Cost to run: electric vs gas

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Here's my newest video on the EGO mower, my findings on the cost to run. I hope you enjoy!
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Luke

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Posted 3 years ago

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Blue Angel, Champion

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Great video!!!

It's been so long since I used a gas mower I have no idea how much fuel it burns... maybe someone can chime in? It would be neat to see by how much $/year the Ego beats a gas mower.
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Steve Valdes

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It's great to know it's only about $.02 a full charge for the 4Ah (or 5Ah?). Should we include the cost of the battery over it's life span; say 500 charges(or?) also as a cost to replace fuel? I'm on the fence regarding this at this point.
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Blue Angel, Champion

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I was thinking the same thing, Steve. Since everything is easier in US dollars (LOL), $200 divided by 500 uses is $0.40/use. Add in the cost of your hydro and it's $0.42/use.

That's based on 500 charge cycles and zero other maintenance. For someone in a southern climate cutting once a week it's about a 10 year period, and in a northern climate cutting 6 months out of the year it would be a 20 year period.

How much maintenance would a gas mower need in that time? Pure speculation... maybe a quart of oil each year and a spark plug every 5 or so? At $5 each, the southern example would burn through 10 quarts and two spark plugs, or $60 in 10 years, and the northern example twice that, or $120 over 20 years. Divided by 500 cuts you're looking at $0.12 and $0.24 per cut, respectively.

Now who can give me an estimate for how much gas an average mower uses in roughly 30-40 minutes of use?
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Jacob

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Well 1 gallon of gas is 125,000 btus.
There are 49 horsepower hours per gallon.
Small engines are between 30 and 40% efficient(i think).
This gives us 14.7 - 19.6 useable horsepower hours.
Divide that by the horsepower of the mower and lets assume 5 hp(mowers are rated for peak, actual hp is much lower).
This gives us 3 - 4 hours of run time per gallon.
Lets assume 2.50 usd per gallon.
This gives us $0.83 to $0.65 per hour to mow.
Multiply that by the hours the battery lasts, in this case 0.75.
This gives us $0.62 - $0.47 per comparable cost. Per charge.

Alternate route of math is calculate power to power. So input power from the wall in KW/h can be converted to HP (kind of its complicated) its not 0.746 kwh = 1hp as it is normally calculated. Thats old math(from what I understand)

These numbers may be right or wrong. Just my attempt. Real world application would be much more accurate.
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David Cline

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I was going to estimate 1/4 gallon per 45-60 minutes based on my past experience. But Jacob's way sounds much more scientific... and also backs up my much less intelligent guess!
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ClarkM

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Not that it makes a difference but I don't think that you should include the cost of the battery for comparison purposes. If we're comparing to a gas mower - we're comparing the cost of the mower in both cases - which in the case of the ego, includes the battery. Now if you want to really do a comparison - choose the mower and use the excess cost of the ego vs mower x and have that be the portion amortized over the comparison period. 

Unreal that the cost per charge is so low. That's absolutely amazing in my opinion. $1 a year to cut your lawn in actual costs...that's just great.
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David Cline

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A perfect comparison would be total cost of ownership over some realistic time period. The only problem is that you would have to make so many assumptions that the comparison wouldn't be very informative.

I'm guessing average length of ownership for most lawn mowers is between 5-15 years. At 5 years you really have to factor in a resale value, which is relatively unknown for the Ego. Beyond 5 years you at some point have to factor in a battery purchase, but at an unknown cost per Ah.

You also have to factor in some repairs and upkeep, which is a total shot in the dark beyond the warranty period. And if you are more electrically or mechanically inclined, you can probably do a lot more of the repairs yourself than the average consumer.

But there is no Ego customer that ISNT weighing the cost of batteries into the up front purchase decision, so I think it's safe to assume that some battery wear and tear cost must be included in any cost of use calculation to be relevant to most consumers.

If you just want to convince yourself that you saved a ton of money buying an Ego, go ahead and assume that the batteries are free and last forever!
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ClarkM

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They aren't free and they don't last forever???
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Jacob

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Um. No.
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Blue Angel, Champion

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Jacob, great work with the numbers!

I do have to add something though. WRT the efficiency of a gas mower engine, 40% is pretty optimistic! The 2016 Prius made headlines last year when Toyota's Engineers managed to crack the 40% thermal efficiency barrier:

http://www.hybridcars.com/2016-toyota...

The average modern car engine, with all of its advanced combustion chamber design, direct fuel injection and all the other tricks mixed in, is only about 30% efficient. I would be amazed if a gas lawn mower was even 20% efficient.

The other thing to keep in mind about gas engine efficiency ratings, they are all stated for the engine's peak operating efficiency, as in the best spot on the BSFC (Brake Specific Fuel Consumption) curve. When an engine is operating at anything but its optimum combination of speed and load, it's efficiency drops from that peak value... one of the reasons we see more and more speeds in car transmissions (keeps the engine in its sweet spot over a broader range of operating conditions).

In practice I would bet the average gas mower is extremely inefficient. Having said ALL that, we could just use someone's real usage data instead of going grey trying to calculate it. :-)
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David Cline

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Well not factoring some battery cost into the comparison is essentially the same as saying batteries are free and last forever. But that could actually be better than guessing what batteries will cost 5-10 years from now and ending up way off.

It's not at all uncommon to ignore some costs in this sort of analysis (salespeople do it all the time), usually for sake of simplicity or irrelevance to the immediate decision being made.

I'll never forget a physics professor I had who lamented having to control for or exclude variables that could never actually be ignored in the real world. He would grumble, "this is like studying massless elephants on frictionless planes."
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ClarkM

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I hope you know that was a joke...
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Jacob

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Ugh good. People on here sometimes are just looking for a reason to cause an argument.
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John William Baxter

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Hello - I am really interested in this topic.  Had a 5.0 AH battery fail recently.  When you get older the years just spin by, so my initial perception is we just bought this yesterday and it cost a fortune and it has failed - what a rotten design.  Then I realized the battery was nearly three years old!  My next thought was hey I am willing to spend a fortune to avoid polluting as much (although this all depends on where my energy comes from).  Ego stuff is EXPENSIVE, but so are Teslas and people buy them.  My gut feeling is you don't save a cent, and you probably pay more to mow your lawn.  But I could be wrong, and even if it is more expensive the convenience and feeling that you might be helping just a tiny little bit to head off planetary destruction makes it all worthwhile.
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szwoopp, Champion

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I would say it costs more to go with battery power over gas. I accept that due to the quiet and convenience, no gas, no smell, no spills, no pulling a rope to start the motor, no gummed up carburetors, etc

With the chemicals involved in a battery I am not sure if there is any great environmental benefit other than the immediate area of the mower.  But again I am all in for the quite and convenience!

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Jacob

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I agree with szwoopp. I've got 2k in batteries (8 5ah batteries), I've saved probably 600 in gas vs electric cost over 5 years
Keep in mind I mow a large lot.
Also lithium cells will probably be the next worst thing to the planet.
But they are also not in the air so that is a plus side
I have also read that gas outdoor power equipment pollutes as much as 20 cars. Is it true? I don't know but they sure do stink like they do.