Michael Deaves

Hi all,

I'm looking at buying an off-grid property and building a cabin and likely a few other buildings there over the next decade or so. The cabin will have a small solar system with batteries to power a fridge, lights, etc.

During the build I'm obviously going to need some power tools up there, circular saw, a chainsaw to clear the land, drill, reciprocating saw, etc.

At first I had my heart set on a Makita 18V cordless combo kit. You get a couple batteries with the kit, and they can fit in any of the tools. But then I remembered all the cordless drills I've encountered in my life that are deemed useless because their batteries have run down and can't be replaced. What a huge waste. Do I really want to tie my self-reliance to Makita's battery technology. What happens in 5 years when they come out with a new battery design and I can't replace my batteries. My tools are essentially worthless. It's just another form of planned obsolescence.

Meanwhile my Dad's corded circular saw and drill that he has had since the 70's are still working just as well today as they did back then.

So I would love to go with all corded tools if possible. The problem is I'll likely be building all over the site where I won't have access to mains power.

But then I remembered that I'm going to be buying batteries for the cabin anyway. So why not design the cabin's solar system in a portable way, such that 90% of the time they are inside the cabin, powering lights and a fridge. Then when I need power elsewhere I can unhook the batteries and inverter from the cabin, and wheel them out to another part of the property to give me power where I need it.

Then I can go with all corded tools, saving me a bunch of money and hassle down the line.

I'm wondering if anyone here has any experience with something like this? Anything I should be concerned with (vibrations are bad for the batteries maybe?)

Thanks for your input, I'll be sure to post pictures when it's up and running.

Hi Jay,

I meant 1.25 kWh, as in kilo-watt-hours, not kilowatts per hour. Kilo-watts per hour doesn't really make any sense, since a kilowatt is a unit of power, which is already a rate of energy consumption, so we don't need to make it a rate again by saying kilowatt PER hour (that makes as much sense as saying MPH per hour). A kilowatt-hour is the energy that is used by running something that has a power of 1 kilowatt for one hour.

I have no idea why battery and solar people talk in kilowatt-hours, it's just confusing to everyone involved. The SI unit of energy is the Joule, that is what should be used. Power is the rate of energy use, or Joules per second. We call this a Watt. So if you are using one Watt your are consuming energy at a rate of one Joule per Second. A Kilowatt is then 1000 Joules per second.

Now comes the weird part. A Kilowatt-hour, is the amount of energy used by running a 1 kilowatt appliance for one hour. Since a kilowatt is 1000 Joules per second, and an hour has 3600 seconds in it, a kilowatt-hour is actually 3600*1000 Joules, or 3600 kilo-joules.

I think the confusion is made worse by the cooling industry, which rates their appliances in BTU-hours, when in reality what they really mean is BTUs PER hour, which is 1055 Joules per hour, which is 0.293 Watts (Joules per second).

So in my post I say that in my home I will need 1.25 kWh per day, which is 4500 kilo-joules of electrical energy. A 52 Watt generator will produce 52 Joules per second, or 4500 kilo-joules in 24 hours.

I hope this makes sense.

I just came across these amazing structures in the book 'Architecture without architects', by Rudofsky Bernard

https://en.wikipedia.org/wiki/Dovecote

Apparently they were built with the sole purpose of attracting pigeons to roost in their tops. Then once filled with dung, they were smashed open and the dropping were used as fertilizer.

This seems like an amazing way to increase the fertility of small farm. Why go through all the trouble of collecting manure from off-site, when you can have the birds fly it to you!

Has anyone had any experience with this?

Hi Justin,

I am in a similar situation about deciding what to do with my electricity situation in an off-grid cabin I'm planning. I figure I need about 1.25 kWh per day of electricity to get by. One solution I have found is these guys,

http://www.devilwatt.com/

They make a little power bar thing that you screw to the top of your wood stove to generate ~ 70 W of continuous power, with no moving parts. You just need to supply cooling water, which could be done by gravity feed from a spring or roof top run off. I haven't seen any reviews of these things, so I don't know how robust they are. Just because they have no moving parts, doesn't mean that they don't break. 1.25 kWh/day is only 52 W continuous power.

If you are running the wood stove anyway then it might be a good solution for the winter months.

Hi all!

I've got a question for all you experienced Permies out there.

First off I plan to convert a chest freezer to a fridge using the method described here,

https://permies.com/t/31282/Energy-eficient-Chest-Fridge-Kwh

I am also really interested in sizing my batteries much smaller than a conventional solar system, since I am on a tight budget. One avenue I am exploring is lining the fridge with 2L pop bottles filled with water, then setting the thermostat to ~ 0 C, so that when there is plenty of sun, the bottles will freeze, and act as a thermal storage. Then if there is no sun, the bottles will melt, and keep the fridge nice and cool for hopefully a few cloudy days. One challenge with this is how to keep my food from freezing, while allowing the pop bottles to freeze. I'm hoping that placing the thermostat near the bottles, and insulating the gap between the food and the bottles, will solve this issue. We will have to see.

Anywho, my real question is how prevent the fridge from clicking on if there is a few cloudy days, while still allowing things like my water pump and lights to function normally. Ideally I would somehow like to have two inverters (or DC power lines), one that is live if there is any usable charge at all in the batteries, and the other that only works if the batteries have lots of charge in them. The fridge could then be wired to the latter, so that in the case of a few cloudy days, it would not come on, instead relying on the ice to keep itself cool, and allowing the other loads to use the battery. Then once the sun comes back out again, the battery first recharges itself, then the fridge would click on, refreezing the ice and 'charging' it's thermal storage once more.

Anyone had experience with something like this before? I would like as much as possible to keep costs down, as the whole point of this system is to save me $$$ on batteries. What would be really cool is to be able to schedule different appliances in terms of their importance. Communications could be used all the time, even if it means damaging the batteries, water pumps and lights would be next, fridge next, then wall outlets for laptops and washing machine.

Thanks so much!

Galen, can you elaborate a little bit more about what you mean when you say 'to make our home comfortable'? What inconveniences do you typically have from running a smaller battery bank?

Of course I would lose some efficiency in this kind of system, but looking at it from a holistic view point it might be better energetically, because the embedded energy in the batteries is not needed.

Thanks for the tips, it makes a lot of sense what you're saying.

It sounds like a good route to take would be to buy a few used batteries, for quite cheap, that can't hold a charge for very long, but at least buffer the incoming energy from the panels.

Another route that I could take is to just purchase a few extra panels, instead of the batteries, and then somehow dump the extra power when the items I'm running don't need it. For example lets say the fridge motor is 5A @ 24 V, then when the panels have enough sun to produce 5A, the fridge motor clicks on, but if there is shade or something it stays off. If the panels want to produce 6A, then the 1A difference is wasted somehow. This seems to me that it should be possible without the expense of a battery and expensive charge controllers.

I am planning to build a simple off grid house soon, and as part of that I was evaluating my energy needs and determining the cost of my solar system. What I notice is that the batteries are the most expensive part, and they are prone to wear, and need to be replaced every few years.

Then I started thinking about what I actually need. My electricity needs are as follows,

-Water pump to pressurize water system
-Fridge/freezer
-Laptop
-Charge cell phone
-Lights

All of these system, except for the lights, have built in storage capabilities. If I get a big enough pressure tank, I could conceivably go many days without having to run the water pump. A well insulated fridge or freezer likewise. My laptop/phone have their own batteries. The lights are the own component that doesn't have it's own storage, but a few 5W LED bulbs could be run from some D-cells.

Has anyone tried this route? That is providing other forms of energy storage (thermal, pressure, component batteries), in order to reduce the cost of your solar system.