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Independent Power - What's a Watt?

Basic Electricity Terms

When you read about generators, inverters, and batteries and things like that, the terminology can be very confusing. If you've never really needed to know these terms, then even if you were told before what they mean you may have long since forgotten. Here we're going to explain these terms in a simplified way, and give you some links to other sources if you want to dig deeper into this subject.

There's so much to say on this subject that I've decided to add to this article periodically over time, rather than try to do an exhaustive explanation at this time. If you find it a little confusing, read and return later to read again. The relationships between these terms are very interesting and will help you understand what's going on with specifications and what they mean to you and your power system. Even if you never completely understand it all, this will be helpful.

What is a Volt?

A volt is a unit of electrical force. This force is called Electro-Motive-Force or e.m.f. To get a feel for what the term means, think of it as pressure. As water pressure increases, more water volume will flow through a hose when the valve is open. As voltage increases, more power will flow through a wire and corresponding load.

Like water pressure, voltage itself does no work, but just describes the 'enthusiasm' (energy potential) of electrons. When electrons have a higher potential (voltage) they carry more power and can perform more work. That's why the current can be the same, and yet at higher voltage there is more power delivered to a load.

What is an Ampere?

An ampere is the number of electrons (1 coulomb) that past a given point in a wire in one second.

What is an Ohm?

An ohm is a unit value of resistance to the flow of electricity. Resistance converts electron flow into heat. For example the heating element in an electric space heater is a resistor. This ability to convert power into heat is usually not as pronounced as in a heater, but the principle is the same and a key element in understanding electricity. The lower the resistance, the lower the power losses due to current flow.

What is a Watt?

Current flow is the number of electrons flowing in the wire, and voltage expresses the energy of each electron. The electrical voltage combined with current flow is what does actual work, and we express the work being done in watts.

One watt of heat energy is generated by 1 ampere of current, with 1 volt of electrical force being applied in a wire that has 1 ohm of resistance.

The formula for power is; (Power in watts) = (voltage) * (amperes)

When we talk about a generator or inverter or any other power delivery system, we use the watt to describe how much power can be delivered to do useful work.

Voltage Drop

Whenever electricity is conducted through a wire, some power is lost. From the previous descriptions you may already see the reason why. All wire has some resistance to the flow of current. In fact the loss of voltage can easily be described in the following formula.

(Voltage lost) = (Current squared) * (Resistance in ohms)

As you can see if the current doubles through a wire there is a four fold loss of power! So as the voltage is reduced the current required increases in order to deliver the same amount of power. This results in a BIG increase in voltage loss from the wire.

This is the big reason for using 24 volts rather than using 12 volts. A 12 volt system requires twice the current for a given load resulting in 4 times the losses in the wiring. Many large systems use 48V battery systems, and now you can see the reason why. My system here is 24 Volts, which I think is a good tradeoff however it's important to see why higher voltage battery systems are used.

AC & DC

Power that we get from an outlet in our home in the US is AC, 120 Volts, 60 Hertz. The "60 Hertz" refers to the frequency at which the voltage is oscillating. A hertz is equal to 1 cycle per second. This means that AC power is vibrating at 60 cycles per second. AC or "Alternating Current" has a constantly varying voltage. AC voltage varies from 0 to plus and minus 160 volts. The standard 120 volt rating is the RMS voltage that can be used to predict the actual delivered power based on AC amperage. One ampere of AC current at 120V AC will generate 120 watts of heat.

This property if oscillation means that power can be "transformed" from 120 volts to much higher voltages for low loss transmission of power to our homes. We can also build efficient and low cost power supplies by using a transformer to lower voltages to a more appropriate level for the device we are operating.

Since virtually all home appliances, lights, and etc. are designed to run on 120v AC 60Hz that's the power we need for our home.

The problem is that there is no efficient way to store power as AC. Batteries that we use to store electricity are DC. Their voltage is constant and does not oscillate. DC stands for "Direct Current". Batteries are wired together to form a single source of 12, 24 or 48 volts. Then we use an inverter to convert the DC voltage to the 120V AC we need to power our homes.

KW Hrs

KW stands for kilowatt, which me