Working on home solar estimate - Is my math right?

Okay, I got the numbers on our usage over the last 13 months. I live in Virginia, so the winters aren't too harsh, but summers are pretty bad. I have solid cost numbers for the last 6 months and the rest are projected from an average of those $/watt values. I am projecting that (if we change nothing to be more efficient) we will need an average of 1500 kwh/mo.


No, I might spend some $$ on adding better insulation to the building, maybe improve my use of electronic devices (I work with computers and we seem to have 8 running at all times), but I bet those numbers are solid. I wanted to target 1200 kwh/month (orange line) just because that covers 75% of our usage, but we have a townhouse and roof space is pretty limited. I'm probably only going to get 18 panels. So, I will get top-of-the-line panels rated at roughly 340 watts, and see how much power I can actually generate. I believe it will still be worth it, but here's my math. Please, someone who knows what you're doing, check me out to see if I got this right.

18 panels x 340 watts/panel x 5 sun hours/day x 30 days = 920 kwh/mo Theoretical yield
Assuming 80% efficiency I can expect to yield 730 kwh/mo.
So, with an average of $0.12/kwh I'm saving just $88/mo.
On a $22,000 system cost - $5700 (fed tax incentive ... I am looking into others) = $16,300 approximate cost of system
Amoritized at 4% over 5 years with a payment of $300/mo it will cost me at total of $18,000.
With a savings of just $88/mo it will take 17 years to pay itself off.
If I were to pay the whole thing up front, it still takes 15 years to pay off.

I think the bottom line is that the townhouse has too little space to allow me to gather enough sunlight and too much energy usage per month. I'd have to work some real miracles to reduce my consumption enough to get a shorter payoff period.

Anybody see any errors in my math?

$22,000? This is a DIY forum.
I paid that sort of price (more actually) 15 years ago. But don't spend that today.
You're considering 6kW of PV panels. I paid $2000 for that much earlier this year.
An appropriate inverter with latest grid-support features required in my area should cost $2000. If not a requirement in your area, older models for $1000.
Beyond that you need mounting racks, conduit, wire.
Total maybe $5000 or $6000.

Issues:
How much of the roof area are you allowed to use? We are now required to leave a 3' wide walkway on two sides and the top.
Are you required to have "rapid shutdown", module level isolation? Either an inverter per panel (e.g. Enphase) or a string inverter plus a simpler box per panel. That adds $30 to $50 per panel.
Are you forced onto a time-of-use rate schedule? If you don't make enough power, your bill will go up not down.

You'll probably generate 27 kWh/day. That's 808 kWh/month.
With summer consumption of 1800 to 2200 kWh/month, and peak time rates triple the off-peak rates, generating less than half your consumption could cause you to pay more money not less.

In the past, most cost-effective system would offset our highest tier rates, not the lower tier. Rules get changed to the utility's advantage.

Hedges. Thanks for your comments. First and most importantly, I'm just trying to understand the math and make sure I have calculated everything correctly. I see that you are 808 kwh/month which is 87% efficiency. So you are bit more generous than I was.

As for the cost, please help me understand your math. If I can generate $88 of electricity that is still $88 I don't have to make. The costs go up and down a bit, but it's pretty stable year round at $0.11-$0.12 per kwh. I don't understand how this will make my bill go up in the long run.

As for the costs. I'm trying to cobble together some numbers, but if I buy 24 pansonic N340's that's retailing for around $7600. Maybe a set of Q Cell 420 panels for $6000. And then there is everything else. $22000 is a pesimistic number, but I'm just trying to figure out a worse case for cost.

Briand81 said:

Hedges. Thanks for your comments. First and most importantly, I'm just trying to understand the math and make sure I have calculated everything correctly. I see that you are 808 kwh/month which is 87% efficiency. So you are bit more generous than I was.

As for the cost, please help me understand your math. If I can generate $88 of electricity that is still $88 I don't have to make. The costs go up and down a bit, but it's pretty stable year round at $0.11-$0.12 per kwh. I don't understand how this will make my bill go up in the long run.

As for the costs. I'm trying to cobble together some numbers, but if I buy 24 pansonic N340's that's retailing for around $7600. Maybe a set of Q Cell 420 panels for $6000. And then there is everything else. $22000 is a pesimistic number, but I'm just trying to figure out a worse case for cost.

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The way it raises your rates is the power company now doesn’t have to pay for power generated, you still have to pay the entire bill... and they don’t have to pay you for power generated when their demands are low, AND they don’t have to pay CLOSE to what they charge... and YOU still have to pay for the solar installation...

but ya get to feed green energy to the grid... reducing the carbon footprint of the power company... so feel good at least...

If your rates are presently $0.12/kWh but because you install solar your rate plan is changed to one where you pay $0.36 between the hours of 4:00 PM and 9:00 PM, then your bill will go up. That is how things are now in my area.

You haven't mentioned what type of inverter you plan to use (and you haven't mentioned batteries.)
The most cost-effective systems use grid-tie inverters and net metering, where surplus generation is delivered to the grid and "stored" as accounting, applied as a credit for power you draw at another time. You need to see what sort of net metering terms are offered by your utility.

I paid about $0.35/watt for several pallets of SunPower panels, including delivery to a trucking terminal. Other people here pay $0.25 to $0.50 depending on their preferences. I've paid $0.05 to $0.12 for older model grid-tied inverters (some brand new in the box.) I could probably assemble a system for $1.00/watt total, doing the work myself. Amortized over a decade the cost is about $0.05/kWh. Alternatively compared to $0.10/kW rates, payback is 5 years. With my cheapest rates being $0.15, payback is 3.3 years (or less, because some times of the day my rates are $0.30 or $0.45

Battery backup, if you want that, costs much more. I have a large amount of PV (generate all my own power) but very small battery backup.

I think your math is sound. Not sure about some of the input data. It sounds like you are planning to have a solar company install the system. Like Hedges says, This is a DIY forum and for us that installed cost is something we can't relate to very well. Unless your cost per kwh is pretty high, it is hard for an professional company install to pay for itself. They have to pay their labor pool and made profit to keep their business alive and feed their families.

Your production will be related to your latitude, the diretion your roof faces, and the angle of the roof. If the system designer does a good job in their planning they can tell you what the system should actually produce. But they are "in sales" so let the buyer beware. Does your quote tell you where the panels will be on your roof, the direction they will face, and the angle they will be set at?

" 18 panels x 340 watts/panel x 5 sun hours/day x 30 days = 920 kwh/mo Theoretical yield ", you need to understand the math behind this statement. Is this based on the directions the panels will be facing on your roof, the angle to the horizon they will be mounted, and your location/latitude?

Supervstech said:

The way it raises your rates is the power company now doesn’t have to pay for power generated, you still have to pay the entire bill... and they don’t have to pay you for power generated when their demands are low, AND they don’t have to pay CLOSE to what they charge... and YOU still have to pay for the solar installation...

but ya get to feed green energy to the grid... reducing the carbon footprint of the power company... so feel good at least...

Click to expand...

User cases do vary. I live in a state with a great energy law. The power company must give full credit for excess power, by law. My power company is an rural coop, non profit company. I do pay a base connection fee of $22 a month, to have service at my home. Even if I make 100% of my consumption, I still pay $22 per month. But to my great surprise, I get the full retail credit of about $0.10 per kwh for any excess and I make about 300 kwh excess per month on average. So I have not paid a penny to the power company since installation about 14 months ago. Long term credit you can case out at wholesale value after the credit is 2 years on the books. I feel blessed. My DIY install with the federal tax credit has an ROI of just over 6 years.

DThames said:

User cases do vary. I live in a state with a great energy law. The power company must give full credit for excess power, by law. My power company is an rural coop, non profit company. I do pay a base connection fee of $22 a month, to have service at my home. Even if I make 100% of my consumption, I still pay $22 per month. But to my great surprise, I get the full retail credit of about $0.10 per kwh for any excess and I make about 300 kwh excess per month on average. So I have not paid a penny to the power company since installation about 14 months ago. Long term credit you can case out at wholesale value after the credit is 2 years on the books. I feel blessed. My DIY install with the federal tax credit has an ROI of just over 6 years.

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That is great.
Federal laws are in the pipeline to do what my post says... and THAT sucks...

Briand81 said:

...18 panels x 340 watts/panel ... $22,000 system cost

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Cost seems high... 18x340 = 6120W, so $22k is $3.60/W. Tesla is advertising around $1.50/W with tax incentives ($2/W w/o) and discounts on powerwalls installed at the same time.

One way to beat the rates and laws is to store power in batteries, so it is never handled by the utility.
You don't suppose the Great Industrialist of our age has anything to do with rules that push us into the hands of solar battery manufacturers?
And then, rates will have demand/standby charges (presently waived for solar net metering customers) because we will occasionally still need to draw full power from the grid when sunlight is obscured.

Batteries really don't solve the problem for us. Unless the spread in rates is greater than in my area ($0.45/kW on peak, $0.15 off peak), batteries costing at least $0.16 and as much as $1.00 per kWh (purchase price divided by kWh capacity divided by cycles until worn out) really don't save money.

Controlling loads to make use of power when it is generated is the best way to go. It is also what the grid needs, not storage.

For Briand81 in Virginia, it may be that net metering laws are presently favorable:


Just glancing through, it does require that (s)he keep liability insurance in the amount of $100,000
He should be aware of the doom and gloom we've discussed. Net metering laws will be changed over time to the benefit of the utility, and the benefits seen by consumer in the beginning won't remain.

Which inverter systems provide no-export operation for situations where net metering either isn't allowed or isn't cost-effective?
Rather than storing power in batteries, my math shows it is cheaper to just put in excess PC capacity. Use power from PV while it is made, discard excess (reduce what is converted from DC to AC if you can't use it immediately), and buy from utility whatever you need but don't make.

Supervstech said:

...Federal laws are in the pipeline to do what my post says...

Click to expand...

I'm like DThames, state law that protects net metering.

Do you have a reference for the federal laws in the pipeline? Last threat I knew about was the NERA pettition to FERC, and fortunately that was shot down. But it's hard to keep up with and one of the reasons I'm here.

I know California, Hawaii, and Peurto Rico all allow the grid operators to throttle back renewable energy, but AFAIK that's not on a federal docket yet (although I believe some countries like New Zealand and Canada have already passed it so a very real threat). I'm actually okay with grid operators doing that to support a stable grid, but I fear political abuse like bridgeGate; or worse FERC mandating solar pricing because fossil-fuel plants that never should have been built have the political clout to recoup their costs.