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So far, each example has been tailored to a DC-only circuit. What if you require AC power? An inverter must be used to convert the power in a DC-only system to AC power. Inverters consume power as they convert DC power to AC power, and in doing so, contribute to the system load. The less power an inverter consumes the more efficient it is, which is how its efficiency rating is determined.

If an inverter is to be used, the following questions must be answered:

  • What size inverter do I need for my load?
  • How much power does an inverter use while operating?
  • How does it affect the total load requirements in my system?

Microwave Oven” by Mk2010 is licensed under CC BY 3.0

Understanding Inverter Efficiency

Imagine you have a microwave oven and the manufacturer’s specification plate indicates that it requires 800 watts to operate. We can easily determine the DC amperage it would require if it were to operate directly from a 12 V battery:

W ÷ V = A
800 ÷ 12 = A
= 67 A

If inverters were 100% efficient, all of the power flowing from the battery would be inverted for use by the load, and the inverter would consume no power at all. In reality, however, inverters consume some of the power that flows from the battery while inverting. This requires an increase in current flow from the battery to run the inverter and support the load, and results in an increase in the total load requirements.

Let’s assume that an inverter has an efficiency rating of 80%. This means that only 80% of the power that is flowing from the battery can be used for the load. If the load is 800 watts, then the flow of current from the battery must be increased to keep the inverter and the load operating:

800 W = W ∙ 80%
800 W ÷ 80% = W
= 1000 W

The total load in the circuit is 1000 W.

Effect on Total Amps Flowing from Battery

Now that we know the total load on the circuit, taking inverter efficiency into account, we can calculate the amps:

1000 W ÷ 12 V = A
= 83 A

The inverter and the microwave will collectively require 83 A of current from a 12 VDC battery source to operate properly.

Another Method for Sizing an Inverter

If you do not know the efficiency rating of your inverter, a general rule of thumb is that the inverter have a minimum power rating of 125% of the total load. So for the 800 W microwave, the following formula would apply:

800 W ∙ 125% = W
= 1000 W

In this case, an inverter with a continuous output of 1000 W should be used.

Congrats, you made it!

Explorer’s Log

  • Designing a portable, battery-based electrical circuit requires calculating the average load, the battery capacity required to support the load, and the power generation required to keep the batteries charged.
  • The load in nearly all systems is dynamic.
  • Proper battery capacity in an electrical circuit enables you to supply the total power required by the load in a system, fully recharge the battery bank from the chosen power generation source or sources on a regular interval, and ensure there is enough energy to power the load between charges.
  • Any power generator selected for a hybrid power system should be able to fully recharge the battery once per day.
  • When using an inverter in a portable, battery-based electrical circuit, it is recommended the inverter has a power rating of 125% of the total load.

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