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Forum Index : Solar : Solar power only, is anybody doing it?
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oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Allan, your correct in your contention that MPPT will give more power out of your panels. If you use the 190watt 5a panels then they are useful from a power perspective.... but I would not use 5a panels. For best bang for buck, your best to use the 6x6 cells with 60 cells per unit... why? Your mppt can only try to use the buck converter to get the use of any voltage over and above the battery terminal voltage... mine seems to be mostly 55v and up from 10am onwards. This means I have only got a differential of maybe 5 volts between mpp and terminal votage.... the panels will be acting as a current not a voltage source, so we can say that we will lose 5x8=40 watts from our 250 watt panel.. we will still have the 210 watts left for direct charging at no cost financially... so in best conditions we may save 5-15% by using mppt.... so the technical argument is still for mppt. However, the system argument is strongly against it. For 20kw of solar you want 20kw or more of mppt... in your case maybe 4000 dollars all up.... thats 8000watts you could go and buy now instead.... and if you use 60 cell panels your so far in fron it does not matter. But more importantly, you do not want all that power when the sun is out, only when it is not.. in the worst light conditions you will find 5% is closer to the output, so 10kw will only do 500 watts, and now mppt will be usefully higher than pwm would be..... but 40% more panel, and no mppt will easily come out in front... for no extra cost So for the expense it is not worth it if you choose the right panels, and if you have a small system, then you really want the extra 2kw you could have bought for the price of the midnights. So remember, there is only the difference between vmp and terminal x current to be gained.... 30%max if they are 72 cell panels, and maybe 10% if 60 cell panels ( 72cells use around 5-5.5amp chips, and 6x6 use 8-9amp chip...for the same power panel.... your current differential is already there for you before you start with no mppt. ie a 60% increase for doing nothing . Your system sounds good.... but I would try to sell the 72 cell panels, and use exclusively 60 cell 6x6 units, then sell the midnights, and buy more panels... simpler, and will work just as well with nothing to break. If you really want efficiency, then use high voltage 400v strings and GTI inverters, and the efficiency will go up much higher than the midnights..... but there is no reason to do so if you have more panel.... always more panel, the panels are the only part of your system that will not fail ( very rarely)... all the rest will fail, and the money spent will be then have nothing to show for it. Even so, in poor light conditions, the HV GTI with very good mppt, will struggle to get 5% in dark o/cast conditions into a matched load..... but with a big array, the sky does not need to lighten up very much to see that go to 15-20% and still pretty dark.... but now 1-2kw and rising ... a bit lighter and we are in the 3-5kw... sun pops out for a few minutes and we are in the 7kw range.... more than I want to handle really.I does not take long at these levels to replace the o/night usage. So yes MPPT raises efficiency by 30% at the very best, as thats all there is to get from a 72 cell panel ( (36-24)xImax ) and thats 66% output for the worst possible terminal case... for a 60 cell panel it will be ( 31-24)x I max thats about 80% for the worst case... and a bank at 24v is abnormal when you have this kind of panel available... mostly in the 90% range for the 60 cell units.... mppt probably won't get much of that last 10% because of inefficiencies in the buck converter ( yes they claim very high efficiency, but I have not seen any useful increase from a 60 cell panel into a 25v bank using mppt units MPPT is very useful for a normally designed system... otherwise pwm and more 60 cell panels will do better in the long run. The other thing is that when you have big arrays, it is only maybe 6 - 10 times per year when your going to struggle ( very very dark sky all day... not just overcast but dark), and it's then the mppt versus pwm will have any possible relevance, the rest of the time there will be far far too much power, and you really want losses then. So the higher differential between terminal battery and terminal soar, the more useful the mppt will become, so best to keep differential low ( panel choice) and spend the mppt money on panels..... this only works because panels are so dirt cheap now... otherwise tracking arrays and mppt..... oztules Village idiot...or... just another hack out of his depth |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I thought I was truly going mad when I first made this observation, buy it has been proven to me over and over again. Like you I have Midnite charge controllers and I think they are fantastic, with the monitoring I can see the difference between one controller with flat panels and the other where the panels are angled. In really cloudy weather the flat ones win every time, when there is lots of sun who cares there is too much power available anyway, even in winter. When Oztules first told me that I was better off without MPPT I was not convinced, however, I have to agree with him now. Especially when you look at the financial aspect, take the cost of the MPPT controller and turn it into more panels. You will gain far more power than any possible gain from the MPPT. A new Midnite bought in Australia will buy 1500 Watts of new panels, if you are buying second-hand panels you will get maybe 3 - 5KW more panels. When Oztules has time to show us how he makes his PWM charge controllers I will be having a crack at that also. With a bit of time and effort I will also be able to add some of those useful features you mentioned regarding the AUX controls and data logging on the Midnites. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I am beginning to think exactly the same way myself. There are two types of MPPT systems, the first uses a microprocessor to hunt up and down to find a peak. the second just holds the panel at the maximum power voltage marked on the panel. Its probably pretty correct, although temperature effects will make a difference, but not a big enough difference to worry about. The first is complicated, the second requires only an op amp and a few resistors. the difference in real performance is bugger all, because the power peak is not really a peak, but a cusp. A couple of volts either way makes hardy any difference at all to the actual power. You can very easily prove this to yourself with an adjustable load and a power meter. Better to just use the simple system and add one more panel, that will very likely give you more power for the money spent than going to some supposedly high tech super software that is supposed to be 1% better than anything else, but costs significantly more. Also agree with the concept that just because you have (say) 8Kw of panels, you must use an 8+Kw rated MPPT controller. In winter you may only get a few hundred watts, and if that is enough to get you through winter, you probably do not need much more than that during the summer months with very long sunny days. So I might humbly suggest 8Kw of panels is a wonderful thing to have, but you may only need (for example) a 2Kw current limited MPPT controller to work pretty well all year round. Sink the cash into more panels. At mid winter you will be very glad you did. I am also amazed at how well, up facing panels work all year round. In summer that is where the sun is, so no surprise. But in winter when the sky is an evil dull grey in all directions, and the sun is supposed to be low in the north somewhere (?) Up beats having low angled northern panels easily. When the winter sun does finally come out, the low pointing northern panels obviously win. But in really wretched dull cloudy weather up will give you more. I have my low looking northern panels mounted directly above my up panels, so that rain water runs off the sloping panels right onto the flat up facing panels. So far so good. Cheers, Tony. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Everyone has different expectations, I just installed an air conditioner that draws up to 3 KW. 8 KW is not enough I am going to double that, then I will have plenty of power when it is really cloudy and when the sun is shining and it is hot I have more than enough power and can easily run the AC. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| M Del Senior Member  Joined: 09/04/2012 Location: AustraliaPosts: 155 |
Borrowed the latest renew magazine (Oct 16) from the local library to read over the break. I used to buy it but it has become too commercialised lately. Instead of a do it yourself live small type magazine, it has become a live large with big houses and massive amounts of tech. In an article. A 10kw off grid panel system, 65kwh of batteries and associated parts costs $55k or so. Plus a gen set and gas cooking/hot water. And you can use up to 10kwh of power a day, the way it is worded that is day and night. That partly explains why the guys I mentioned at work think anything under that amount won't cut it. My experience tells me (small hobby systems) that once your batteries are topped up in the morning then the panel output is yours to use, so on a sunny day with a 10kw system once batteries are full at 10am you have 10kwh(?) to spend every hour till the panel production starts to drop off. The commercial system sellers seem to be claiming the full daily usage limit is 10kwh maximum, even after batteries are fully charged early am. Could someone confirm my experience/thinking, or have I missed something? edit HAPPY NEW YEAR TO ALL Mark |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
It depends on your expectations, and anticipated way of life, I don't think you can really generalize. Climate and location comes into it too. You can live on the unfortunate side of a mountain range or in a deep valley for example. Or your location may have terrific potential for a wind or hydro installation. Probably the biggest factor of all is you and your families level of motivation. A great deal can be done with very little with the right approach. Cheers, Tony. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
10 KW of new panels would cost under $10k without any rebates, batteries are normally rated in amp hours except by people who live in a grid tied world. 65 KWH is about 3200AH which is approximately $30k add an inverter, charge controllers and installation and you are close to $55k. Currently I have 7.5 KW of panels and a 1600AH battery bank. I run all of the following and only need to draw power from the grid if we have 4 plus days of very cloudy weather. Large side by side fridge with ice maker, large chest freezer, electric stove/oven, water pumps, solar HW pump, computer network 24/7, computers, 65" led TV, full size colour copier/printer plus vacuum and all normal appliances. Recently I have added a 10KW AC that draws up 3 KW but drops down to around 2 KW once the house has cooled down. I can run this only on sunny days while the sun is shining but I do have sufficient power to do it. I am planning to add another 5 KW of panels to my system shortly. I have spent well under $10k buying second hand most items and DIY almost everything. That includes a 8KVA Lister Diesel Generator I bought for $400 not working and rewound the generator myself to fix it. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
After several very cloudy days with the remnants of TC Debbie and less than 2KWH's per day during this I have come to the conclusion PV solar alone is not sufficient. At least in our climate close to the coast, although these 2 days are okay we do get weather like that for a few weeks sometimes. If I had 40 KW of panels we could get by but that is not practical. There is also the slight risk of hail destroying the panels, this happened near here a couple years ago with orange size hail stones. Apparently, panels are okay up to about golf ball size. My conclusion is for me at least is that some other backup is required even if it is a decent petrol generator to get through the occasional bad weather event. Nice to have power though when 35,000 homes in the area don't have power and not expecting it to be restored for 3 plus days from when it went out. In 2013 we had another ex-cyclone hit the area and some people were without their grid power for 2 weeks. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah looking a the number of people without power at the moment thanks to cyclone Debbie, I'm also glad I live off grid. And I think its time more people do the same. No matter what your opinion of the current energy debate, coal verses renewables, a storm like Debbie is still going to wreak the grid and the only fix is to go off grid. Currently I have two business client web sites off line because a phone exchange near Mackay ( Farleigh ) has lost power, and has run its batteries down, and there is no date for restoration. I'm pretty confident I'll loose one client because of this outage. Not happy. A forward thinking phone company would put solar panels on their exchanges, they already have the battery bank installed. But that costs money, and share holders dont like it when the company spends money. I myself have a backup generator and 48v charger. I have 3.8kw of solar and a wind generator, but after 2 or more days of heavy overcast, the batteries are pretty discharged. I can cut back on power usage, and still be comfortable, but its the batteries I worry about. So I fire up the generator and put a couple of kWh into the battery bank, just to get through the night without giving the batteries too hard a time. The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Its not just cyclones, but bush fires and floods can leave people without power for weeks. Even a very small system might be just enough for some critical power for refrigeration and minimal lighting. Cheers, Tony. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I was thinking exactly the same about the phone exchange as ours only lasted about 18 hours after the power went out. Yet there were blue skies all day up to 11 AM when the phone lines went dead. I could use my mobile phone to connect to the net but I was wondering when that option would fail also. Power has been restored here now, we have not been affected anywhere near as bad as those people up north. I wonder how long their mobile phones worked from the towers that survived the cyclone. As for the cost of panels on phone exchanges, surely the numbers would stack up unless they get their power at some really low rate. It is good to see talk of batteries being introduced to the grid, not that it would make any difference to our blackouts in the last few days. Yesterday I heard on the on the radio that some company is planning to put in 150 hectares of solar panels and 100 MWH batteries somewhere in Australia. At least that it is what I gathered from the tail end of it that I heard and it is not associated with the Elon Musk proposal in SA. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
The grid connected battery systems would give people connected to the grid that ability also providing they have the right Inverter. Seems to be quite a bit of talk about a push to get a lot of them installed around the country to help with the current power crisis. It would be frustrating having a 5KW bank of panels on your roof and the food in your fridge going bad. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
I do wonder how well some peoples panels are installed. Recently saw a bank of 6 on the ground down here 30° South, so they were pitched a bit. My Weather records for February show 72km/h guts for the day it came down. Haven't checked yet to see which part of the installation failed, but vaguely recall a bolt torn out of one of the alloy RHS struts; that could have been a secondary failure though. Going on other damage around the place, the wind was from the SSW, and the shed they are on was purposely built running N/S. It was the front panel row that tore loose, first of 7 rows. Bit curious if the aero effects caused that to be a higher loaded than the others. I'd assume panel installations up North would need to be cyclone rated, but do wonder how well this is adhered to by some installers. Could be that the grid might be minus a fair bit of solar feed up that way for a while. Cheers Phil. |
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| hotwater Senior Member Joined: 29/08/2017 Location: United StatesPosts: 120 |
This is an old thread, but it serves as an introduction. I live off grid 5 months a year with about 1200W of panels. Granted that is summer with propane stove and a generator for doing laundry. I get all my refrigeration lighting and hot water from only from PV and a car battery. The battery is from a vehicle we don't take with us. Anytime you add a battery to the system, the solar advantage is lost. This battery is used almost only for starting current of motors. Total cost of the home built control system less panels was about $100. For years we have not had to run the generator for any purpose but laundry. It is surprising how little power you can get by with. The whole system is microprocessor controlled. Any excess power goes into heating water. I can divert as little as 10W efficiently into heating. A 10 & 20 gallon tank are in series. Primary heating goes into the 10 gallon and any excess goes into the 20 gallon tank. No electrons are wasted and the panels are operated at power point. The fridge only operates during daylight hours and stores cold in a large mass of liquid. The system has enough panels to operate on a rainy day. All the electronics is custom built. Soon I will be adding a modified dishwasher with the hearing element designed to work on raw panel power. I don't think living totally off PV or wind is the issue. It is far more economical to power high demand items with an occasional generator start. I will be adding another 600W of PV soon. Primarily this will be used for development work on some electronics, not because I actually need it. With some lifestyle adjustments you can live comfortably. I've fine tuned things enough that I don't care if my wife leaves lights on. Th extra 600W will make things a little easier as the days get shorter. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
My setup has no issue with high demand unless it is for extended periods at night or on rainy days. So my 3000W AC or 2400W clothes dryer can both run together, I now have 9.5KW of panels so these things are easy to run on sunny days. Not everybody is prepared to live with the bare basics, as Oztues said Living off grid not surviving off grid. I have come to the conclusion that sunlight where I live is not reliable enough to rely on PV alone unless the PV array is ridiculously large. 99% of the time PV is fine but there is the occasional period of 3 plus very cloudy/rainy days in succession. Yes I could cut our consumption down to much less by getting rid of things like the dishwasher, AC, Dryer Electric Oven etc but we are not prepared to go that. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
At least starting out with just the bare basics with all its limitations may be a financial necessity for some of us. Not everyone has a spare $50K to splash on having a top of the line system professionally installed ready for the first day you move in. Anyhow, its fun and a challenge slowly building up and improving a system over time, and adding to its capability as more is learned. Gas as a source of raw heat is always going to be cheaper than electricity, especially natural gas. And bottled gas cheaper than installing a lot of extra additional battery capacity. If you really must have an electric stove, oven and dryer, and electric hot water, fine..... But modern gas appliances work just as well, arguably better. Cheers, Tony. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I have scrounged and bought most things very cheaply so I would have less than $6,000 invested in total. The electricity we use is free, gas is not. Gas has a place but I am trying to keep it to very minimum. There is also the argument of renewables and non-renewable energy. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| hotwater Senior Member Joined: 29/08/2017 Location: United StatesPosts: 120 |
I am confused at times what my mantra for being green is. My neighbors 10 feet on either side have grid power. Cost saving was a major issue as the residence is mostly unoccupied. I couldn't resist the challenge of designing an off grid system. My goal was to have a system that operated with the most minimal battery possible. I succeeded with that opting for more panels. Over a KW of panels into a car battery is not the norm. Everything works real time (only when the sun is shining) with a microprocessor scheduling loads depending on generation capability. Diverting any excess power into heating water works beyond my expectations. Just because your controller says it has diversion doesn't mean it works very well. Almost all off grid systems are over designed and just waste a lot of potential power. Now that everything working well, I am being lured into the land of waste. Adding a dishwasher is certainly possible with my current system. Panels are so cheap now that I am thinking of adding more panels to do laundry with a modified LG washer. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I think you will find the over design is to have sufficient power in cloudy conditions. Designing a system with just enough panels when there is full sun is going to result in frustration. Also the batteries should get to float voltage regularly, if there is barely enough panels it will struggle to get the batteries charged and it best to keep lead acid batteries near full charge as much as possible. You can have a satisfying green smile when there is a black out and your neighbours are in the dark and you just carry on as normal. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
My experience has been that if you have no battery, you need about eight to ten times the panel capacity to supply power on a totally dull grey cloudy day. And in mid winter you can have several of those in a row. Its just not practical to size a battery to support a full week of unbroken cloud cover. Say about 2Kw of panels if your daytime average load runs to about 200 watts. And then only have a very small occasional short consumption spike, maybe the refrigerator starting up, needs any grid assistance during the day. In mid winter you would have about seven hours of useful daylight and draw almost zero grid power during that time. There will be a significant morning and evening peak in consumption unless you sleep for seventeen hours ! And during the night about the only major load will be refrigeration. Even in mid winter your grid load will reduce to about half of what it was before you added solar. As there is a fixed service charge, your electricity bills may drop by about a third. In summer the situation is reversed with about seventeen hours of usable daylight and your over powered solar should supply about eighty percent of total power consumption. That is what you should be aiming for. Solar panels are fairly cheap, batteries are always going to be the most expensive part of the system. So the trick is to build up a system that requires minimum help from the batteries, and only at night. If you run it for a year, or at least through one winter without a battery, the power you draw from the grid will give you an excellent idea of what sized battery will be required to go completely off grid. Also remember that you should only use some fraction of total battery capacity if you expect long battery life. If you are patient and build up a system gradually, a smart meter will do all the monitoring for you. It then becomes fairly straightforward to sensibly size a battery. Cheers, Tony. |
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