portable generators: how to know what load you're imposing such that you can deduced fuel endurance?

Thread Starter

ThirtyWest

Joined Jul 15, 2017
150
Looking at a portable generator to power some essentials for hurricane season.

The specs usually list something like: 9 hours @ 50% load.

Aside from generating a list of all my potential consumers (including the LRA values to get startup wattages for things like a refrigerator) is there a way to monitor the gen and get an idea of the current load that's being imposed upon it?

I'm not using a transfer switch but rather a locking panel on the breaker; the large HVAC consumers will be shut down, leaving only a few things in the house. Still, I'd like to know the load so I can estimate how long that 8 gallon tank will really last.
 

crutschow

Joined Mar 14, 2008
30,100
The easiest would likely be to measure the load of your various devices, such as the fridge, TV, etc., with a power meter such as the Kill-A-Watt.
Those can actually measure the total kWh over some time period to give an accurate long-term energy usage value.

Note that the inverter-type generators, although more expensive, are more efficient at light loads, since they can throttle down the engine speed, which a standard generator can't do.
 

Ian0

Joined Aug 7, 2020
5,502
Note that the inverter-type generators, although more expensive, are more efficient at light loads, since they can throttle down the engine speed, which a standard generator can't do.
If that were true, an ordinary wouldn't be able to regulate speed (and hence frequency). Most have mechanical governors which adjust the injector pump to keep the speed steady. The latest ones, which must meet Euro 6 emission standards are electronically speed controlled.
A typical diesel generator is 32% efficient at full load, petrol and LPG generators are rather less efficient.
The fuel usage vs. power graph is a straight line. At zero power output it will use about quarter of the fuel it uses at full power (and of course the efficiency is then zero), and half power it uses 62.5% of the fuel it uses at full power.
So, if you have the fuel usage at 50% load figure, then the fuel usage at full load is 1.6x
 

Ian0

Joined Aug 7, 2020
5,502
If appliances in your country are labelled the same as they are in Britain and Europe, then there will be a sticker which tells you the number of kWh it will consume in a year. Divide by 8766 to get the long-term average power.
 

crutschow

Joined Mar 14, 2008
30,100
If that were true, an ordinary wouldn't be able to regulate speed (and hence frequency)
That's not what I meant.
An inverter generator can reduce its engine speed to increase efficiency at partial load, since the output frequency is fixed by the inverter circuit.
A non-inverter generator, of course, has to maintain the same speed, independent of load to maintain the frequency, so is not as efficient at partial load.
 

Ian0

Joined Aug 7, 2020
5,502
That's not what I meant.
An inverter generator can reduce its engine speed to increase efficiency at partial load, since the output frequency is fixed by the inverter circuit.
A non-inverter generator, of course, has to maintain the same speed, independent of load to maintain the frequency, so is not as efficient at partial load.
Yes - Agreed.
I wonder how much difference it makes.
 

crutschow

Joined Mar 14, 2008
30,100
I wonder how much difference it makes.
Probably it would be mostly below a 50% load.
One reference said it's up to a 40% saving.

Since some loads are cyclic, like a fridge or furnace, an inverter generator can throttle down between the times those loads are active.

Another advantage, is that the high frequency drone of the motor is reduced at light load.
 
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crutschow

Joined Mar 14, 2008
30,100
If you are in a cold area where winter storms may knock out the power, you might also want to check your furnace/heater's power usage.
 
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