PV Gap

William Said:

How do I get started to set up a 500 watt a day or so PV solar system?

We Answered:

First thing:  you need to correct your units.  A watt is a rate, like a mile per hour.  What you want is watt-HOURS, which a quantity like distance (miles).  You can get 500 watts out of something the size of a flashlight battery, but you'll only get it for minutes at the most.

Getting units right is important; if you use units wrong, you can get answers that are completely misleading and waste a lot of money on something that won't work.

If you want 500 watt-hours per day (say, 100 watts of loads running for 5 hours/day) you'll need at least 100 watts of PV panel (after correcting for the losses due to the angle of sunlight!).  The average sun on a sunny day is equal to about 6 hours of the sunlight at noon, so 100 watts of panel will give you up to 600 watt-hours of juice per day, or maybe 500 Wh after losses.

It's unlikely that your sun situation is going to be that good all the time (unless you live someplace like Phoenix!) so you'll need more PV.  If you have 150 watts, you can go on 4 hours of noontime sun; 200 watts lets you get away with 3 hours.

Next is batteries.  You don't want to discharge your batteries more than about 50%, so to coast for 1 day of no sun you need 1 kWh (83 amp-hours @ 12 volts) of battery.  Coasting for 2 days would be a good thing, so maybe 170 amp-hours of 12-volt deep-discharge battery would serve.  You can get suitable batteries for a starter system in lots of places.

The next thing is the charge controller and inverter (if any).  If you use 12-volt lamps, radio etc. you can dispense with the inverter and its losses.  You can also take all your gear camping with you!  The charge controller will help manage the batteries and prevent damage from overcharging.  Having an undervoltage cutoff or alarm would be good to prevent over-discharging.

I'm looking to set up a system soon.  My goal is to run emergency lights and a backup sump pump.

Dale Said:

In California, under net metering laws, are you allowed a profit if you install a PV solar system?

We Answered:

Normally you will start to profit after the pay off of the system. Most utilities will only swap you kWh for kWh. However if you play the game right in Cally you can use the tarffs to your advanage. I would do my best to only use power at night while the rates are low and let the solar power system backfeed the grid all day while the pay back is high.