Comment by wrycoder | Hacker News Reader

Comment by wrycoder | Hacker News Reader

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I'm running a small hot tub in central New Hampshire. I figure it costs me $15 per month in the winter.

Edit: Service in the US is generally 200A or 400A. I'd say 400A is overbuilt for what they have, but that doesn't mean they actually use much of that capacity.

For example, I average 1000 kWh per month, which is about 1.39 kW 24/7, or 12 amps average on my 200A service. The 200A is to have headroom for peak demands, and we probably never get close to 200A.

If I were to rewire my panel (it's full), I'd go for the 400A - it's just heavier copper in the three lines to the entrance panel. That would not increase my monthly power bill right now, but it would provide lots of excess capacity in case I want to switch to heat pumps and electric vehicles.

Since we heat with wood and propane and don't use electricity for either and have mostly LED lighting and a gas stove, I do wonder where all the power is going, especially at night. Too many computers, I guess. Add analysis to my to-do list!

NH electric rates are $.20/kWh [1]. If you are spending $15/month for heating the hot tub, then you are consuming ($15/month)/($.20/kWh)=75kWh/month to heat it.

Assuming you are using are using a resistance heater (100% heating efficiency), to raise 200Gal of water (typical small hot tub size) from 50F to 100F requires about 25kWh of energy.

To only use 75kWh/month (255kBTU), you would need to only raise it to 100F less than 3 times a month, and that assumes that the water temp doesn't naturally drop below 50F (unlikely in NH).

It's likely you are spending significantly more than $15/month to heat it with electricity.

Propane is about $30/MBTU, which would work out to about $9/month for the equivalent amount of heat.

1. https://www.electricrate.com/residential-rates/new-hampshire...

2. https://bloglocation.com/art/water-heating-calculator-for-ti...

I pay $0.13 per kWh in my town. (That's my marginal cost.)

The tub is well insulated, with an insulated cover, so you need to account for that. My estimate on the hot tub cost is by comparing my kWh usage after the tub was installed to the prior year use.

Most small tubs are electric, like mine.

> I pay $0.13 per kWh in my town.

So assuming $15/month, you are using 115kWh, which is 392kBTU of heating energy. It takes about 83kBTU to heat 200gal of water from 50F to 100F, which would be like 5 full heat ups from 50F. Of course there are the other convolved factors you mentioned (insulation).

Believe me, it's worth $15 per month to my wife! So happy.

It's funny how removed we are from reality. Even if $15/month might be cheaper than a propane bill, it would be far far more expensive against the planet to heat it with gas/coal burned at a factory 100 miles away, converted to electricity, then converted back to heat inside the spa water heater.

I think that an air/water heat pump on the tub, with say a COP of 8, would be less expensive to operate than propane or straight electric. But, I couldn't justify the capital expense. (Small hot tubs are electric.)

I like my wood/propane mix for heating, because it's immune to power outages. At some point I'll get too old to cut/split/haul wood, though, and will have to figure something else out.

I'm not sure why there's a push to eliminate gas cooking in favor of electricity. Perhaps some people are concerned with methane leaks in the natural gas distribution system.

> I'm not sure why there's a push to eliminate gas cooking in favor of electricity.

It can make a lot of sense in urban areas. In addition to removing the methane leaks (with their high GWP), it reduces the need to install/maintain expensive natural gas distribution infrastructure, and also improves indoor air quality by eliminating NO2 and CO production within the house. If the power goes out, you just use a propane grill in the backyard, or a small butane stove.

> Small hot tubs are electric.

S̶p̶e̶c̶i̶f̶i̶c̶a̶l̶l̶y̶,̶ ̶f̶o̶r̶ ̶t̶h̶o̶s̶e̶ ̶w̶h̶o̶ ̶w̶e̶r̶e̶n̶'̶t̶ ̶a̶w̶a̶r̶e̶,̶ ̶s̶m̶a̶l̶l̶ ̶h̶o̶t̶ ̶t̶u̶b̶s̶ ̶a̶r̶e̶ ̶t̶y̶p̶i̶c̶a̶l̶l̶y̶ ̶h̶e̶a̶t̶e̶d̶ ̶t̶h̶r̶o̶u̶g̶h̶ ̶f̶r̶i̶c̶t̶i̶o̶n̶ ̶a̶n̶d̶ ̶w̶a̶s̶t̶e̶ ̶h̶e̶a̶t̶ ̶f̶r̶o̶m̶ ̶t̶h̶e̶ ̶c̶i̶r̶c̶u̶l̶a̶t̶i̶o̶n̶ ̶p̶u̶m̶p̶s̶ ̶a̶n̶d̶ ̶d̶o̶n̶'̶t̶ ̶h̶a̶v̶e̶ ̶d̶e̶d̶i̶c̶a̶t̶e̶d̶ ̶w̶a̶t̶e̶r̶ ̶h̶e̶a̶t̶e̶r̶s̶.̶ ̶Y̶o̶u̶ ̶c̶o̶u̶l̶d̶ ̶a̶d̶d̶ ̶a̶ ̶s̶e̶p̶a̶r̶a̶t̶e̶,̶ ̶m̶o̶r̶e̶ ̶e̶f̶f̶i̶c̶i̶e̶n̶t̶,̶ ̶w̶a̶t̶e̶r̶ ̶h̶e̶a̶t̶e̶r̶ ̶a̶n̶d̶ ̶r̶u̶n̶ ̶t̶h̶e̶ ̶p̶u̶m̶p̶s̶ ̶l̶e̶s̶s̶ ̶o̶f̶t̶e̶n̶,̶ ̶b̶u̶t̶ ̶i̶t̶'̶s̶ ̶n̶o̶t̶ ̶a̶s̶ ̶s̶i̶m̶p̶l̶e̶ ̶a̶s̶ ̶r̶e̶p̶l̶a̶c̶i̶n̶g̶ ̶o̶n̶e̶ ̶k̶i̶n̶d̶ ̶o̶f̶ ̶h̶e̶a̶t̶e̶r̶ ̶w̶i̶t̶h̶ ̶a̶n̶o̶t̶h̶e̶r̶.̶

Never mind. Apparently I was misinformed. The circulation pump moves the water through the heater, but isn't the primary heat source.

I don't think this is true. Could you source this? I've had several small outdoor electric hot tubs, and all of them have had dedicated heaters. The couple times I've had problems with the heater, the symptoms involved the pump running constantly and the tub never reaching temp. I'm sure there is some friction heating, but I don't think it's anywhere near enough to reach standard temperature.

Apparently you're correct, and I was misinformed. You could implement it this way—like how Tesla's newer designs can run the electric motor in a less-efficient mode to provide heat for the battery pack—but the designs I investigated did have separate heaters as part of the circulation system. (For some reason the specs for these were not listed in the product summary, but rather only in the manual.)

> I'm not sure why there's a push to eliminate gas cooking in favor of electricity. Perhaps some people are concerned with methane leaks in the natural gas distribution system.

It's that, as well as the fact that having a gas stove means you have a gas hookup in the first place, so you're more likely to also use it for heating. That in turn leads to more emissions than other heating methods produce.

Well, even with a catalyst, my wood stove puts out a lot of emissions. But, I don't live in an area with an inversion layer.

So in NH I am paying just over $4.20 a gallon for propane and about $0.18 a kwh for electricity. With those prices a propane heater for my hot tub would cost more to run. Also a heat pump heater for the hot tub would optimistically get around a 3-4 COP in warmer weather and 2-3 in the colder weather but either of those would lower your price to heat it considerably but the capital expenses neglect those savings.

Source: https://www.amsenergy.com/fuel-cost-calculator/

Interesting, thanks. Looks like burning shelled corn is the way to go. Or coal.

That area is almost exclusively hydroelectric power.

Er, which area is that? NH is almost entirely nuclear from the Seabrook site.

I just skimmed the article, but I thought it said it was in BC.

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