What You Need to Go Off the Grid
6 of 7 in Series: The Essentials of Planning Your Solar Power System
Off-grid solar PV systems are expensive, so find any means you can to reduce the energy requirements. Most off-grid houses use a wide range of energy resources, in sharp contrast to the typical all-electric suburban home. Solar hot water heaters are always good candidates because they're cheaper per kilowatt-hour than an off-grid photovoltaic system, and solar lighting systems are always wise. This variety can be an advantage in that you aren't completely disabled by power blackouts. In fact, you'll be completely impervious to power outages, and even if one of your resources goes down, the majority of your lifestyle will still be intact.
Here are the functions of each part:
Charge controller: The charge controller feeds current into the battery bank at the required voltage. Good charge controllers draw the best performance out of the batteries and are very important for economics because they influence efficiency.
Battery bank: The battery bank is typically made up of six or more individual batteries connected with stout cables in either series or parallel arrangements.
Inverter: The inverter changes DC to AC voltages suitable for use with household equipment. An inverter is optional if you use DC loads exclusively.
DC loads controller: You may be using both DC (boat, RV, and auto appliances) and AC loads (standard household appliances). The DC loads controller maintains the proper currents and voltages into the DC loads.
AC generator: As a backup power supply, the AC generator isn't strictly necessary but is usually part of any off-grid system in order to prevent blackouts when the sun is weak for extended periods.
Transfer switch: The transfer switch alternates the power source between either the inverter output (when battery power is available) or the AC generator.
AC loads controller: This device includes appropriate fuses and switching means and maintains the voltages and currents used by the AC appliances connected to the system.
Which type of current you choose depends on what you want to run. If it's just a few lights at night, with a coffee maker and a fan or two, DC is fine. However, the market for DC appliances is far smaller than 120VAC, so you may go for AC if you're using standard household appliances (which is the most common way to go and is cheaper and better because of the widespread availability of AC appliances compared to DC appliances).
DC, which is more efficient because batteries use direct current, is usually the choice for small cabins and small power systems. You can use DC appliances for RVs and boats, so envision your cabin like a big RV, and you get the picture. But DC also requires larger wire diameters, which can be very costly if you need to run lengths of more than 50 feet or so.
But when you install a system with batteries, you have to stay on top of things. The battery or battery pack is the core of any off-grid system, and it drives the system's cost. All action comes and goes from the battery, and much of the safety and control equipment is designed to protect either the battery or the balance of the system from the battery. You absolutely have to understand batteries, or you'll end up paying an arm and a leg for new ones all the time and you won't get decent performance out of the ones you have.