How to Understand Generator Ratings Part 2

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[If you haven't read Part 1 of this article , you should!]

When the generator's voltage is more than 120V (around the peak of the sine waveform), an automatic step-down action will be taken by the inverter transformer to convert AC to DC and charge the battery. On either side of this section of the curve, no current is available to charge the battery - the instantaneous voltage of the generator rises to 150V which reaches the inverter's battery-charging transformer; the voltage is stepped down accordingly by the typical battery bank voltage. The output voltage from the transformer is 15V AC which goes to the rectifier to convert AC to DC. Since this voltage is higher than the battery's voltage of 12.19V, current flows in to charge the battery.The sine wave peak "squash" denotes gensets with weak generators that are incapable of charging the battery. The genset is subject to wear and tear with a longer charging time and a higher fuel consumption to put each watt of electricity into the battery.

Some sample commercial genset charging timesAn example is that of a family with an off-grid residence who replaces their old propane fridge with a "spare" electric model. This infrequent use off-grid system cannot handle the load. In fact, the family needs to reconsider its off-grid capacity with their new expected load patterns to determine the right renewable energy production capacity. Since there isn't enough current, there is a clear need for a generator to stand in for an increase in voltage to match their energy consumption.

Installing a new 4,000W generator , the family has difficulty in fully charging the battery. This is because the sine waveform phenomenon comes into play; even though the generator is "loaded up", ready to charge the battery, the current quickly drops to 15A. Hence, the purchased generator is too small and weak to produce the current needed to charge the battery.

An industrial unit of 7,500W would be more appropriate to charge the battery with reduced charging time and lower fuel consumption. It's easy to see the recommendation in most situations: purchase a bigger genset (in this case of at least 7,500W (120V/62A or 240V/31A)). Confirm the presence of an electronic voltage regulator which should be included in the generator. This is also known as the electric excitation component. High-quality generators give a peak voltage adjustment to allow for quick battery charging and start-up surges for heavy motor loads. Low-quality generators operate on passive excitation or voltage control that is not conducive to charging the battery. Consult experienced sales staff on the type of generators for battery-charging applications. Request a return guarantee if the unit does not function as required and remember that a small generator is of no use to you.

Voltage Selection

Genset voltage selection is determined by the loads connected in the home. Check what levels your installation needs.