Off Grid Cabin

[pbennison]

Let your loads dictate the size and scale of your system. A good general rule of thumb is if your loads are never more than 1000W then 12V if OK. If less than 2000W, then 24V, and more than 2000W, then 48V. I've built all three. Currently I'm operating a 24V system with 2000W of solar for my workshop, and a 48V system with 4500W of solar for my main cabin. 24V would have been good enough for most household items, but I designed my cabin to run my 240V well pump, which it does very well.

I would recommend you start out with lead-acid. Don't worry about the 50% depletion issue. It is so much easier to scale lead-acid that you can double the capacity and still pay far less than Li. When working with lead, you can literally scale a single string from 100Ah to 2500Ah, depending on what what batteries you buy.

When scaling for your loads, look at the max discharge the batteries will take. Let's say you want to run a 2000W load on your 48V system. You don't want to drain the batteries at more than 1/10 of their capacity. The math would work out to be...
(2000W/48V system voltage) / 0.1C discharge rate =416Ah. So, a battery of about 400-420Ah would work fine. A 6V Trojan L-16 fits nicely. Rolls also makes 6-8 batteries in this size range.
You can multiple that number by the days of autonomy you want, to account for rainy days when there's no solar.

Now, determine how big an array you need to keep this battery charged. Let's say you went with the 400Ah battery. The math would be....
400Ah X 0.125C charge rate X 52V charging X 1.25X losscorrectionfactor = 3250W of solar panels.

As a general rule of thumb, I use the 2X rule for most of things I need. If I need to run that 2000W load, I'd buy 4000W of panels. If I need to run 2000W on the inverter, then buy at least a 4000W inverter. If I need about 2600W to charge the batteries, then buy 5.2kW generator. I'd get an inverter(s) that can produce standard NA split-phase 120/240VAC. Look at brands like Magnum, Outback, Schneider, and Sunny Island. I would NOT advise you to buy any of the cheap Chinese high-frequency inverters. They typically can not handle stuff with high startup surges, typically things with big motors. IE: the refrigerator, air conditioners, pumps, ect.

BTW, I designed most of my solar arrays as single-pole ground mounts, which lets me point the panels at the morning and afternoon sun. I built them primarily out of HomeDepot unistruts that I welded together. They were designed to hold 4-6 250W grid-tie panels. So, 2-3 of these arrays would completely supply all of your power, even in the depth of winter, when you have sunny days.


Keep in mind that the numbers I presented are for what works for me. Find your own numbers, and plug them into the forumulas if you like. The numbers may change, but the math stays the same.

Ok, this may sound foolish, but here goes. How do I get a 6V battery (Trojan L-16) to produce 48V? Do I get 8 of these?

[organic farmer]

The solar-power lifestyle is a different lifestyle that may take a while for your family to adjust to.

In the mornings my wife wants to percolate a pot of coffee for herself. So long as it is clear and sunny she can do that with no problems, as we have sufficient power. On cloudy days she must make her coffee on the woodstove.

When she was working, my wife insisted that when she got home from work, she wanted to eat a meal, relax and watch TV for an hour, and then to take a hot shower. But by then the sun will have gone down, so everything is done from battery power. I tried for years to convince her that when she got home, the first thing she needed to do was to take her shower, while the sun was still up. She would kill our battery charge every night with her shower.

If you can work out a lifestyle where you only consume power during daylight, then solar power can be beautiful. But once it is dark, you need to stop consuming power.

[MichaelK!]

Let your loads dictate the size and scale of your system. A good general rule of thumb is if your loads are never more than 1000W then 12V if OK. If less than 2000W, then 24V, and more than 2000W, then 48V. I've built all three. Currently I'm operating a 24V system with 2000W of solar for my workshop, and a 48V system with 4500W of solar for my main cabin. 24V would have been good enough for most household items, but I designed my cabin to run my 240V well pump, which it does very well.

I would recommend you start out with lead-acid. Don't worry about the 50% depletion issue. It is so much easier to scale lead-acid that you can double the capacity and still pay far less than Li. When working with lead, you can literally scale a single string from 100Ah to 2500Ah, depending on what what batteries you buy.

When scaling for your loads, look at the max discharge the batteries will take. Let's say you want to run a 2000W load on your 48V system. You don't want to drain the batteries at more than 1/10 of their capacity. The math would work out to be...
(2000W/48V system voltage) / 0.1C discharge rate =416Ah. So, a battery of about 400-420Ah would work fine. A 6V Trojan L-16 fits nicely. Rolls also makes 6-8 batteries in this size range.
You can multiple that number by the days of autonomy you want, to account for rainy days when there's no solar.

Now, determine how big an array you need to keep this battery charged. Let's say you went with the 400Ah battery. The math would be....
400Ah X 0.125C charge rate X 52V charging X 1.25X losscorrectionfactor = 3250W of solar panels.

As a general rule of thumb, I use the 2X rule for most of things I need. If I need to run that 2000W load, I'd buy 4000W of panels. If I need to run 2000W on the inverter, then buy at least a 4000W inverter. If I need about 2600W to charge the batteries, then buy 5.2kW generator. I'd get an inverter(s) that can produce standard NA split-phase 120/240VAC. Look at brands like Magnum, Outback, Schneider, and Sunny Island. I would NOT advise you to buy any of the cheap Chinese high-frequency inverters. They typically can not handle stuff with high startup surges, typically things with big motors. IE: the refrigerator, air conditioners, pumps, ect.

BTW, I designed most of my solar arrays as single-pole ground mounts, which lets me point the panels at the morning and afternoon sun. I built them primarily out of HomeDepot unistruts that I welded together. They were designed to hold 4-6 250W grid-tie panels. So, 2-3 of these arrays would completely supply all of your power, even in the depth of winter, when you have sunny days.


Keep in mind that the numbers I presented are for what works for me. Find your own numbers, and plug them into the forumulas if you like. The numbers may change, but the math stays the same.

[organic farmer]

... I was thinking 4 12V. My next decision will be AGM or Li. I really want to go Li while understanding how expensive they are. From everything I have researched, if you can afford them they are the way to go.

Since your cabin will be vacant all winter long and un-heated, I would suggest that you plan to remove the batteries each fall. So they are not subjected to the temps the cabin will experience.

[pbennison]

I recommend looking at a site like PV Watts

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjq5an_udDuAhUSip4KHfphD6QQFjAAegQIARAD&url=https%3A%2F%2Fpvwatts.nrel.gov%2F&usg=AOvVaw1XtCRzsAUzjiUrPd4XuBSC

to see how many watts a day your panel can produce and then see if your area can produce 5000 WH of power for two days backup you ask for. I have only had my system operational during the shortest days of the year. I also have 1000 watts of panels and I feel I’d be hard pressed to make 5000 WH of power in one day. I have used up to 1800 wh of power and I have been fully charged by 1pm. Those 10 hour long days where my panels only produce power for 8 hours, but only make 30 to 40 amps for about 8 of those, really make 5 kw of energy production in a day tough,

If you use lead acid batteries, you could get the storage you’re asking for out of 8 golf cart batteries with a 50% reserve. The batteries I mention will have over 10kwh of storage, so can’t be depleted below 50% which really cuts into storage.

I have a 12 volt system. If I had a 3000 watt inverter that was used to its potential, I’d really want a 48 volt system. If the 3000 watt inverter was not used to its potential, like to power a microwave that pulls 1800 watts, and I fused the inverter for around 2000 watts, then I’d go with 24 volts, but then I really ought to just put a 2000 watt inverter in.

Thank you. I will check this out.

[pbennison]

And how many batteries do you plan to include?

I was thinking 4 12V. My next decision will be AGM or Li. I really want to go Li while understanding how expensive they are. From everything I have researched, if you can afford them they are the way to go.

[chrisski]

I recommend looking at a site like PV Watts

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjq5an_udDuAhUSip4KHfphD6QQFjAAegQIARAD&url=https%3A%2F%2Fpvwatts.nrel.gov%2F&usg=AOvVaw1XtCRzsAUzjiUrPd4XuBSC

to see how many watts a day your panel can produce and then see if your area can produce 5000 WH of power for two days backup you ask for. I have only had my system operational during the shortest days of the year. I also have 1000 watts of panels and I feel I’d be hard pressed to make 5000 WH of power in one day. I have used up to 1800 wh of power and I have been fully charged by 1pm. Those 10 hour long days where my panels only produce power for 8 hours, but only make 30 to 40 amps for about 8 of those, really make 5 kw of energy production in a day tough,

If you use lead acid batteries, you could get the storage you’re asking for out of 8 golf cart batteries with a 50% reserve. The batteries I mention will have over 10kwh of storage, so can’t be depleted below 50% which really cuts into storage.

I have a 12 volt system. If I had a 3000 watt inverter that was used to its potential, I’d really want a 48 volt system. If the 3000 watt inverter was not used to its potential, like to power a microwave that pulls 1800 watts, and I fused the inverter for around 2000 watts, then I’d go with 24 volts, but then I really ought to just put a 2000 watt inverter in.

[organic farmer]

And how many batteries do you plan to include?

[pbennison]

We recently bought a cabin in northern NY with no grid power. I got an estimate to run grid power but it was three times the cost of the cabin. The cabin is small with a wood stove, propane heater, propane refrigerator and I plan to install LED lights. My plan is to have an off grid solar generator system with backup gas generator. I ran the power consumption calculator and estimated that I will use approximately 2000WH/Day. I have added a factor of 25% for a total of 2500WH/Day. I have spent the past couple months researching on forums like this and below is my first stab at a couple options. I would appreciate any and all advice. Plan would be to use the cabin during the spring, summer and fall. If needed I would disassemble and break down for winter.

Thank you...


2500WH/Day
2 Days Autonomy

Option 1
48V
4 250W Solar Panels
30A/48V Controller
3000w Inverter/Charger

Option 2
24V
4 250W Solar Panels
50A/24V Controller
3000w Inverter/Charger