New Off-grid house

Re: New Off-grid house

ok a few questions here. are you looking to eliminate the need of the generator altogether or do you intend to keep it as backup (recommended)? would it not also be better to go with the 320kwh worst case to base your pvs needed, noting that this may not matter as much with the generator being run occasionally?

it seems viable, but it may be better as an option to use a smaller inverter to keep high loads off of the batteries and this would help you to learn to spread out the loads. odds are you may want to up the number of pvs as most do expand their loads sooner or later anyway. this would also benefit the batteries by keeping high loads operational to times of good solar production by being more able to throw a charge into the batteries and handle the loads directly upping efficiency.

Re: New Off-grid house

the 320kwh would be the worst case draw you figured you'd need and if you design low you will need to run the generator more often. in fact you can even cut it all in half if you want to and run the genny more in the day while the batteries charge up to handle night time loads. it's all in what you feel is ok to do.

as to a smaller inverter it would stop you from popping a large load to your batteries at one time forcing a bit of awareness of your loads. instead of using 6kw at one time you can use 4kw instead and lessen the impact on your batteries by that 2kw which represents about 41a at 48v. the other 2kw in loads you would use at another time when you aren't using as much power. besides the 4kw inverters are generally cheaper, but there are also 6kw inverters that are cheaper than that too. all up to you how you want to do things and which you would prefer, but i'm just throwing out possible options to lessen battery strain.

you could even supersize it and just about eliminate any need for the genny, but even then i'd hang onto it as mother nature throws curve balls.

Re: New Off-grid house

tyagel wrote: »

Thanks NOrthguy -- Thanks for you reply on the batteries. I can definitely look at batteries other than the Trojans (and watering 72 seems unappealing). I see that you have 8 Trojan IND13-6V. What was the reason for choosing those? Why 6V over 2V? Would it be preferable for my system? I also noticed you were using two controllers. Is that generally preferable? I will try to do some research online, but I'm not yet familiar with how to appropriately size controller for system.

My batteries are weired. I bought them because they promised longer life and deeper discharges. Howere, the way they're designed is absolutely unsuitable for solar applications. I wouldn't recommend them to anyone. It was a mistake to buy them.

Usually 6V batteries consist of 3 2V cells connected in series, so you get the same thing as if you had 24 separate 2V cells, but in a different pakaging. I would prefer separate 2V cells. They are easier to move around or replace, but they require more wiring. Noone makes them as small as I needed, so I had to go with 6V.

About the controllers. A controller can only handle so much. Mine can do 3.5kW each, so I had to use two for 6kW array.

Re: New Off-grid house

Also with inverter size, the internal AC charger size varys . The 4Kw inverter may have a 60A charger, the 6Kw may have a 100A charger.

Then you have to read the fine print on the Power Factor of the charger, which varys from .9 to .6 between different brands. That Power Factor affects your generator size and running costs.

Having 2 generators, one small and one large, gives you many more choices. I use a 30A 240V dryer cord to connect to my different generators, or you could use a utility box and interlocking switches for connecting. (outlets are mounted on the generator, plug & cord on the breaker box)

Re: New Off-grid house

tyagel wrote: »

Neil,

Having slept on it your points all make sense to me I believe.
- I was thinking only of the wattage of the array and not the batteries. The batteries need to be sized for summer even though there is more daily wattage coming of the array. So I'll redo the battery calculations with the 320 number.
- I also get the inverter point now. I was Sizing the inverter to the solar array. Your point is that a 4kw inverter will easily handle the average load with room to spare. It then just leaves the question of what max loads we want to plan for, right? With the same load does a larger inverter put any additional strain on battery over a smaller one?
- now another part of the equation also seems to be coming into focus... If I plan my array for what I am expecting to be daily usage with efficiency losses and accounting for winter sun (6kW array) then how do I recover without always having to run the generator to recharge? Do I plan for twice the array needed so I can pack in one extra battery day charge per day? Or maybe 50 percent over? Frequently the system will draw minimal power during the week but even stil 4 days to fully recover from 60 percent would seem reasonable. Thoughts?
- In terms of the batteries, is there a way to calculate the maximum number of watt / hour you can charge? Also is there a strain on the battery the faster you charge them?
- Ok so I'm still lost in terms of calculating the right batteries to use... 2 4 6 12v. There seems to be some serious cost trade offs... But I'm assuming there should be some economic benefits in Sizing the 'right' batter array. If any battery system I'm going to install will only last 5 years, I'd rather go with the lowest initial price tag... I'm sure someone has done these trade off calculations... I'd be very interested to know people's thoughts? (or a link since maybe it's been discussed)

Thanks again! This is truly great being able to discuss with such a knowledgeable group.

actually everything starts with your daily load requirements that determine your battery bank size and ultimately your pv size. a generator can be run daily if you wish to keep the costs down on what the solar array and batteries may cost as systems designed to handle full normal daily loads can run up the price quite high. to further this in attempting to eliminate the generator periodically could go super expensive as that would be very tough to achieve. most allow up to 3 days autonomy for battery backup, but this is 3x the size of a battery bank that is based on a day's usage that could be supplemented with the generator if the next day is too cloudy. most of us base everything on a day for loads and charging as a starting point and some opt to handle it with cloudy days on the genny. this may be more use of the genny than you may find acceptable and hence upping the battery capacity for each day of operation that solar may not be available for.

opting for higher autonomy does add to the basic charging requirements of a pv system to being able to supply said loads and charging the batteries back up after the autonomy period is up and you now have available sunshine. i have times where the sun is gone for weeks and that would be a ridiculously over sized battery bank. the over sized array can make the autonomy needs smaller by giving more power in less than optimal times and when the sun appears it can supply both load power and battery charge power.

as you can see it is basically simple, but with so many ins and outs it adds complexity.

as to the inverter wattage rating, that is basically the most power to be drawn at one time. it is possible with many higher power electric loads running in tandem with the many normal smaller loads to hit 6kw of draw and that can be a huge strain on a battery bank even though the instances of the loads hitting the 6kw point might be rarer. a simpler 1kw inverter, only as an example, if run for a day at max would be 24kwh and at a 48v battery voltage would be a 500ah draw for a 1000ah battery bank set to max recommended 50% depth of discharge (dod). that would obviously be too small for some of your all at once loads and nobody does that at max continuously, but if you figured every load on you could have to present to an inverter it could probably exceed the 6kw inverter too. it is not desirable to do without some loads that would be an on/off thing in duty cycle in having continuous loads, but it isn't necessary to go with the max total possible load you can put to it either as you can spread out the loads. instead of running the microwave while running the sweeper and washing machine/dryer combo at the same time, you could run them at different times or even at the time of receiving good solar power with an over sized pv system able to provide power to the loads while also providing power to the batteries for charging.

rather than keep you head spinning on all of this go with a 6kw inverter anyway if you figure you'll need it and if you wish to limit the wattage draw i believe you can do that even with a lower sized main breaker to pop off to remind you of your high draws. some may not like that option as it may pop at an inconvenient time. in fact go with the basic system you've figured and just add more pv to it and if necessary another mppt cc to handle it and most circumstances will be accounted for. it's the variables that can come into play that give one a headache. you can add to the pvs system later too if you like.

Re: New Off-grid house

You can charge from the inverter/charger with a generator, the solar charge controller, or both at the same time which is an excellent strategy in winter in your climate.

tyagel wrote: »

Mike, on the inverter concepts you are discussing is it the case that one can charge batteries either from inverter or from charge controller? Or maybe what this means is that you need the inverter to go from generator AC to DC into the charge controller? Or do you go direct?

Re: New Off-grid house

good point as phantom loads can be a royal pain at times. some loads you may want switchable with power strips and in some cases you could utilize a switched outlet.

as to the figures going from the battery to ac loads i don't have the link to the info as bb brought that link to that info onto the forum and you may find it somewhere here. we often have cited efficiencies going from the pvs to the batteries at roughly 77% so you may be able to extrapolate from that the info you want.