Dear Mr Rawles,
Mr J’s submission makes interesting and useful reading, however I would like to add a few observations on the subject to possibly aid the decision making/wallets of people who are building/upgrading photovoltaics systems.
– Batteries: if at all possible get used golf cart batteries that have been properly maintained. Around here they get recycled for a $25 core charge which is easy to beat. My deep cell batteries are now 4-5 years old (2 seasons in various golf carts and 2.5 years in my battery bank. They are in like new condition as they take really well to charging with photovoltaic panels. The biggest enemy of deep cell batteries is repeated fast discharge / too deep discharge cycles. The smaller your bank the more likely it is that your batteries will be subject to these.
3 batteries delivering 300Ah may sound like a lot of power but isn’t. If you are pulling a 500W load @ 85% efficiency, the batteries must deliver (500/.85)/12.5 = 47A. That is 16A each, while their safe discharge rate (C20) is about 6A (114/20). If the wiring in the bank is not ideal, 1 battery may even have to provide half the load (see last year’s submission on photovoltaics for details). This is the real reason why batteries don’t last in small banks. I know this sucks but if you think Wal-Mart won’t be there to provide cheap replacement batteries in 2 years, one needs to pony up now and get as big a battery bank as one can afford. On the bright side: a basement full of batteries beats getting bailed in ….
– 24 volt systems have certain advantages as stated in the article. However when used with photovoltaic panels you will need to create a serial-parallel setup of panels to charge them. This isn’t a big deal for a static installation but less than ideal for small portable systems. Personally I decided to go with 12V on all systems because I don’t want to deal with multiple voltages. Furthermore 12V tools, lighting, etc. are plentiful but 24V or 48V units are harder to come by.
– Though its true that larger panels are cheaper per Watt; they also are less mobile which may or may not be a problem depending on how you plan to ride out the storm. My setup includes 95W, 60W and 15W panels so I have more options. Even 1W panels are useful for charging phones, rechargeable batteries, trickle charging car batteries, etc. Our lawnmower uses a 5 yr old motorcycle battery for starting. Last year we had some starting problems with that battery toward the end of the season, so this year I hooked it up to a 1W panel that was shaded by a roof overhang. Never had a problem all season and, to my surprise, at the end of the season the battery showed a resting voltage of 12.8V. Don’t overlook these little things – you need reliability first and foremost.
– Mr. J. got a pure sine wave inverter which is great if you have the money for it, but it isn’t really needed unless you run sensitive electronics like medical equipment. Power tools and consumer electronics can handle dirty power supplies up to a point. Having said that, I agree with Mr. J. that its quite possible for your power tools to sound ‘raw’ and feel underpowered. I believe this is due to the size of the inverter being used; most likely the number/size of its capacitors is inadequate to sufficiently smooth out the inverter’s stepping at higher load factors. I have 300W, 1000W, 2000W and 2500W inverters at my disposal. The 1000W unit really causes trouble with medium size power tools (rated @ 500-600W) but on the 2000/2500W units, the same tools sound/behave like they are running on power line supply. So, for the same money I prefer to get twice the size inverter even if its modified sine wave.
– Another note on inverters: though its always good to be careful with your wiring, there is no need to get obsessed about it. The inverter’s micro controller monitors the outputs and will shutdown the inverter at any sign of trouble like shorts. And it reacts a lot faster than a standard fuse or circuit breaker. This is a key difference between inverters and generators or power line supplies where only passive components stand between you and possible disaster. Sticking to the manufacturer’s installation guidelines should be sufficient.
– My system is stand-alone so I don’t need transfer switches and the like, but I will add my voice to JWR’s note: be very careful with male-to-male wires and other un-standardized solutions. You do not want to come home to find your precious inverter has blown its mosfets (or did something far worse) because someone plugged that cable in the wrong outlet. The second danger is that you will make that mistake yourself because after being half awake for a few nights you will not be thinking as clearly as you like to believe. How seriously do I take this? Some of my circuits are monitored by micro controllers. If I flip a switch the controller will go through a checklist to see if my request can be executed under the circumstances without causing harm to the system, if not I just get a blinking light.
– I can see someone topping up their batteries with a car’s alternator in a pinch. However using a 14.4V alternator to run an inverter to run a battery charger to charge a 12.6V battery doesn’t sound particularly efficient. Expect to lose 15%+ on each step. Why not buy the heaviest starter cables you can find and make a direct battery to battery connection? The thing is that being inefficient is not a big deal when gas is plentiful. Once you are going through your last tank, a person may feel different about it. If at all possible try to take such scenarios into consideration when designing your system’s layout. At the end of the day you are going to find something will limit your system. The question is: can you live with that bottleneck? If the answer is ‘no’; well, there is still time to do something about it.
I hope my notes will be of help for some of you. – D.P.