Help me with this food truck math problem
March 16, 2022 8:24 PM   Subscribe

I'm trying to do the math of moving a food truck from a generator to electricity on the grid (i.e. plugging into the grid). For whatever reason, the math is not working out - maybe you know better?

This is the information I have:

The food truck's current situation is that they have a 6.5 kw gasoline generator.
They pay $20 a day in gas at a location where gas is $5/gallon.
They run their generator for about 9 hours a day.
They only run a cash register, exhaust fan, fridge and lights using the generator (not the stove and no microwave).

Electricity on the grid costs $0.30/kwh.

I tried to do this equation by thinking about how much gas they run through a day, but that number came out really high and it doesn't seem like they have appliances requiring that much power.

Then I tried to figure out how much power each appliance (fridge, fan etc.) uses and that number seemed really low.

Can you help me figure out how much it would cost for this food truck to move to the electrical grid (i.e. plug in) in $/day?

Bonus question: how do generators work? Do they burn the same amount of gas independent of how much power is actually being drawn on them?

I won't threadsit, but I'll stick around in case there are any follow up questions. Also, this information is to the best of my knowledge, but I may be misunderstanding something - so let me know if something is not making sense. I also realize that different appliances will have different power requirements, so estimates based on average or typical is welcome.
posted by Toddles to Technology (14 answers total) 1 user marked this as a favorite
Power is measured in watts (W) and kilowatts (kW). So the generator, for example, can put out a maximum of 6,500 watts when it's running full speed. It doesn't normally run at full speed, though. A good generator saves gasoline by avoid excess power generation. So you shouldn't use the rating as a value in your math. The only number you should use from the generator is the $20 a day to keep it filled.

To get a real number I'd start by looking at the actual power consumption in Watts of those four items (register, fan, fridge, lights). There are devices like "Kill-A-Watt" you can buy or borrow to measure.

If you can't measure that easily, then look for a power rating label on the big items. Take a fridge, for example: You'll see writing somewhere like "110V 2.5A", which means it's pulling a maximum of 2.5 amps of 110 Volt grid power. Watts is (volts x amps), so when that fridge is starting up it may pull 275 Watts to get going, or it might pull 275 Watts all the time. Again, you're trying to get a rough estimate. Other things like light bulbs are easier to add up, they're already labelled in watts. If you have LED bulbs, look for the actual draw and not the packaging that says "40W equivalent" when it's really pulling 8W.

Add it all up (including the light bulbs) and you'll get a total in watts. Divide by 1,000 to get kilowatts.

Then you pay by time. So if the food truck pulls 1,000 watts for an hour, that's one kilowatt/hour or $0.30 in charges (does that include all the fees and taxes?) If the truck pulls 500 watts for an hour, that's $0.15. A simple divide or multiply.

I'd try to work up a worst-case cost scenario. Total kilowatts x 9 hours x $0.30. That should get you an idea of cost.

You would have to pull 7,400 continuous watts for 9 hours to hit the $20 budget and since your generator can't go over 6,500 the math seems to point to the grid being cheaper from the POV of this random internet person. Good luck!
posted by JoeZydeco at 8:45 PM on March 16, 2022 [5 favorites]

I was googling and it said that a generator does indeed use more gas when it needs to produce more electricity.

I also found this factoid. A 5-kilowatt gas generator burns roughly 0.75 gallons of fuel per hour. This analysis uses an awful lot of assumptions but all things being equal that would mean a 6.5 kw generator would use 1 gallon per hour.

The current load of the generator doesn’t use 1 gallon per hour, it only uses one every 2.25 hours. So again handwaving but that implies using only 2.9 kw every hour.

2.9 kw x 9 hours x 0.30 $/kw-h comes out to $7.83.

I don’t know anything about generators so take this with a grain of salt, but I’m inclined to agree with JoeZydeco that it would be cheaper to be on the grid. Even if it wasn’t cheaper it might be worth it to not have to procure and store multiple small containers of gasoline every day.
posted by cali59 at 9:08 PM on March 16, 2022 [1 favorite]

The generator also has a minimum power level it will run at, so most likely they are using much more fuel than the ideal wattage of those four devices.
posted by nickggully at 9:09 PM on March 16, 2022 [1 favorite]

Dr. Google tells me that modern gasoline generators have an efficiency of about 20%. 4 gallons of gas has about 130kwh of energy. 20% efficiency mean you are using about 25kwh during a typical day. That would cost you $8-9 on the grid.

Going at the problem from the other direction, if the poor generator were going flat out, 6.5kw over 9 hours is 58.5kwh, which would be $17.50 on the grid. Even in the worse case scenario you are saving money (although perhaps not enough to make it worthwhile). In the more realistic case you are halving your fuel costs.
posted by It's Never Lurgi at 9:28 PM on March 16, 2022 [6 favorites]

I get the same math as It's Never Lurgi. Though with the caveat that I'd be putting pretty huge error bars on the efficiency estimate. But that also agrees fairly well with cali59's estimate by other means. So it's basically impossible for getting the electricity directly to cost more, and there's good reason to suspect it would cost less than half as much.
posted by Zalzidrax at 9:36 PM on March 16, 2022 [1 favorite]

Add in generator maintenance and the eventual cost of replacement, and the grid looks even better.

And when workers realize how much stress the noise and the fumes were adding to their jobs, they'll be shocked.

But I wouldn’t get rid of it; food trucks have to move fairly frequently on average, and it may be needed again.
posted by jamjam at 9:47 PM on March 16, 2022 [6 favorites]

FWIW the average house in the U.S. uses 28.9 kWh per day (24 hours).

28.9kWh * .30 = $8.67 - that's what the average house would spend in electricity over a 24 hour period.

Your one light, one fan, refrigerator, cash register used for only 9 hours a day will use less than that - probably less than half.

So that $8.67 is an upper bound and we can say pretty confidently that your usage will be markedly less than that. Probably 1/2 or even 1/3 or 1/4 of that - as a decent ballpark estimate, anyway.

I know you're probably already thinking this through, but personally I would plug in wherever possible even if the cost were the same. It's less noise, less pollution (that is coming right into your own personal environment, meaning whoever is working the food truck is getting most of the negative effects of it), less wear & tear on the generator, less gas to horse around with, not subsidizing Russian oligarchs or giant gas companies in general, and so on.

Saving probably more than 3/4 the daily cost would just be a cherry on top of all that.
posted by flug at 9:48 PM on March 16, 2022 [1 favorite]

There's no doubt that running a small gasoline generator is more expensive than grid power. It never makes sense to run a generator instead of using grid power, even with expensive grid power at $0.30 per kWh. If it did make sense to run a generator, people would use generators for their homes - but no one does that except in an emergency because it is very expensive, because generators are very inefficient. So you don't really need the details here because you know the grid power will be way cheaper for whatever amount of power is being used.

That said, I would be surprised if the total power being used is more than 1kW on average. A typical small commercial fridge only runs about 500W maximum, probably much less than that with normal use (a good household fridge runs about 1Kwh per day in typical use). An exhaust fan would be a few hundred watts at most. The cash register and lights probably only add 100W or so, unless they are very inefficient. Total should be somewhere between 500-1000W. That's 4.5-9kWh in a 9 hour day, or $1.35-$2.70.
posted by ssg at 10:24 PM on March 16, 2022 [7 favorites]

ssg has it - the actual power draw is very little and the cost of the gas 10x the grid.

Small generators are terribly inefficient for this kind of thing, because they need to be oversized to handle the startup power draw of "reactive" loads.

A reactive load is an electrical device that does not draw a consistent amount of power, it draws varying amounts. The most common is anything with an electric motor, like the motor in the compressor of the fridge. Electric motors pull a huge amount of power for a fraction of a second at startup, often 5x the normal running power. So a 500w commercial fridge could pull 2500 watts for 1/5 of a second as it starts up. This fridge will start up every couple of minutes or so while in constant use, being opened, etc.

Generators need to be sized up to handle the startup rush, so they spend the rest of the time outputting power far less than their maximum rating. Even when idle, the generator burns a certain amount of fuel just to fire the cylinders. The larger the generator, the larger the idle fuel cost.

There are more efficient types of generator (inverter) that can actually slow down the gas engine when idle or producing low power, and complex electronics keep the frequency of the output alternating-current power at exactly 60 hz needed for the appliances. They are still not perfectly efficient at low or idle loads.

The 6500 watts seems suspiciously high for this application. Are you sure there is not an electrical heating device somewhere? Even a small electric toaster oven or coffee pot can use 800-1800 watts. One or two of those plus the startup watts for one fridge could require 6000 watts easy. Usually heating appliances are non-reactive loads, they pull the same amount of current when on, but they do use a lot of electricity to generate heat. This is why those food trucks often have big propane tanks for a cheaper, longer lasting source of heat.
posted by sol at 3:17 AM on March 17, 2022 [2 favorites]

You MAY be able to mitigate the situation by inserting a stand-by battery between the generator and the power use. Most of the time, low power use, they draw power from the battery. If they're near the grid, they charge the battery from the grid. If they're away from the grid they charge the battery from the generator but only if it's below 25%. But battery comes with its own maintenance issues and replacement costs. Maybe use a large 12V battery like an 8D automotive battery and an inverter instead?
posted by kschang at 5:44 AM on March 17, 2022

6500/7000 watt generators are one of the "standard" sizes in the US (because then you can have a 240-volt outlet with 30 amps per leg) - Honda has probably sold millions of their 6500 series gennys over the years.

Do they burn the same amount of gas independent of how much power is actually being drawn on them?

Depends - most modern small gas generators have an "eco mode" where the engine runs harder with greater power draw, which in turn means it uses less gas if there's a smaller power draw. This is fine for power tools on a construction site, not so good for other applications where a stable electric current is important, so there are also (as mentioned) inverter generators that produce constant power regardless of what the engine is doing. Those you can run in "eco mode" and easily get 8-10 hours out of a 5-gallon tank in a generator.

As others have said, it will definitely be cheaper to run off the grid from a "dollars and cents math" perspective. But for a theoretically mobile food truck the question is whether it is practical - can you run an extension cord along a route where people won't trip over it or pull it out? Is the outlet in reasonable distance from the truck (voltage drops with distance, how much depends on the cable gauge (thickness))? Is the available outlet its own circuit or did Uncle Harry the Handyman just tack an outlet onto the same circuit that's running 65 other things in the building? (Refrigerators have a surge/startup power rating where they draw more power when the compressor kicks in, which happens on startup and possibly at other times, like if it's a really hot summer day, and that can be 2 or 3 times the "constant run" rating, so adding a fridge to an already heavily loaded circuit could trip the circuit breaker.)
posted by soundguy99 at 7:08 AM on March 17, 2022

The roof of the food truck is probably a great place for solar panels, which might not allow them to abandon the generator or electrical power, but might really reduce gas usage. Cash register, exhaust fan, and lights are manageable. Propane refrigerators are available. A decent battery bank can be charged overnight for backup. Gas or diesel generators produce a fair bit of pollution. There might be grants for solar conversion.
posted by theora55 at 9:06 AM on March 17, 2022 [1 favorite]

Don't forget the cost of the generator - how many hours will it run before it has to be replaced?
posted by canoehead at 9:57 AM on March 17, 2022 [1 favorite]

They could spend $1.5k on a solar generator ($1000 generator, $500 2 panels) and charge it via the grid at night or panels during the day and that would probably do it. This guy will power my big ass Samsung fridge for about 8.5 hrs without a panel, adding a panel or two would make it last all day according to the manufacturer.
posted by julie_of_the_jungle at 8:03 AM on March 18, 2022 [2 favorites]

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