Last year I said I was working on an alternative energy book. A do-it-yourself guide for an off-grid home. The project got derailed by my months off-line and all so much catch-up work since. The book is still on my project list but recently I have been getting quite a few questions about doing primarily PV solar systems. While no substitute for the book, I have desired to scribble down some of the considerations here.

Multiple people have asked me "What did you do?" Let me address that question first. Planning an off-grid electrical system is very different from saying my tract house has 240VAC, 200A service just like every other house in the tract. You really need to do your homework first. That is, figure out how much energy you need, when you need it and what its source will be. What I did fits what I calculated I needed. Here are some ideas to get you pointed in the right direction.

• Figure out what your total power usage will be in watt hours. For example, an electric coffee maker which consumes 1500 watts but only runs for 15 minutes a day consumes 375 watt hours (1500 * 0.25) but a refrigerator which consumes 500 watts but runs half the time consumes 6000 watts (500 * 12).
• Rather than saying "my kitchen in Spokane has 200 watts of light in it so that is what I need", look for efficient alternatives. For example, my kitchen has three watts of overhead lights on one circuit and three separate circuits of under counter lights which add up to a total of five watts. Look at LED lighting for efficiency and placement of lights to give you what you need where you need it.
• On a PV system, power you use at night costs more than power you use when the sun is shining. A battery charge/discharge cycle is not 100% efficient. 80% is a realistic starting point for lead-acid, significantly lower for Edison cells.
• For lead-acid batteries, you seldom (with never being a better choice) want to discharge them below 50% of their capacity as this will significantly decrease their life.
• While it depends on the weather where you live, a reasonable guideline is to have battery capacity sufficient to run your normal loads with three days of no sun without getting below the 50% charge guideline.
• Besides needing to know your typical watt hour usage, you also need to know your maximum demand and at what voltage. For example, your peak demand at 120VAC may be 2000 watts and at 12VDC (typically for lighting) may be 100 watts.
• You need to establish the PV panel output can you expect on a full-sun day. Where I live, that number is about six times the peak power output of the panels. Specifically, my peak panel output is 1920 watts and the maximum output I have seen in a day is about 12,000 watts.

That will get you started with your arithmetic. Beyond the arithmetic is just making some logical decisions. For example, I have a rice cooker and a hot-plate. If I have energy to burn I use them but when I don't it is more cost-efficient to cook with gas. You may also discover, for example, that a very efficient (but expensive) 12/24VDC refrigerator is a big win when you calculate the systems cost increase to run a much less efficient traditional unit.

Put some sort of backup power into your plan. A gas or diesel generator are the typical approaches. While you don't want to have to use them regularly, if they are never needed you probably built a bigger PV system than makes economic sense.

Size your system to fit your needs. A much too large battery bank can be damaged through sulfation from lack of use. An inverter (to convert DC to AC) that is much larger than needed will be less efficient that a right-sized unit. A low voltage (12V, for example) system with high current demands will result in much higher losses in cables and conversion than a higher voltage system.

That should get you started. I encourage you to ask questions as that will help me make sure the book covers everything that is needed.