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Old 23-12-2020, 15:53   #1
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Electric galley kwh sizing based on propane usage

I've been thinking about all-electric and part-electric galleys for a future boat in the 40' range. As has been the topic of other threads, this has become a viable alternative to propane as battery, inverter, and solar technologies have improved.


I am trying to quantify the typical daily usage and have been measuring and timing power draw and run times in my kitchen ashore. It occurred to me that 2 gallons of propane per month is a pretty widely reported use rate for the galley and that with some conversions I can arrive at an equivalent amp draw.


2 gallons of propane at 100% efficiency produces the same amount of heat as 40 kwh of electricity at 100%. Generally propane for cooking is less efficient. While this is difficult to quantify exactly, propane being 75% as efficient as electricity is perhaps a close guess, which means that 30 kwh in the battery is roughly equivalent to 2 gallons of propane in the tank, from a cooking standpoint.


30 days in a month, so that gives us 1 kwh a day is roughly equivalent to 2 gallons a month. That's about 80 AH at 12 volts.


How closely does this match the experience of those with electric galleys who have tried to measure how much electricity they use?
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Old 26-12-2020, 12:15   #2
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Re: Electric galley kwh sizing based on propane usage

A similar calculation I found suggests one gallon of propane is equivalent to 27 Kwh. California seems to think that people average 42 Kwh/month cooking, or 40% above your estimate.

There are bunches of variables (heat-up time, shape of your pot bottoms, what kind of cooking you prefer) to throw in. It really messes up an engineering level of calculation.

Most attempts at this deal with up-front and use costs, with propane stoves costing more up front but less to operate. I think I'd slap 2 Kwh/day into computing the necessary energy to produce each day in order to avoid waking up in the morning with dead batteries.

The real questions, I think, are where are you going to get your energy and how do you want to distribute the energy jobs? If you have a big genset, then do you prefer cooking with electricity or gas? Either is available to you. If your electrical generation is limited, might you shift jobs like heating water and cooking to propane? At the extreme, some cruisers even use propane refrigerators and save solar for the electronics. Lithium ion batteries give you more storage (at whopping expense), but the critical question is not how much you store, it's how much can you produce each day.

Both of us prefer gas cooking, and don't like running the genset at night. Ergo, propane stove and propane occasional heat, electric water heat when on shore power but engine heat for water heating when putting. We don't have solar. That's our set of choices.
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Old 26-12-2020, 12:37   #3
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Re: Electric galley kwh sizing based on propane usage

The Morgan's Cloud website just had a discussion on power requirements for induction cooking, https://www.morganscloud.com/2020/10...-it-practical/ that you might be interested in reading. It is a two part article and I think it then led into the two part discussion on 12v/24v electrical systems.

I think the article is behind a paywall but the site is well worth the cost.

Later,
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Old 26-12-2020, 12:52   #4
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Re: Electric galley kwh sizing based on propane usage

Quote:
Originally Posted by Jammer View Post
30 days in a month, so that gives us 1 kwh a day is roughly equivalent to 2 gallons a month. That's about 80 AH at 12 volts.


How closely does this match the experience of those with electric galleys who have tried to measure how much electricity they use?
We don’t have an electric galley but have run the numbers in consideration and fell in the 80-100 AH range for our average planned use.

Would be curious to see what others actual numbers are as well.
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Old 26-12-2020, 13:27   #5
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Re: Electric galley kwh sizing based on propane usage

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Originally Posted by Jammer View Post
Generally propane for cooking is less efficient. While this is difficult to quantify exactly, propane being 75% as efficient as electricity is perhaps a close guess,
From what I recall, the US DOE figures that a propane stove is about 40% efficient (that is 40% of the energy in the fuel goes into heating the food), whereas normal electric cooktops are 70-75% efficient. Induction is over 90%.
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Old 26-12-2020, 14:27   #6
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Re: Electric galley kwh sizing based on propane usage

A bit of real life experience here. I have about 700 amp 12 volt lifepo batteries. Boiling a large pot of water (5 liters) for cooking pasta on an induction stove takes away 8% of the capacity. Water is from the tanks so ambient temperature start.
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Old 26-12-2020, 15:07   #7
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Re: Electric galley kwh sizing based on propane usage

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Originally Posted by Jammer View Post
30 days in a month, so that gives us 1 kwh a day is roughly equivalent to 2 gallons a month. That's about 80 AH at 12 volts.

How closely does this match the experience of those with electric galleys who have tried to measure how much electricity they use?
1 kWh per day seems about right (if you're a bit frugal). I had:

- 0.27 kWh for pasta
- 0.25 kWh for a cauliflower dish
- 0.13 kWh for half a pizza
- 0.12 - 0.14 kWh to boil one liter water

Some inductions stoves are darn noisy, really make sure you buy a good one! (For the name of a model that works great and is almost wisper quiet send me a PM, contrary to the Morgan's cloud I don't charge... )
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Old 20-01-2021, 17:52   #8
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Re: Electric galley kwh sizing based on propane usage

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Originally Posted by Rimica View Post
A bit of real life experience here. I have about 700 amp 12 volt lifepo batteries. Boiling a large pot of water (5 liters) for cooking pasta on an induction stove takes away 8% of the capacity. Water is from the tanks so ambient temperature start.

I’m looking at a 2-element induction cooktop and wonder what wattage is needed for decent boiling times? Many smaller units have maximum 2kW per element, though some more expensive ones provide 2.8kW as boost power. What do you think?
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Old 20-01-2021, 20:12   #9
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Re: Electric galley kwh sizing based on propane usage

Everybody talks about "efficiency", but most of the time they are avoiding the REAL question (at least to me) what is the cheapest? I have no real interest in the amount of energy input or how much power is lost, all I care about is the dollars expended.

The equation is really simple for most boats. Most boats will not be able to cook using just solar power, they need to run a generator. If you are running a generator, there is NO WAY your cooking will be cheaper than propane--if you are honest with the costs. Fuel, maintenance, and depreciation for generator and batteries will be significantly more expensive than propane. How expensive is a complex question, that depends a LOT on how it is used. Even safety is not that clear cut. Far more boats have fires from their electrical system than from propane. To be honest, neither one scares me. I know if I do things right and take proper care, both systems are as safe as a land based house.

Now, you might see advantages that outweigh the cost issues. For some things, I know I do. I have a 3000 Watt electric kettle that heats my coffee water in 3 minutes in the morning. I know I pay extra for this, and I am happy to. I have an induction cook plate that supplements my propane stove for some kinds of cooking and even a convection oven and microwave, but for most things, propane rules both as a chief chef and as chief accountant.

For simple cost, it is VERY hard to beat propane unless you can supply all the power you need from solar, and that is a very large and expensive solar system. Likely not possible on anything other than a cat with acres of deck space that can be paved with walk on panels at a bizzillion dollars per generated kW-hr.
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Old 20-01-2021, 23:10   #10
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Re: Electric galley kwh sizing based on propane usage

Quote:
Originally Posted by Jammer View Post
I've been thinking about all-electric and part-electric galleys for a future boat in the 40' range. As has been the topic of other threads, this has become a viable alternative to propane as battery, inverter, and solar technologies have improved.


I am trying to quantify the typical daily usage and have been measuring and timing power draw and run times in my kitchen ashore. It occurred to me that 2 gallons of propane per month is a pretty widely reported use rate for the galley and that with some conversions I can arrive at an equivalent amp draw.


2 gallons of propane at 100% efficiency produces the same amount of heat as 40 kwh of electricity at 100%. Generally propane for cooking is less efficient. While this is difficult to quantify exactly, propane being 75% as efficient as electricity is perhaps a close guess, which means that 30 kwh in the battery is roughly equivalent to 2 gallons of propane in the tank, from a cooking standpoint.


30 days in a month, so that gives us 1 kwh a day is roughly equivalent to 2 gallons a month. That's about 80 AH at 12 volts.


How closely does this match the experience of those with electric galleys who have tried to measure how much electricity they use?

I tested a microwave (46%), butane burner(50%) and induction hob (75%) for efficiency heating water.
To measure how much energy went into the water I measured start and end temps and weighed the water.
For the microwave and hob I used a watt-meter to measure how much energy was used.
For the butane burner I measured the before and after weight (to nearest gram) of stove and fuel can to determine how much fuel was used and from that calculated energy used.

I then used that and a spread sheet to calculate the A-hr/d equivalency of a tank of a given size lasting a given number of days.

If you want to accurately use this you need to weight the tank after its filled using a luggage scale and again after it's empty. You need to log when you start using the tank and the day or fraction of day it lasts. Then go into the following table for an estimate of daily electrical usage.

The weakness of the table is that I have not accounted for the efficiencies of baking/roasting with propane vs a convection oven. I'm pretty sure the relative efficiencies will be similar to the stove top because a propane oven needs to be well ventilated or it will chock on its own combustion gases. I I could the convection oven being twice as efficient, but until I test it won't know for sure. That testing is being planned and prepared for.

Here is a link to a previous post of mine about this: https://www.cruisersforum.com/forums...ml#post3306142
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Old 20-01-2021, 23:21   #11
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Re: Electric galley kwh sizing based on propane usage

Quote:
Originally Posted by BillKny View Post
Everybody talks about "efficiency", but most of the time they are avoiding the REAL question (at least to me) what is the cheapest? I have no real interest in the amount of energy input or how much power is lost, all I care about is the dollars expended.

The equation is really simple for most boats. Most boats will not be able to cook using just solar power, they need to run a generator. If you are running a generator, there is NO WAY your cooking will be cheaper than propane--if you are honest with the costs. Fuel, maintenance, and depreciation for generator and batteries will be significantly more expensive than propane. How expensive is a complex question, that depends a LOT on how it is used. Even safety is not that clear cut. Far more boats have fires from their electrical system than from propane. To be honest, neither one scares me. I know if I do things right and take proper care, both systems are as safe as a land based house.


Now, you might see advantages that outweigh the cost issues. For some things, I know I do. I have a 3000 Watt electric kettle that heats my coffee water in 3 minutes in the morning. I know I pay extra for this, and I am happy to. I have an induction cook plate that supplements my propane stove for some kinds of cooking and even a convection oven and microwave, but for most things, propane rules both as a chief chef and as chief accountant.

For simple cost, it is VERY hard to beat propane unless you can supply all the power you need from solar, and that is a very large and expensive solar system. Likely not possible on anything other than a cat with acres of deck space that can be paved with walk on panels at a bizzillion dollars per generated kW-hr.

If you are replacing a working system, electric is a money loser.
If you are starting form scratch, the electric system is probably10-20% more expensive in capital costs.
I did the math in a post something like 8mo ago and it's lost.
I could update it tonight but I don't have the time.

Regarding energy production and storage.
If you need aobut 100Ahr/d to cook on that means you need about 300W more panels and 300Ahr more battery capacity. If you wanted to be conservative 400 & 400

The long term costs are not just savings on the cost of the propane, but the spending the time, taxi money, etc in getting it. How much is your time worth? For a couple hours chore, it probably isn't worth much. If you are looking at spending half a day getting the propane, then your time starts to have a bigger monetary value. At least it does to me.
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Old 20-01-2021, 23:26   #12
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Re: Electric galley kwh sizing based on propane usage

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Originally Posted by fxykty View Post
I’m looking at a 2-element induction cooktop and wonder what wattage is needed for decent boiling times? Many smaller units have maximum 2kW per element, though some more expensive ones provide 2.8kW as boost power. What do you think?
I saw a video on YT showing a guy comparing a 1300W hob from Costco I believe boiling water in 3/4 or less the time than the biggest burner on his Viking or similar gas stove. 1800W split between 2 burners would be fine. A counter convection oven is 1800W in the US too.
America's Test Kitchen tested counter top convection ovens and recommended Breville (sp?) for about $250.

The big cost is you're going to need a 4,000-5,000W inverter and cabling to support it else you run the risk of popping the overload if you or a guest turnes on the oven and both burners on high all at the same time. Oh, and the inverter needs to be a true sine wave, induction doesn't get along with modified sine wave power.
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Old 21-01-2021, 00:04   #13
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Re: Electric galley kwh sizing based on propane usage

We switched to induction cooking over 2 years ago and as liveaboards in the Tropics, it was our best decision ever.

Fast, clean, with a heating efficiency that keeps the galley much cooler.

If you harmonize cooking meals during max solar production, power consumption is not really an issue.

I decided to replace my original Smeg 4 burner gas stove top, with a 4 hob Elba stove, as it was the same footprint.

We ocasionalky use 3 for a big breakfast or a back hob simmer of stew/soup etc, while cooking a meal upfront.

If unused Elba hobs are there.....they provide extra counter space for plating, adding ingredients as they are clean and none conductive.

So I find the 4 hob a plus over a 2 hob arrangement.

But I do have the Solar and inverter capacity to manage a staggered use of 4 hobs
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Old 21-01-2021, 04:23   #14
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Re: Electric galley kwh sizing based on propane usage

Electric cooking and water heating is an excellent option to have. We use an average of around 1.5 kwhrs a day at a rough estimate when using the system exclusively.

If you are powering this from solar, the only limitation is that you cannot then cruise areas of poor solar insolation. This limitation may or may not be a significant problem depending on your cruising plans, but keep in mind cruising plans are best written in sand at low tide so versatility is helpful.

Our solution is to have the multiple options of electric induction, propane/butane, and diesel (via our Reflex stove). Each of the options has advantages and drawbacks. This multi system approach is not hard to set up and enables us to use the best fuel for the job depending on the circumstances and cruising area/season.
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Old 21-01-2021, 06:39   #15
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Re: Electric galley kwh sizing based on propane usage

Thank you all for the replies.

I have been doing some additional reading and have also performed actual measurements in my home kitchen using induction, resistance appliances, natural gas, and butane. While I have a more lengthy writeup in my notes I may share at some point, that also addresses the soft "cooking enjoyment" aspects, here are my findings in summary:
  • My tests involved boiling water from room temperature.
  • Induction 73% efficient
  • Natural gas 15% to 22% efficient depending pan diameter
  • Butane campstove 35% efficient
  • Appliances with integral resistance element (electric skillet, electric kettle) 83%-86% efficient
It is possible that the gas stove is not running at the nameplate gas use level which would explain the very low figures. Industry figures are 30-35%. Another factor is that industry tests include simmer times, not just boiling water, where the heat transfer may be more efficient.



A closer reading of test literature combined with my own experience shows that there is no efficiency benefit to induction cooking over any other form of electric cooking. The 90% figure Lodesman quotes upthread has not been reproduced in other studies.
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