When you want to run an appliance such as a fridge, most of us need the grid or propane
By Ron Grune
A Solar/Battery configuration is great, for small electronics. But when you want to run an appliance such as a fridge, most of us need the grid or propane. The reason is simple: W = V x A. Most people understand the units: a Watt measures power consumed; a Volt is the EMF of the power system, and an Amp is the amount of current.
Consider a small fridge rated at 48W
For a 12VDC system: 48 W = 12V x 4A.
But for a 24VDC system: 48 W = 24V x 2A.
Wow, a 12VDC system will consume TWICE the amount of Amps of a 24VDC system, for the same amount of power delivered.
So why is this important? Simple: the Amps is what your battery can provide. Once you choose the V, you forget about this and focus upon the A. The more A you draw, the faster your battery will run down to zero. You can look at this 2 ways: either your 12VDC battery will last only HALF as long as a 24VDC battery; or you will need TWICE as much 12VDC battery capacity compared to 24VDC.
Actually, the situation is worse. Half and twice apply for a perfect world. But solar and batteries are not perfect: they suffer from inefficiency and decay, which will greatly increase the differences between 12VDC and 24VDC over time. Also, the wiring needed for 12VDC will be much more expensive than 24VDC, as you increase the power capacity of the system [say 48W to 100W], because the 12VDC system will be moving more than 2X the A needed for a 24VDC system. More A means much heavier wire gauge is needed, which is now very costly.
All of this has rather big implications for your system design. But one thing that becomes easy is your AH calculations. The AH is “Amp-Hours”, which is the total A your device will consume for the number of hours you run it. Example; using 24VDC, a 48W fridge will consume 48AH over 24 hours. So simple now to determine what battery capacity you need for your 48W fridge over 24/7: 48AH !
Well, actually, due to decay and inefficiency, your battery should be larger than 48AH. How much will depend upon your conditions, which will be changing daily if you use solar. A fair estimate is to 1.5X the battery capacity to 60AH, for a 24VDC system. This is feasible. But if you were running a 12VDC system, you would need to double the 60AH again, up to 120AH !! woah, you mean to say a 12 VDC system could need up to 3X the capacity of a 24VDC system to provide equal power? Yes!
So how to design a 24VDC system? See next article.
Happy trails…. !
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