How Many Solar Panels Do I Need?

The Bid

It’s taken us some effort to get to this point, but we’re actually very far along, in that we know the exact system-size that is required to eliminate our expensive PG&E kilowatt hours.

I’ve said exact, but let me qualify that claim. All these predictions are estimates. I’m showing specific numbers — like 5,412 — so that the math can be followed, but remember, weather changes from year to year, and the sunshine in one neighborhood can be quite different from that in another neighborhood. So don’t bog-down in the details. Do NOT demand that a few of your panels be larger or smaller, so that your system’s projected production matches your target. Predicting solar panel performance is an inexact science. Just like predicting your electrical bill.

This site cannot estimate what your solar installation will cost. But let’s look at a few of the factors that contribute to cost:

This is an easy roof:

The truck delivering the panels can pull right up to the roof, and a man standing in the back can hand them right to the installer!

This is a difficult roof:

This home is in Mill Valley. It’s multi-story, the roof slope is steep, and the panels will need to be carried to the house, then up a ladder.

This is an easy roof:

All the required panels will fit on one roof surface.

This is a difficult roof:

This is a home in San Rafael. Although the home is over 4,000 square feet, the roof is chopped up into a few dozen different surfaces, making any sizable install expensive.

This is an easy roof:

It’s comp-shingle — short for composition shingle, also called asphalt shingle. These roofs are easy to walk on, easy to drill-through, and easy to seal.

This is a difficult roof:

It’s a flat roof. Though flat roofs are generally associated with commercial properties, plenty of homes in the Bay Area have them as well. One extra cost is the tilt racks — shown above — that are often used on flat roofs. But another cost is the special equipment and skills required to seal the penetrations. Tile roofs, foam roofs and slate roofs are also more expensive to attach solar to.

This is an easy install:

It might look like soup, but these are new panels, capable of handling solar.

This will result in a more expensive install:

This is a Federal Pacific — an ancient service panel that is inadequate for any modern home, never mind one contemplating adding solar. Service panel upgrades add cost.

We’ve just identified six adders — factors that trigger additional costs calculating the cost of your solar installation:

  1. Roof Access
  2. Roof Height
  3. Roof Slope
  4. Number of surfaces
  5. Flat / Tile / Foam / Slate
  6. Electrical service panel size

Other adders might cover the extra cost of driving beyond the installer’s primary territory, or requests for particular hardware — like Enphase microinverters.

Still other adders might reduce cost. Perhaps the installer wants to keep a crew busy in the off-season. Or perhaps your install will be visible to neighbors — and likely trigger additional business. Perhaps you just seem like an easy-to-work-with client.

Let’s wrap-up by estimating a price for the 16-panel Enphase system discussed earlier, and deciding if this purchase is a good investment or not.

In our 16-panel scenario, we have three important things going for us:

  1. The electricity we wish to replace is expensive: 30 cents and 34 cents, when I made these screen-prints. (Note: rates change every 3 months. For years they shuffled around, up and down a penny. But effective the end of 2014/beginning of 2015, PG&E rates went crazy, with double-digit increases and decreases. Just remember that PG&E is a for-profit business; they're not your friend. Solar likely makes sense.)
  2. We’ve said that the property had a southwest-facing, sloped roof that was sunny all day. (Note that in the closeup of the EPBB calculator’s kilowatt-hours of production, there’s additional data showing the “cost” of imperfectly oriented panels. Our “realistic” angle and orientation cost us only 3 %.)
  3. It’s 2015, and panels are very inexpensive right now.

With equipment costs fluctuating, and labor costs different everywhere, there’s no way for this site to predict what your local installers will charge you for this system. But I ran this scenario by one company, and the owner said, without any adders, he might bid $17,000.

I’m not sure if that sounds high or low to you, but the actual cost would not be $17,000.

That’s because there’s the 30% federal tax credit. It’s a credit, not a deduction, so the actual cost is .7 x $17,000 = $11,900.

For years, California has had a state rebate. It has been steadily decreasing in generosity, and was down to about 3% of the cost of the install. Now the funds are depleted. I mention this because some literature will mention “the tax credit and state rebate.” Assume no more state rebate, though some counties and cities are creating their own incentives…

Let’s do a quick calculation to see how quickly solar pays for itself.

Let’s look at that slide-bar one more time:

Our objective was to eliminate the electricity above tiers 1 and 2. We can see that roughly 1/3 of that expensive electricity is 30-cent electricity, and 2/3 is 34-cent electricity. So the average cost of the electricity we wish to replace is ~33 cents. (As mentioned earlier, this PG&E graphic is showing old rates.)

We did the math and determined that we’re sizing our system to eliminate 5,412 kilowatt-hours of this expensive electricity per year, so, at 33 cents, that calculates to $1,786 per year.

$11,900 divided by $1,786 savings per year means that our system will pay for itself in 6.7 years. Then we get free electricity for the rest of our life! Yes — solar is that good an investment…

(For a more detailed analysis of the financial advantage of getting solar, visit

Your assignment: have one or more companies come by and prepare formal bids for installing solar at your home. If you’re in Marin or Sonoma, call me. I can help. If you’re in other parts of the San Francisco Bay Area, I may still be able to suggest a vendor or two. (For example, for the city of San Francisco, I’ve heard nothing but good things about Luminalt.)

Go Solar! Today.