Sizing a Generator for Your RE System: Page 2 of 2


Inside this Article

An 1,800-watt, Portable Coleman Generator
An 1,800-watt, portable Coleman generator
A 30,000-watt Cummins/Onan Generator
A 30,000-watt Cummins/Onan unit.
Appliance Rating Label
While some appliances use rated watts as a selling point (for power or efficiency), multiplying the amp rating(s) times voltage will give you the generator’s apparent load and peak surge.
Engine Generator Product Label
This engine generator displays its continuous power output rating of 5 kW and its surge capability of 6,250 W. For volt–amps, you’ll need to check the documentation.
Generator Outlet Panel
While full generator output (7,200 W) can be accessed from the 120/240 VAC, 30 A round receptacle, using the 120 V, 20 A receptacles reduces available output to 4,800 W (2,400 W from each).
An Autotransformer
An autotransformer will allow a 240 VAC generator to power 120 VAC loads, or vice versa.
Outback Power's Inverter-Charger
Current-limit settings on an inverter–charger allow maximum use of generator output without overloading.
An 1,800-watt, Portable Coleman Generator
A 30,000-watt Cummins/Onan Generator
Appliance Rating Label
Engine Generator Product Label
Generator Outlet Panel
An Autotransformer
Outback Power's Inverter-Charger

Allowing for Surges

Lastly, generator power specifications also emphasize their “surge” capacity, or the VA that can be delivered for 30 minutes or less. This number tends to be about 20% higher than a generator’s continuous VA rating. In this example, a 5,400 VA requirement may necessitate a generator with a surge rating of about 6,500 VA. However, this surge capacity can come in handy when starting motorized loads, whose start-up surge current is often several times the normal running current specification.

So even starting with a 3,600 W load, it’s not unusual to need a generator rated for at least 6,500 VA, especially if the loads have low power factors and are operated at high elevation. 

Other Considerations

Attention to system voltage, split-phase load balancing, and ratings for circuit breakers and outlets may be required to optimize generator size and performance.

RE systems that operate 120 VAC loads generally require a 120 VAC generator, and systems that operate at split-phase 120/240 VAC typically need a 120/240 VAC split-phase generator. However, there are times when a different configuration needs to be considered, perhaps because an old (but still serviceable) generator is available. For example, a 120/240 VAC split-phase generator can be used to power a 120 VAC system by wiring an autotransformer to the 240 VAC generator output. 

One-half of the total power available from a 120/240 VAC split-phase generator is available from each leg. But too many loads connected to one leg may overload part of the generator—even though the total load is less than the generator’s rated power. An autotransformer connected between full, 240 VAC output and the loads will balance the load across the generator’s 120 V legs. Popular autotransformers are available from Schneider Electric (formerly Xantrex), OutBack Power Systems, and others.

Sizing for Battery Charging

A critical load to consider when sizing a generator for an off-grid system is an inverter’s built-in battery charger. Assuming 85% efficiency and a 95% power factor, a battery charger rated at 25 amps DC delivering 1,450 W to a 48 V nominal battery bank (charging voltage is actually 48 V) equals a 1,800 VA load. Adding this load to a peak combination of essential loads can dramatically increase a generator’s size calculation. 

Carrying the example through, adding 1,800 VA to the original 4,300 VA load estimate results in a new estimate of 6,100 VA. Applying the same deratings and surge multipliers, the revised generator “rating” is increased to 9,200 VA!

At the high end, a 9,200 VA generator could simultaneously meet projected load demands and operate the battery charger at full capacity, leading to reduced generator run-time and less noise. However, because peak loads (i.e., microwave ovens) can be short duration and charging loads taper off as the battery fills, much of a large generator’s capacity may go unused, reducing its fuel efficiency. Large generators are also relatively expensive.

At the low end, a 6,500 VA generator could meet projected AC load demands, but operating the battery charger at the same time could create an overload. Fortunately, some inverter–chargers include useful features to manage such a load combination. For example, built-in chargers typically include settings that can limit battery charge current to reduce the charger’s load.

A more sophisticated tool is an inverter-charger’s AC source input current-limit setting, which limits the total current that will be drawn from the generator. If the sum of the home’s AC load current and the charger’s AC current exceeds the setting, the charger “backs off” the AC current it draws. In effect, the charger becomes a variable, “opportunity” load.

This solution allows a smaller, less-expensive generator to be used, although battery charging time (and therefore generator run time) will likely be increased due to the lower battery charge current. For example, if the household loads draw 30 A (AC) and the charger is set to draw 25 A, but the input current limit setting is 30 A, the inverter will reduce the charger load from 25 A to 0 A to keep the total load current at the limit setting. However, if a 15 A load is turned off, reducing the downstream AC load total from 30 A to 15 A, the inverter will automatically increase the charger load from 0 A to 15 A, and the generator’s total load current will still be at or below the 30 A input limit.

Choosing a Generator

Understanding how to accurately estimate VA requirements, accounting for environmental factors, and knowing how to work with generator ratings and inverter–charger settings can help ensure that a generator can provide the power needed for your system. A little up-front computation will save you dollars (and headaches) in the long run and ensure that you buy right from the get-go.


Jim Goodnight has more than 35 years of design and project management experience in a broad range of technical fields. He has been designing and optimizing PV systems since 2002, and providing technical and field support since 2004. In 2010, Jim joined Schneider Electric as a senior sales application engineer.

Comments (1)

tnorman's picture

I was thinking of using a generator as the power source to recharge an off-grid battery system but can not find any info on how long it actually takes to recharge the batteries? How fast can a generator recharge the batteries to full power? Is there anything like a "quick-charger" that can be used?

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