Telescope Reviews: Solar power for an observatory

Hi Ken: I see you have generated a fair bit of response to your question. Please forgive me for adding yet more to the chat, but I have been living on solar power for over 10 years now - we live on a remote island without the usual grid power. One thing that we occasionally enjoy in our perverse way is watching the TV news proclaiming about how many thousands of people are without power after each storm hits us. It's a real treat to know that you are not going to be without power no matter how hard the wind blows. Anyway, if I may add a few general hints so that if you decide to go ahead you may be able to avoid some of the early pitfalls which commonly happen. When we first put our system in, we decided to do it right from the word go. Now, you have been given advice about not using an inverter more than once, and this may or may not be right. It really all depends on what power you will be using, i.e. 12 volts or 117 a.c. If you are going to be using anything which requires the 117 a.c. then I would suggest that you get an inverter. It is true that the inverter will consume more power than the relative amount used by a d.c. powered item, however the accepted efficiency rate for the whole system is usually about 85%. In reality, this is very acceptable. So, before you decide that you do not need an inverter, consider what you will be using. One thing I will say about inverter power, is that it is considerably cleaner and more consistent than grid power. Voltage fluctuations on the grid can sometimes be really wild. You will not find that with an inverter. With respect to your battery bank, solar panels, disconnect, charge controller and wiring, everything needs to be oversized. It is true that you can work out each detail with mathmatical precision, however, in the real world, it doesn't work that way. Let me give you one simple example. Suppose you have a nominal 12 volt panel which is rated at 4 amps. In theory, that would be a 48 watt panel. In fact, that will not be the case. Typically, most panels put out around 16 volts, with an open circuit voltage of around 21 volts. Yes, that is twenty-one, not twelve. As soon as the panel is attached to something to provide actual power, the voltage drops dramatically, so you will never see 21 volts going into anything. And neither will you likely see 16 volts going into anything. Do not worry about the ingoing voltage, as the charge controller will do an excellent job of taking care of that end of things. From the panel, the power goes through your controller either directly to your battery bank, notice I said battery bank and not battery, or through your inverter then to the bank. It depends on the type of system you have. I would suggest that, since you have direct access to a 117 a.c. line, that you initially start by using your system to power your 12 volt d.c. items. An inverter can always be added later. Perhaps one of the most important items in the system will be a d.c. disconnect. This is just like the main breaker on your panel for your house wiring. However, there is a difference between an a.c. breaker and a d.c. breaker. Under no circumstances should you try using an a.c. breaker in place of the d.c. breaker. You will find out, if you have not already, that d.c. breakers are expensive. For example, I have a 250 amp d.c. main breaker, on a 24 volt system which cost $400 over 10 years ago. Do not skimp on the main breaker. Just like the mount on the scope, an e.q. 3 will hardly hold a 14" scope. Same thing with the power system. The wiring for d.c. is vastly bigger - read thicker, than anything used for a.c. Be aware that d.c. power does not travel well for any distance. Unless your d.c. voltage is around 32 volts or higher, which yours will not be. So before you undertake anything, plan your layout so that the wiring runs between the panel[s] and the batteries/controller/d.c. disconnect are as short and direct as possible. This is very important. If your battery bank is going to be inside the observatory, then put them in a battery box which is vented to the outside. Most people simply build something which looks like a built-in bench seat with a long lid so you can access the batteries on a regular basis for watering. Without going into a long discussion on the pros and cons of the various battery types, I will just say that probably 99% of all successful solar systems are using flooded lead acid batteries. The standard, if you will, is the Trojan T-105. It is a 6 volt battery which will take an amazing amount of abuse. I would suggest that you should get 4 batteries, hooked up, obviously in series/parallel. So two pairs each hooked up in series, then they are in turn hooked up in parallel. You will have your nominal 12 volt power, but with somewhat more reserve, and this you will find you will need. Do not try to mix up different types of batteries, which has already been mentioned, nor batteries of a different age. Unfortunately, with lead acid batteries, you cannot start with a pair and then add another pair next year. You are far better off to have the battery bank considerably oversized than just barely adequate. As far as the controller is concerned, your system should be OK with something on the order of a 20 amp controller. But because the price of a larger 40 amp controller is not really that much different, you may be better off going to that size. It would not really be necessary, but in future, if and when you added more solar panels, your controller would be up to the job. I have always thought that with things electrical, it is much better to have them oversized and underworked than the opposite. A bigger controller will have the ability to decrease the amount of charge going in to the batteries as they reach full. Someone has already referred you to the homepower website earlier, and I can say that whatever you need to find out about solar power, you can find there. So to briefly - if that is possible for me - sum up, Panels, you can start small and add. Batteries - get plenty. Controller, get big enough, same goes for the wiring and disconnect. Inverter, forget the small 400 watt toys and buy a real one, 1,000 watts plus. It will have a battery charger, very useful for keeping your battery bank up when we have those infernal cloudy days! I've likely forgotten a bunch of things, but if you have a question, just say so. And good luck! Keith