Why not use 12v DC for Off-Grid

Many people when  first contemplating  going off grid  expect to use  12V DC  power because they  see 12V batteries  and 12V lights used in RV; so they think this is  how  power is used in  off-grid applications.  It’s is a reasonable assumption but fraught with technical problems. Except for lights, few 12V appliances are durable like their 120V AC household equivalents.  Take a 12V coffee maker as a common example. You can buy such devices in RV shops and truck stops. In order to heat up enough water for two cups or one big mug full takes a certain amount of power.  300 watts is about the minimum   that will heat up water in anything resembling a reasonable length of time. 

At 120V the 300 watt coffee maker will draw 2.5 amps. At 12V DC the coffee maker needs to draw 25 amps to heat up the same quantity of water in the same length of time.

2.5 amps only require a thin wire.  The #16 gauge wires often used in automobiles is rated to carry 10Amps so it is perfectly safe and remains cool in use.  However, in order to carry  25 amps requires at least #12 gauge wires  provided  you use high temperature  insulation or better yet  #10 Ga wire which is rated for 30amps.

The 12V appliance will not last as long because the construction does not stand up to prolonged use and typically fails in a few months instead of lasting a few years as is common with normal household appliances.  The same can be said for all 12V appliances.

A quick review of products reveal   the cheapest 12V coffee maker is around $35 whereas the cheapest 120V AC model is $12.95 not to mention which the range of choices for 12V models is limited while the number of AC products in almost limitless by comparison. There is also a wider range of styles available.  A similar situation can be seen for just about any product you care to mention. 12V appliances are often only found in specialty stores.

Another aspect has to do with wire losses using the same example of a 300 watt coffee maker and let’s say there is 25 feet from the power source to where the coffee pot is plugged in.  You need to use #10 Ga wire and this results in a 10.79% loss in the wiring.  This will mean it takes that much longer for the water gets hot and you are wasting 10.79% of your solar power.

Let’s look at the losses for the 120V coffee maker. Voltage drop at 120V is only 0.69% and you can get away with using #18 Ga wire. The electrical code does not permit using any smaller and in any case such small wires are too fragile for general wiring work.

So it should be obvious that not only can you use smaller wires which cost less to buy but the losses in the wire are less than if you used 12V DC.

Conventional 120C AC wiring is very common and the fittings are readily available in just about any hardware and home repair supply stores. This makes it attractive due to low price and being readily available. If you are not sure how to wire it there are any number of self help guide books including guides as to what the code calls for.

If you choose to wire with 12V you will need specialized hardware, fuses and or breaker or breakers. In many cases special  (read expensive) terminals bus bars and definitely larger and thus more expensive copper wires. The cheap stuff is simply an invitation for a fire down the road.

Inverters in varying sizes are available at reasonable cost. It is possible to get inverters able to power entire house holds.

There are still cheap junk products in the market but there are also plenty of quality brands with a long history of excellent performance.  One hint never load the inverter to 100% of the rating. If you need 1000 watts select a 1500 watt inverter because the inverter will run cooler and last longer.

Some of my designs are even intended to power air conditioners, assuming you have a large enough battery bank and recharging capacity.

An added benefit to wiring for 120V comes from the fact you can start up a regular genset and feed this power right into the off-grid home wiring circuits. Even the best solar system will see cloudy days and the battery bank will get depleted eventually.

Having the ability to run a back-up power source in those conditions is a benefit.

Inverters will automatically shut down when the battery voltage goes low. This will protect your battery from excessive discharge and subsequent damage.

Having a DC load directly connected to a battery may result in permanent damage to the battery if the DC load continues to deplete the battery below safe levels.

 

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