In May 2007 we installed a solar power system at our house. This page describes how we sized the system, and then shows our actual installation. Our house is in Mountain View, CA and PG&E is our provider – some of the logic here may be unique to California and PG&E rules.

System Summary

We have a 6.5 kW grid-tied system. There is no battery – when we make more than we need, we sell it to the utility company, and when we need more than we make, we buy it back.

 

Some electrical terms you should be familiar with

A Watt (or Kilowatt, kW) is a unit of power. Some electrical devices are labeled directly in watts (or kilowatts). For eample, a common lightbulb is a 60 watt bulb. Hair dryers are often 1000 or 1500 watts (1.0 or 1.5 kW). Others are rated in amps, leaving you to figure out how many watts they consume by remembering that W = Volts X Amps. Volts is usually 110 (line voltage). A device with a 3 amp power supply will consume up to 330 watts.

A Watt-Hour (or Kilowatt-hour, kWh ) is a unit of energy. It is the way most people are billed for electricity from the utility company. A kWh is the amout of energy used by consuming a kilowatt of power for a full hour. If you leave a 100W lightbulb on for 10 hours, you’ll use 1 kWh of energy.

Sizing The System

There are several different ways to approach sizing a system:

• Save as much money as possible, over the long term
• Generate as much solar power as possible (even if it’s not cost effective)
• Constrained by available space (e.g. the useful part of the roof)

We choose to save as much money as possible, by bringing our electrical bill as close to zero (without going negative) as possible. Fortunately our roof has a large, southern-facing area that doesn’t get any shade. It was able to hold the number of panels we needed.

We looked at the average number of kWh we used last year. You don’t need to produce that many kWh to break even in dollars, for 2 reasons:

• PG&E uses tiered pricing. The more KWh you use in a month, the more you pay for them. Kind of like income taxes. If you eliminate 1/2 of your usage, you might eliminate 3/4 of your bill, if you are up in the highest tiers.
• Time-of-use (TOU) pricing. On a TOU plan, you pay more for electricity used at peak hours than non-TOU pricing. You pay less than regular price off-peak. If you produce a surplus at any moment in time, then you sell it to PG&E at the same TOU prices – higher at peak, lower off-peak. Most PG&E customers with solar power are on PG&E’s E7 rate plan, which has peak times 12pm to 6pm Monday thru Friday. Peak time happens to overlap nicely with prime solar power generating time. Every net KWh you generate during the peak period (that is, more than you used) is worth 2 or 3 KWh you can buy back from PG&E at night, when you aren’t generating.

Putting these together, you could theoretically produce a fraction of your total usage and still break even on electricity costs. However, this is an ideal generating scenario. There will be days when you get much less solar generating efficiency due to clouds, even smog. If you happen to use the bulk of your electricity during peak times (e.g. air conditioning during the hot afternoons), you won’t benefit as much from TOU pricing.

Our contractor had an elaborate spreadsheet that let us play with all of these variables, and we ultimately settled on a system as follows:

• Raw panels: 44 panels X 175 watts each = 7700 kW DC output
• Actual AC output: 6.5kW (factors in loses during conversion to AC)
• Average peak-equivalent sunlight hours at our location: 5.3
• Annual production: 10,500 kWh / year

which is about 2/3 of our annual consumption.

Note: At least w/ PG&E, if you produce more than you consume on an annual basis, they will not write you a check for the difference. That’s why the goal is to get as close to zero without going over. While there’s an ecological benefit to producing more, there’s no economic benefit. Also note that PG&E is always playing with rate plans, and for a while the E7 plan was closed to new customers.

Costs

As of May 2007, solar power costs around $10 per watt, installed. We paid under $7.80/watt as part of the Solar City program. In California, there is a significant rebate program. The rebate will drop as more people install it. It’s currently around $2.50/W. There is also a $2000 federal tax credit available. Our final out of pocket cost was about $33,000.

Installation

Our contractor was Solar City. They have programs in several California cities that provide an extra discount if enough people in a city get together and order as a group. They took care of everything: permits, coordinating with PG&E, and doing the installation. The actual install took about a week. It took another week for PG&E to inspect the system and swap our our electric meter for a time-of-use (TOU) meter.

Step 1: Installing posts to support the panels. This must be done carefully or you run the risk of a roof leak.

 

Step 2: Add the panels. We have a total of 44 panels (BP Solar 175), each rated at 175W DC output, for a total of 7700 DC watts.

 

Step 3: Connect to an inverter, to turn DC power into AC power. We have 2 Xantrex GT3.8-NA inverters. They each run half the panels. There is also a master circut breaker for the solar power system – the smaller box next to the inverters.

The interver will tell you all kinds of things, especially how much you are producing at the moment, and how much you’ve produced so far today. Here’s an example from 1 unit (each unit reports independently), Here (in the late afternoon) it is reporting 1.780 kW instantaneous power production, with 21 kWh energy produced so far that day.

Step 4: Connect into your main electrical system. Basically, the inverters are connected to our main electrical panel as if they were just another circuit taking power from the utility. The fact that the inverters generate rather than take power doesn’t really change how you hook it up.

If you took enough batteries, and connected them thru an inverter into any socket in your house, then you would offset the total power you take from the utility company. If you had enough batteries (and used little enough power elsewhere in the house), you’d see your “meter spin backwards”. A solar power setup works the same way. (Of course, the utility company frowns on people doing this w/o their permission.)

Step 5: Switch to a digital, time-of-use (TOU) electrical meter. Technically not necessary, but in practice advantageous for solar power producers. PG&E does this as part of their inspection and permission to generate power. Your utility company may work differently.

The TOU meter can show several different pieces of data – here it is showing the net instantaneous usage – we are selling 2.51 kW back to PG&E right now:

In Operation

Coming soon – live data on power production.

Other People’s Installations

Here are some links to other people that have installed solar power – I am only collecting articles by individuals, not by solar companies:

Links to solar power home pages

 Other Solar Links

Good explanation of PG&E pricing and billing