all your appliances
(not just your fridge) and have a battery bank big enough to store the
power for as long as you want to stay self sufficient. This will be the
topic of an upcoming article when I look at the total system design of
our own GMC motorhome my husband and I are currently building.
This information is a result of our own experiences in building and living in motorhomes with solar systems. There is a lot to learn on the subject and for further information a great resource is Collyn Rivers 'Solar That Really Works" and "Motorhome Electrics". Collyn is a "guru" when it comes to this subject. You can now purchase these books direct from this web site. Click on this link