Costs are dropping on these systems, and like computers, the best approach is always to get the least you can get by with now, and add on to it with cheaper upgrades as time goes by. The prices you actually pay on the market are usually set less than the real costs by tax incentives and rebates that the fed.gov and your local utility give. These, of course, are real costs - you're just getting all of the taxpayers in the country and the customers of that utility to pick up parts of the bill. There is a type of system called a "grid tie", where you sell electricity back to the power company - who is required by law to buy it from you. I'm not going to emphasize that because I'm talking about a SHTF power supply. There just might not be anyone left to sell to.
Among the approximations built into yesterday's analysis was that you actually use the full 1800 Watts 24/7 from your 120V 15A outlets, and I don't have to tell you that you don't. Your electric utility doesn't sell you power (Watts), it sells you energy (Watt-hours, and our bills are expressed in kilowatt hours; thousands of W-hr). The system that we outlined yesterday was actually a 3600 Watt-hour system, and it's probably best to start thinking in terms of Watt-hours, understanding that 250W solar panels are specified to deliver that all the time, as long as the sun is bright and sky is clear. Over the course of a month, should you really run it at full output, that would be 3600 Watt-hours, 24 hours a day for 30 days. That's 2592 kW hrs (kWh).
One reason to start thinking this way is that it makes choosing batteries easier. Since a watt is the product of voltage and current, 120V at 15 amps equals 12V at 150A. Batteries are sold by the amp-hr, and 150 A-hr is not an extremely big battery.
Enter the world of planning. My house is an 1980-era house and the electric panel has never been upgraded. We have about 20 kW service here. That essentially means 20kW every hour, or 480kWh a day, and 14,400 kWh in a month. My electric bill just came, and it said we used around 1/10th of that last month. The important point is: you may need some extra capacity to run other things, and you may need extra capacity for starting some motors, but you don't need to build out the full fuse panel in your house. That first number I threw around (for a 20 kW house - that 14,400 kWh system at over $100,000 -based on the last time I looked at prices, a few years ago) is not the way to approach this question. The system we looked at yesterday, instead of being one circuit in my house, would provide more power than I bought last month.
The way to calculate how much capacity you need is to list the loads you need to run and determine how many watt hours are there. People that live on small boats or live in trailers are good at this. If you are going to use electricity to cook, you don't need the thousands of watts the stove uses all the time, but you need to bank those watt hours in batteries somewhere. Better yet, cook on gas, or wood, or a microwave if you must use electricity. If you're going to run some nights at light, you need to bank those watt hours into the battery during the day.
This is probably a good time to introduce the first law, or golden rule or whatever you want to call the most important thing to remember:
The cheapest watt in a solar power system is the one you don't need. Conservation is cheaper than solar panels.Admittedly, you can go a little nuts this way, but if you want to run a lot of lights, for example, LEDs are expensive but extremely low energy consumption compared to incandescent bulbs.
This is boring work, looking at the power consumption on everything and entering it into a spreadsheet, but probably essential. An example for the communications backup might look like this:
|Load||Amps @ 12.5V||hours/day||Needed A-Hr|
|HF Radio - RX||1.3||6||7.8|
|TX at 50 W||10||0.2||2|
This tells me that I only need about 11 A-hrs to run my Emergency Communications station. And I could get by with less.
More to come...