The difference between AC and DC current

Electricity, power or whatever you want to call it…

Many questions about power always come up when I mention living off the grid. Unfortunately, the terminology used is often confusing, and very few of us are electricians and have only some understanding of the basics, let alone understand power well enough to use it in non-standard applications like solar, wind or other power generation systems.

I myself am not an electrician, but out of necessity I’ve learned a few things about power (sometimes the hard way).

Here are a few basics:

There are two (general) kinds of power for use in electronics and appliances; Alternating Current (AC) and Direct Current (DC). To keep it simple, AC is supplied to your home and is what comes out of your power socket, usually at 110-120 volts in North America (and 220-240 volts in many other countries). And DC is most known as the power used in your car or truck, like when you plug things in to what used to be a cigarette lighter.

In both “kinds” of electricity, there are two main components: Voltage (v) and Amperage (a). Wattage (w) is simply voltage multiplied by amperage. So for example, a 60 watt solar panel will usually output 12 volts (DC) at 5 amps.

In a typical house in the USA, the voltage is around 120 volts (AC). Most circuits are 15 amp circuits, meaning that circuit can run a maximum “load” of 1800 watts (120 x 15). If you plug in a heating element rated at 2000 watts, it will “throw/trip” the circuit breaker because that circuit cannot handle any more than 1800 watts of “load/draw”. Similarly the fuses in your car are rated at a certain amount of amperage (amps). If you open your fuse panel in your car you will see numbers like 5, 10, 20, etc. These are for 12v (DC), and the numbers correspond to amps. A single 20 amp circuit in your car can handle only 240 watts (12v x 20a) before the fuse burns, breaking the circuit.

When living off the grid, one needs to be familiar with the basics above, plus also how to change power from AC to DC, DC to AC and how to change voltage (either AC or DC) to match your appliances/fixtures.

There are three main ways to talk about changing power: Transform, Rectify, Invert.

Warning: The terminology used for changing power often gets mixed up and mislabeled, so use this as a guide only!

Transformer: Changes power to a different voltage within the same type (example: 240v AC to 120v AC).

Rectifier: Changes AC to DC

Inverter: Changes DC to AC

You will also hear people talk about converters. Converters are simply one of the three types above. Some talk about transformers in the same manner, misusing the term. For example, a “power brick” that plugs into your home power (120v AC) and changes the power for your laptop computer or game console is a step-down rectifier because it changes AC to DC and then also brings the voltage down to around 20v (DC).

You may have heard the term “wall wart”, those heavy, bulky little black boxes that plug things in and take up your entire outlet. Those are also rectifiers that change AC to DC and then step-down the voltage, often to low DC currents.

So why does anyone but an electrician need to know any of this? Well, if you plan to either save energy while on the grid, live partially off the grid, or go off the grid completely, these are important things to understand for this reason: Almost all alternative energy systems (solar, wind, water turbine, etc.) use DC, and everything in your “on the grid” home plugs into AC (even if it gets converted to DC with a “brick” or “wall wart”).

An important thing to remember is that every time electricity is changed it wastes a bit of electricity in the process as heat. That is the reason those little “wall warts” and “power bricks” are warm (if not hot) to the touch. In general, the bigger the change, the bigger the loss, which means loss of efficiency and (eventually) loss in terms of money, too.