Power outage

You are right, of course energy. It is measured in J or Wh. Efficiency in any case is the ratio of useful energy to potential, it is without units. 5.6%

 

My main reason for 'nit-picking' (at least Azusa will probably call it that) about use of the proper measurement units is that that's the discipline that most helps people remember to do the right math.

For example, if you assume that when Azusa wrote "2,000 watts" he really meant "2,000 watthours" (which would be the correct unit), then you come up with 5.6% efficiency (assuming, also, that when you wrote "35,461 watts" you meant "35,461 watthours", which would be the correct unit, and also is close to what I'm finding for a gallon of gas).

On the other hand, if you assume Azusa really meant "2000 watts" and was drawing power at that rate for 10 hours, that would be a total energy of 20,000 watthours, and your efficiency result would have been 56%, not 5.6%.

If that's what Azusa meant, it could have been made clearer by saying "2,000 watts for 10 hours" (instead of in 10 hours). (When you say "I used X for Y hours", it sounds like X was the constant rate for all of that period, while "I used X in Y hours" sounds like it took you Y hours to use X, and X should be an amount, not a rate.)

So a little bit of careless writing with units can bring us to a factor-of-ten discrepancy in possible result. Is it 5.6% or 56%?

Personally, I lean toward interpreting what Azusa wrote as 20,000 watthours. The other interpretation leads to the 5.6% efficiency figure, which strikes me as implausibly small.

By the same token, a 56% figure is clearly also implausible (because the gen 3 engine on its own isn't rated any better than 38.5% efficiency, so that's an obvious upper bound on what the answer can be; that's what fuzzy1 was getting at in #18 about limits). But at least we're more in the ballpark here.

I'd guess the most likely way to patch up the story would be that Azusa's loads were not really drawing as much as 2,000 watts for the whole 10 hours, and so the total was something under 20,000 watthours, and the efficiency will come out something well above 5.6% but also well below 38%.

But one moral is it's helpful for people to be careful in the use of measurement units, so others don't have to make guesses at what was really meant, or how to patch up the story.

 

#6

I asked AzusaPrius a question about the amount of energy received and he answered 2000 Watts in 10 hours.
I assumed that he spent 2 kWh of energy.
He had previously described his home consumers, I did not see very powerful consumers there to mistakenly count 20 kWh.

For trip reports, my program calculates the energy spent and gasoline consumption.
Energy from the petrol engine 0.96 kWh
Energy Consumption 11.92 kWh/100 mi
Fuel Consumption 66.006 mpg
Fuel Usage 0.082 ga

Potential energy in fuel: 127 MJ*0.082=10.414 MJ=2.8927 kWh
Energy from the petrol engine 0.96 kWh
Engine efficiency is approximately 33%

It is more profitable to spend gasoline on trips than on generating electricity.
But when there is no electricity due to an accident, the Prius is useful.

 

As long as we are speculating…

I calculate 75 minutes using 3 minutes to charge after 20 minutes discharge for ten hours.

Giving Azusa the benefit of the doubt (even though underlying confidence has long ago faded), I would speculate 200 watts an hour for 10 hours would equal 2,000 watt hours as stated in shorthand.

There is the Azusa we have come to expect.

Given the best engine Toyota has ever made is 41% thermal efficiency and there is typically significant losses in electrical conversions in vehicle and in aftermarket inverters, I would ballpark(er) the best possible conversion efficiency to be less than 25% and 5.6% is certainly possible.

Personally I would be more impressed with whole house backup from a quiet vehicle or generator. That said, I would take what I could get during a power fail.

 

The conversion efficiency of 5.6% can be explained as being so low due to the use of several conversion stages.
Gasoline engine, motor generator, inverter, high-voltage battery, converter, 12v battery, inverter 12/120 + Additional losses in wires

For comparison, I took data on the first portable gasoline generator with a capacity of 2000 W
Specifications

• Engine: 2.5HP
• Displacement: 79.7cc
• Total Harmonic Distortion: <3%
• Voltage: 120V
• Amperage: 20A(120V)
• Starting Watts: 2200w
• Rated Watts: 2100w
• Tank Capacity: 1.1gal (4.1 liters)
• Run Time: 8hrs@1/2 load
• Oil Type: SAE10W-30
• Weight: 52 lbs (23.5kg)
• Dimensions: 20x12x19in (50.8x30.4x48.2 cm)

During 8 hours of operation, it consumes 1.1 gallons at half power. The output is 8 kWh of energy.
8 kWh / 35.461 kWh = 0.2256
Efficiency 22.56%

 

Another example of different math coming from not enough attention to the word choices.

If Azusa writes the engine starts every 20 minutes and runs for three, that's nine minutes every hour, or 90 minutes in ten hours.

If rjparker writes "3 minutes to charge after 20 minutes discharge", that's three minutes every 23, or nine minutes every 69 (about 78 minutes in ten hours).

So that just looks like Azusa said one thing, and rjparker rephrased it just differently enough to get something else.

But i shortened the Azusa quote to make it look that clear. A longer quote shows the two interpretations were already fighting each other in it:

The first part seems clearly to say "kick on every 20 minutes" (which would mean a 3 on, 17 off duty cycle), but the later "then silence for 20 minutes" contradicts that and means the 3 on, 20 off duty cycle that rjparker used.

So the original post from Azusa was just imprecise enough to say two different things in the same sentence, and rjparker picked one of the two.

Still working on the units though! Watt hours for energy, plain watts for power. 200 watts for 10 hours would equal 2,000 watt hours.

"watts an hour" would be some kind of unit for a slew rate, like a wind farm as a storm approached.

 

Sooo.... You can run the things you "need" for about 12 hours on less than a gallon of gasoline.
Sooo.... Less than $3.00 for half a day.

 

That seems to go pretty well with my 2019 result that the engine uses about 0.6 gallons per hour when running to charge the battery (as opposed to about 0.3 when it runs only to keep coolant temperature up). 0.6 gallons per hour ✕ 84 minutes comes to 0.84 gallons.

'course, it'll all come down to the things you "need" and what power they draw.