# A 500 watt solar system

So, what does a 500 watt solar system look like and how much does it cost? Let’s start from the top and work our way down.

First, the sun does not shine every day in Indiana and statistically there are less than 6 hours of solar power available on a sunny day with a fixed-panel installation. You may improve that with a solar tracker, but there is additional cost there that we are not going to include in this basic solar panel system.

Next, we need to determine how much solar power we want to store for the evening hours. How much do we want to whittle away at the electric bill? Lots of questions, so let’s go with this plan. We want to take a good sunny day and store enough solar power to deliver 500 VA until the next day. So, 24 hours of 500 VA is the goal. This will be a Grid-Tie-Interactive system to keep the inverter costs down – it works only when the grid is available – it is not a backup system.

Twenty four hours times 500 VA is 12,000 VA. Lets convert that to the amount of current drawn by a 500 VA 120VAC inverter connected to a 24 volt battery bank. (I would rather use 48 volts, but 24 volts is easier.) 12,000 VA hours / 24 Vdc = 500 AmpHours of battery capacity. That doesn’t sound like all that much as many 24V battery systems are 270 AmpHours and larger. so, we would need four 12 volt 270 AmpHour batteries configured for 24Vdc operation. We also don’t want to discharge the battery bank 100% each day, so there needs to be some additional storage. Let’s add 50% and make that six 12 volt batteries. (Note that stacking two 12 volt batteries rated at 270 AmpHours gives you a 24 volt system that still delivers only 270 AmpHours because they are connected in series. Three of these sets in parallel gives a 24 volt system with 810 AmpHours of battery capacity.)

Now, how many solar panels do we need? Well, if the sun shines 6 hours a day and we need to re-charge the batteries during that time plus provide an additional 500 VA per hour we will need to have enough solar panels to deliver 3500 VA per hour for 6 hours or fourteen 250 watt solar panels. (I’ve put the worksheet at the bottom of this article.)

Wow. That is scary. $7000 just for the solar panels. And I thought this could be done for a reasonable amount. I wonder what I was smoking. The whole system cost would be at least $10,000 and we only get back $36 per month. I think that is the end of solar. There is no way that a sane person would put that much money into a technology that has a 23.6 year payback period.

I think I’ll investigate a wind system next.

Notes are here. Solar Calculation Notes