Sizing Your System
Determining your system’s size can help prevent overuse or damage to system components down the line.  Start by evaluating your appliances in your home.  Record how many hours a week you use each appliance and how much energy it consumes, in terms of watts.  For instance, a lamp with a 40-watt bulb that is on for 5 hours a day will use 40 watts for 35 hours a week, or 1,400 watt-hours a week. 

Once you get your typical energy consumption for a week, you can design your system to generate enough power in a week to keep your batteries fully charged.  Your battery bank should be able to last you at least one week without any energy coming in.  Say you want a battery to cover the above 1,400 watt-hours a week.  At 12 volts (volts*amps=watts or watts/volts=amps), your battery bank will need a capacity of at least 117 amp-hours.

Your sources should be able to keep your batteries fully charged all the time.  Lead-Acid Batteries should never go below 80% capacity for a longer life. In an average day, your sources should be able to completely recharge your bank.  So, for the above example, we will need to produce 117 amp-hours at 12 volts in a week, or 16 amp-hours a day.  Sources will be rated in watts, but their outputs can vary. A 120 watt solar panel is producing 15-18 volts and 6-8 amps.  It will produce less in the morning and evening, but in our area, we can get an average of 25-30 amp-hours each day from our 120-watt solar panel.  National Center for Photovoltaics

So, now we are producing more than we are using.  Is this a problem?  Not really.  This is actually good, and the system should be designed to use this extra energy for something useful.  Water Heaters work great for this, as a diversion load for your system.  When your batteries get full, the diversion controller turns on the water heater and uses the energy coming in from the source. The controller and diversion load should be designed to handle the maximum output of all the sources.  Diversion loads can be stacked to come on at different intervals, giving your system a sliding scale, depending on how much power is coming in at the time.

Once the usage, storage, and source capacity has been determined, you will need to decide how to use the energy.  What appliances will be on 12 or 24 volt DC, and which ones will be run through an inverter?  Your inverter should be sized 25% larger than your biggest draw.  Inverters are rated in watts and have a surge rating, but most users have found that surge ratings are useless.  So, the above lamp is pulling 40 watts, so we would need at least a 50-watt inverter to convert the 12 volts DC coming from your batteries into 120 volts AC.

Wire size can play a big role in the efficiency of your system. For DC, wires should be copper stranded and as large as possible for lower line loss.  The inverter should be as close to the batteries as possible and should use a big battery cable. 

Additional Resources:

• Fieldlines  and OtherPower - The best home-power resources on the web
• System Sizing Worksheet - Use this worksheet as a guide for sizing your system
• Solatron Technologies - A great source for components