Oasis Montana designs and sells residential solar electric power systems

Oasis Montana Inc.
Renewable Energy Supply and Design
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Contact us: info@oasismontana.com
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What features are desirable in a fuel generator when integrated into a home gen-verter power system?

Many of our customers, particularly in the north, use propane or other fuel generators to assist in charging their batteries in low winter light conditions, or in conditions of little wind.  Many who are just building their homes have the need to run large power tools like welders or sawsóloads that wouldnít be cost effectively powered by a PV system.  Upon completion they can add solar or wind power to lower or eliminate their generatorís run times. When a generator is teamed with an inverter and battery bank, system efficiencies increase and less fuel is used. These can be designed to be turnkey enough that, when your batteries reach a certain low voltage set point, the inverter senses this and turns on the generator to run the AC loads while itís charging the batteries.  When the batteries are fully charged, the inverter turns the generator off and the loads continue to run off the batteries via the inverter. When the voltage falls, the cycle starts again. This keeps the generator from running 24/7, or from running inefficiently just to power small loads.

We donít claim to be generator experts, but the inverter systems that we sell have to work with them, so we know how to size them and what features to specify. A lot of people ask us what brand and size generator they should use with their battery/inverter systems, but unfortunately there isnít a one-size-fits-all answer.

There are many factors that should be considered before purchasing a generator, such as: whether you need 240 VAC power for large loads (shop tools, well pump, etc.); the voltage/amp-hour capacity of your battery bank; the maximum charging rate of your inverter(s); other charging sources like solar/wind; how long/often you want the generator to run; and whether you want/need the generator to start automatically.

A rough rule of thumb for sizing a generator for your inverter(s) is to get a generator with a power rating (after de-rating for altitude and fuel type) that is at least 1.5 to 2 times larger than your inverter power rating. The reason for this apparent oversizing is not to make more money for the generator dealer, but because the generator is often charging the battery bank and running your AC loads at the same time. If the generator is too small, it wonít be able to do either job very well.

If you want to find out how deeply you will discharge your batteries each day and how often/long you will have to run your generator to recharge them, you first need to calculate your average daily energy consumption in amp-hours or watt-hours. With this information and the amp-hour capacity of your battery bank, you can also determine how many days of storage your battery bank will provide between generator charging cycles. Next, look at the maximum DC charging amperage of your inverter(s); if it is less than 20% of your battery bankís amp-hour capacity (at the 20 hour rate) then you should consider using an inverter with a more powerful charger or going with multiple inverters (or separate chargers) to increase your charging rate. To determine the hours of generator run time required to fully charge the battery bank from its deepest depth of discharge, you should divide your battery bankís 20 hour amp-hour capacity by 2 (to account for a 50% depth of discharge which is the lowest point we recommend you discharge your battery bank) and then divide that by half of your inverter's maximum DC charging rate. We use only half of the inverterís maximum charging rate because inverters donít charge at their maximum rate over the entire charging cycle. Reducing the charging rate to half of its maximum value will give you a more realistic estimate of your generator run time.

Letís run through an example so you can better understand what the heck we are talking about in the paragraph above: Say you have an Outback Radian 4048 inverter system with eight L-16H 6V 420 amp-hour batteries and an estimated daily energy usage of 4,000 watt-hours. The maximum charging rate of the FM80 charge controller is 80A, which is ~19% of the 420 amp-hour capacity of the battery bank. This is a good match. The total watt-hour capacity of the battery bank is 48V x 420 amp-hours, which equals 20,160 watt-hours. If we only want to draw the batteries down to the half-way point (50% DOD), then we only have 10,080 watt-hours to work with. If we divide 10,080 watt-hours by our daily load of 4,000 watt-hours, we find that the battery bank will provide 2.5 days of storage at this rate of discharge. This is a little on the low side since we like to design systems to have 3-5 days of storage in the battery bank. To calculate the hours of generator run time required to replenish the battery bank from the 50% depth of discharge point, we take half of our batteryís amp-hour capacity 210 (420/2) and then divide that by half of the chare controllers maximum DC charging rate 80A. This gives us an estimated generator run time of 5.25 hours, which isnít too bad if you only have to do this every 2.5 days. Of course if you have some large AC loads on while the generator is charging the battery bank, the inverters might reduce their charging rate to support the AC load, and this will lengthen the generator run time. Clear as mud, right?

Another consideration about which generator may be best suited for your project is the elevation at your site. The higher up you go, the thinner the air is, and it affects the gas-air mix of your generator. So, a generator needs to be derated at high elevations, approximately 5% for every 1000 feet (above 1000 feet above sea level). For example, if you have a residence at 8000 ft. elevation, you would need to derate your generator about 35%. A 10KW generator will only be able to give you about 65% of its rated power at 8000 ft. elevation; therefore you must keep this in mind to be certain you will still have enough power for your loads and battery charging at higher elevations. Wind turbines need to be derated in a similar fashion. If you think you need a 10KW generator, but youíre at 8000 ft. elevation, you will really need one sized at about 13 Ė 14KW.

Whatever brand or size generator you are considering, we recommend that you purchase a low-speed 1800 rpm generator vs. a 3600 rpm unit because it will last longer. A remote 2-wire start capability is a plus if you want your inverter system to automatically start your generator if the battery voltage is low or if a large AC load kicks on. Fuel choice depends on what is readily available and least expensive at your site. Diesel fueled generators typically last longer than gasoline, NG, or LP fueled units, but they are much dirtier, and you need to worry about the fuel gunking up if it is stored for a long period of time. A lot of our customers go with LP fueled generators because they have other heating loads that require LP, and it makes sense to go with one fuel for all your needs instead of two or more. We donít sell generators so we donít recommend a particular brand or model over others. The feedback we get from our customers indicates that Kohler, Onan, Kubota and Honda generators are very reliable. Generators do require periodic maintenance and overhaul, so it makes sense to purchase one from a local dealer that can provide that kind of service. For a basic diagram of a generator inverter system, see www.oasismontana.com/gen-verter.html