Generator Sizing

The example we’ll use is an 1,800-watt PV system installed at an off-grid home in the Midwest. The battery bank is two strings of flooded lead-acid batteries, with eight 6 V batteries in each series string. The battery bank is rated at 700 amp-hours at 48 VDC nominal—or about 60 V under charge.

The minimum charge rate, C/20, is 35 A (700 Ah ÷ 20), but the 1,800 W PV array will produce 30 A at best (1,800 W ÷ 60 V)—less than desired. With cloudier weather, the generator and inverter become even more important to achieve the desired charge rate. An ideal charge rate would be about C/10, or 70 amps. This will minimize generator run time, while still allowing the PV array to contribute 25–30 amps when it can without exceeding the batteries’ maximum charge rate.

We would like a charger, usually integrated into the inverter, that can hold a 70 A charge rate. Sticking to the most commonly used inverter/chargers, the Xantrex XW4548 is rated at 85 A, which is plenty. The Magnum MS4448AE puts out 60 A, which is close enough for our needs. The largest OutBack VFX3648 is rated at 45 A, so one solution would be a pair of FX3048Ts, each rated at 35 A DC, for our desired 70 A total. Let’s assume the Xantrex XW4548 inverter for this example. According to the manufacturer, the AC input charging current necessary for this inverter to create the full charge rate is 24.8 A at 240 VAC, or 5,950 W. This gives us a starting place for sizing a generator to power this inverter.

First, adjust for maximum load. As a rule, most generators run best close to, but not at, maximum output. 85% is a commonly used figure, although some high-quality units can be run at 100% of their nameplate rating. So 5,950 W ÷ 85% = 7,000 W, assuming a quality, accurately rated generator. Next, adjust for elevation and temperature. Assuming sea level, adjusting for elevation is not necessary (see “Elevation & Temperature Effects” below). To adjust for temperature, assume 100°F operation, which is an approximate derate of 6% for a generator rated at 60°F. This results in 7,000 W ÷ 94% = 7,447 W—a 7,500 W generator should be sufficient. If there are loads that will be run while charging the battery, apply the temperature and elevation adjustments to them and then add this amount to the sizing total.

Finally, make sure the voltage output of the generator matches the input of the inverter/charger. Traditionally, off-grid inverters have been based on 120 VAC input and output. In this example, matching the inverter’s input voltage to the generator’s output is no problem since this inverter/charger supports both 120 and 240 V configurations.

Elevation & Temperature Effects. Power output of all generators is rated for use at sea level. As elevation above sea level increases, air becomes thinner, with less oxygen. The result is decreased performance. Most generator owner’s manuals include an adjustment factor, but a general rule is to derate 3.5% per 1,000 feet above sea level. For example, at 5,000 feet, a generator will produce about 82% of its rated output.

As the ambient air temperature increases, the engine and alternator heat up, which decreases both efficiency and power output. Generator manufacturers will rate power output as low as 60°F and as high as 120°F; there is no common standard. Using both rated output power and temperature as starting points, a typical deration is 1.5% per 10°F above rated temperature. For example, a generator with a rated output of 10,000 W at 60°F will be capable of 9,400 W at 100°F.