Seeking help with off grid cabin

Re: Seeking help with off grid cabin

Welcome to the forum.

What you need is lots and lots of money. Why? Because "fridge, chest freezer, lights, laptop, washing machine, toaster" all use power. Refrigeration equipment: 1-2 kW hours per day each. Toaster: nasty 1,200-1,500 Watts while on. Washing machine: big start-up surge, heavy running demand.

Don't even think about trying this on a 12 Volt system. 12 VDC equipment won't save you anything, and probably will cost you more.

Avoid used batteries if you want the system to work. They may function today and drop dead tomorrow.

If you buy those 235 Watt panels you will have to have an MPPT type controller as they are 99% certain to be GT panels with a Vmp around 30 and thus will not match any system Voltage directly. They also will be inadequate for charging anything but a very small system.

A "crappy inverter" is the same as throwing money away.

So ... How about setting up a small "lights & computer" system on 12 Volts for now, and installing a full-power system for everything else later?

Re: Seeking help with off grid cabin

If 6kW hours is the number you're shooting for you should be going with a 48 Volt system. As such you'd be looking at roughly 500 Amp hours of battery @ 48 Volts, a 60 to 80 Amp MPPT controller, and about 3200 Watt array. It might be possible to do this for $10,000 US.

Don't discount the toaster and especially not the refrigerator; you just have to take them into account when doing the design. My system was basically built to handle the full-size refrigerator plus 'extras' and can actually produce around 2.4 kW hours per day. I have certain advantage of climate and elevation combined with seasonal-only use; it would be impractical to try and keep it going in the Winter up here because the available daylight shrinks to 6 hours a day total.

I also have the satellite Internet set-up, water pumps, and even a microwave that can be run midday if wanted. The overnight storage is slim, so when the sun goes down stuff gets turned off. Everything costs too much up here, but if I could I would expand the solar array and battery capacity both to make it easier to get through the marginal days.

Re: Seeking help with off grid cabin

I am not talking about a 3X overbuild or a 4X overbuild: I'm talking about sizing a battery bank so that it doesn't go dead quickly. For 90% of the batteries used this means you automatically lose 1/2 the calculated capacity because regularly discharging to less than 50% capacity is murder on batteries. That gives you 50% capacity to work with, and if you divide the average consumption over two days you get 25% DOD. The remaining 25% capacity provides one "no sun day" of energy and on the third day of clouds you start the generator. This also allows the most efficient use of generator capacity.

PV array has to be sized to charge the whole battery, regardless of SOC, and the shortcut of basing peak current on 10% capacity works most of the time. In some areas you'd want even more, due to less-than-perfect insolation. Cutting it down to the minimum battery manufacturer's recommended 5% is asking for trouble as it doesn't account for concurrent loads and will not charge the batteries sufficiently in the hours of daylight available (which should be based on minimum sun, not maximum).

My advice is based on my experience and includes a lot of repairing of systems designed by people who didn't do the calculation right in the first place.

I have seen people (including Bill, who does not have an off-grid system) advise on the basis of "three days with no sun" which is impractical because you end up with more panel and battery than you need.
I have seen people advise on the basis of panels producing 100% of their rated power, which they don't.
I have seen people advise on the basis of 100% system efficiency, which doesn't exist in the real world.
I have in fact seen people give out a lot of really bad advise which results in system with no margin for error or variation in operating conditions.

And then I have to fix their messes. Some of these are "professional installers" who in fact have only a vague understanding of what they are doing based on being able to connect wires together and have it work momentarily. I wouldn't trust them to change a light bulb. Whereas my success rate is 100%, even with seemingly hopeless situations.

You can improve over-all efficiency with certain management techniques, but you must be able to adapt your usage to them. Until you know whether or not this can be done on an individual system it is better to err on the side of caution.

The two biggest mistakes repeated over and over are insufficient panels for charging the amount of batteries and insufficient batteries for supplying power for the loads.

The sun does not shine brightly all the time.

P.S.: I'm getting sick of people derailing threads of newcomers with bad advise and off-topic discussions.

Re: Seeking help with off grid cabin

znate wrote: »

Ok I think I'm starting to catch on.

So if I want to use those used batteries I could use 4 of them in a 24 volt system which would give me 325 ah which is slightly less than what you said above. Does it make sense to do it that way? I could use the best 4 and keep the rest for another project or try to sell them.

Or should I go with a higher sytem voltage, say 48v? Whats the advantage? slightly higher efficiency? Smaller guage wires? Is it easier to get components for a 48v system?

Thanks for all the help everyone. Some of it's over my head but I appreciate it anyway.

What the application is will make a difference. Particularly the big question of "what happens if the batteries fail?" Is it critical? If so, avoid used batteries. Sometimes the new ones are problem enough. If it's not critical you can gamble on used batteries.

24 Volts works for most applications. It is more difficult to get fuses/breakers rated to handle 48 Volt systems because the system actually functions at ~60 Volts when charging. So if you see those Square D breakers rated for "48 VDC" know that they do not guarantee performance at the higher Voltages such a system actually sees. On the other hand they work fine at 24 Volts.

There is slight improvement in efficiency in 48 Volts over 24, and it is the way to go if you need more power (either stored or on-demand). The 'threshold' for this is debatable, as a lower Voltage system can take an occasional, short-term 'hit' of high current. As with so many things the decision should be made based on the continuous average power need.

I seem to have posted this link several times in the past few days, but here it is again: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

Re: Seeking help with off grid cabin

If you can't recharge them they go dead once the stored power is used up. If they sit around under 70% SOC for days they will sulphate and become unable to be recharged. This is why charging is important.

So, 325 Amp hours @ 24 Volts? About 3.9 kW hours stored. If they are aged don't expect them to have full capacity; the actual Amp hours go down over time no matter how well-treated the batteries are.

Your proposed array should be sufficient to charge them (at about 39 Amps peak current). Do you need an FM80? The FM60 is slightly less money and would still handle the current without problem.

I would not try to run an inverter from a DC-to-DC converter. True, the Suresine should pull less than the 30 Amp rating of that converter but only for the constant power rating; one surge and the converter will pop. A lot of us wish Morningstar would make a 24 Volt version of this inverter!

Have you looked at the Samlex inverters? Pretty good value for smaller size equipment and they have 24 Volt versions down to 300 Watts. These PST models are not good for motors, though.

Re: Seeking help with off grid cabin

They're not all that efficient especially when it comes to 'real' surge power. They don't work that well with inductive loads. This is not to say it absolutely will not work for such use, but that there is the possibility it won't or won't work well.

On the other hand the SA series do, and they aren't exactly expensive either.

Re: Seeking help with off grid cabin

znate wrote: »

I like that idea. Could I set it up so that when the 24v batteries are full the extra power would feed another charge controller that would charge the 12 volt bank?

Yes. It requires an MPPT controller to down convert the Voltage and a second battery.

Is there a dump load type circuit built into charge controllers that could be used to feed the second controller?

Not needed. Although you can arrange to have the second controller switch on only when the main bank is full by utilizing the AUX control of an Outback or MidNite for example. Otherwise it could be operated by a Voltage controlled switch. Normally you'd just include the power usage of the secondary bank in the over-all design and allow it to run all the time.