Solar Power – Pt-2 Inverters

Howdy !

OK, now I know that everyone completed their power requirements worksheets so now we can talk about the size and type of power support system we are going to need to operate all that equipment. First let’s figure out the operating cycle of the inverter. All the duty cycle means is how long the inverter system is expected to operate. For example, if we want to run all our loads for 12-hours out of 24-hours the duty cycle will be 50%. Now let’s look at typical loads.

For example, if the load includes a small television and VCR or DVD , the power requirements are relatively small and the usage is usually short-term (i.e. the duty cycle is low). The support system may consist of the engine battery, whether from a tow vehicle or a motorhome, with one auxiliary deep-cycle battery, the existing engine alternator, and the necessary fuses and cables. With such limited power usage the batteries alone may be able to power the loads without running the vehicle engine and recharging them from the alternator. Ah, one thing to remember here, if you lump the starting battery into the power supply bank you may not have enough “Umph” to get the dang engine to start !

Now, if the loads are larger, such as lights, heater elements for a refrigerator or water heater , or even a standard RV microwave, the power supply system will require a larger battery bank and may necessitate a high output engine alternator, solar panels or even a stinky ol’ generator. The batteries alone cannot supply long-term power to loads like these. Ultimately, the energy must come from someplace other then just the batteries.

Now here is an interesting question; “What voltage do I want to use?” Humm, truth is that 12 VDC is by far the most common system voltage on recreational vehicles. 95% of all inverter installations are 12 VDC. However, at this relatively low voltage, inverter support systems for continuous duty are limited to around 2,000 watts to 3,500 watts. Remember, for every 100 watts coming out of an inverter, about 10 amps must be going in at 12 VDC. Thus, 2,000 watts of loads equals about 200 amps from the support system. Fortunately, 2,000 to 3,500 continuous watts are satisfactory for most recreational vehicle applications (although larger inverters may be specified to handle higher surge or intermittent load demands and here we are getting into a MUCH larger system). Also, the other support system components such as fusing, switching, cabling, connections and tooling, are readily available at this 200 amp current. Some recreational vehicle applications, however, require more than 2,000 watts of continuous power. A 12 Volt inverter would then demand over 200 amps of input current. Not only does this power level push the limits of 12 volt inverter design, but support system components are also more costly and harder to find and install. For these higher power applications a 24 VDC system makes sense. Because 24 VDC is less common in recreational vehicles, however, some special equipment may need to be specified in these installations.

Lets think on this one for a week – –

Later,

The Old Ranger