# Newbie question about DIY solar panels on the rooftop.

Registered Users Posts: 23 ✭✭
Ok after spending some time on YouTube I've learn about wind and solar power. I am really getting into saving power I've dropped approx 500 kwh in just 2 months by burning wood and paying attention to details. Tho I really dig the wind turbines, I've found that solar is more practical in the per watt cost. There is one thing that I can't understand. When I first started reading, I thought I could power my 2000 at home for \$2000 and not have a power bill. I've learned a lot in just 2 days but I've seen DIY people purchasing grape panels from Home Depot at 250 watts each x 4 and claim to make 4kwh per day. Ok, fast forward to last night and a similar setup but with 16 of those same sized panels on the roof were making the meter spin backwards. Now this confuses me. How can 4 panels on one home make 4 measly kwh and A home with 4 x that produce enough power to spin the meter backwards? Shouldn't 4 times the panels make 4 times the kwh? So the 2nd setup should be 16kwh per day? Can someone shed some light on it? No pun intended.

## Comments

• Super Moderators, Administrators Posts: 33,402 admin
Re: Newbie question about DIY solar panels on the rooftop.

The basic grid tie math is:

16 * 250 Watt panel * 0.77 panel+controller deratings * 4 hours (~9 month a year minimum) = 12,320 WH = 12.3 kWH per day typical minimum

The peak (average power) would be:

16 * 250 Watt panels * 0.77 deratings = 3,080 = 3.1 kWatt peak (average) power flow

If the 3.1 kW produced is > power consumed by the home at that minute, then the meter will stop turning or even start turning backwards (note: utility power meters come in many "flavors" this days--Some will never turn backwards, others will have two ore more meters/registors to record power flow, or even two separate meters--it all depends on your utility and billing plan).

Using PV Watts for Greensboro NC (as an example) with a 16*250W=4,000W=4 kW array, fixed, titled to latitude and the rest defaults using PV Watts, you would see:
"Station Identification"
"City:","Greensboro"
"State:","North_Carolina"
"Lat (deg N):", 36.08
"Long (deg W):", 79.95
"Elev (m): ", 270
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 36.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (\$)"
1, 3.97, 385, 32.73
2, 4.52, 384, 32.64
3, 5.44, 498, 42.33
4, 5.67, 484, 41.14
5, 5.68, 497, 42.24
6, 5.84, 476, 40.46
7, 5.59, 468, 39.78
8, 5.61, 474, 40.29
9, 5.31, 440, 37.40
10, 5.04, 448, 38.08
11, 4.13, 368, 31.28
12, 3.45, 324, 27.54
"Year", 5.02, 5248, 446.08

Around 324 to 497 kWH per month, on a long term average with a 4 kW GT connected array.

Note that not many homes use 3kW * 24 hours per day--That is a lot of energy:

3kW * 24 hours * 30 day per month = 2,160 kWH per month

So, with a GT system, think that you are recharging the utility grid during the day and discharging it at night. In terms of customer usage, the utility looks like a giant AC Battery Bank and you are charging/discharging it for your needs.

Just like you run your car+battery+alternator. Sometimes you use energy, other times you recharge the battery and share power from the alternator to run the lights/radio/etc...

Does this make sense?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭✭
Re: Newbie question about DIY solar panels on the rooftop.

Meter doesn't spin backwards all the time; just when panel production happens to exceed household consumption. This is not the same as negating all power use in the home. In order to do that, the panel production during the hours of available sunlight has to exceed the home consumption for the full 24 hour period.

So if your household uses 16 kW hours per day you need roughly 5000 Watts of panel to make that up in four hours of good sun. If you use less, fewer panels. If you have more sun, fewer panels.

Four 250 Watt panels = 1000 Watts and in 5 hours or so could indeed make 4kW hours of electric. If everything in the house was shut off, all of this would be sold back to the utility.

Sixteen of those panels would be 4000 Watts and would in the same time produce 4X the amount of power: 16kW hours. Possibly enough to offset the home usage, and if that is small there would even be some to sell back.

Even at \$1 per Watt just for the panels it would be \$4000. Then there's the GTI, wiring, hardware, et cetera. Ain't no free lunch.
• Registered Users Posts: 23 ✭✭
Re: Newbie question about DIY solar panels on the rooftop.
Meter doesn't spin backwards all the time; just when panel production happens to exceed household consumption. This is not the same as negating all power use in the home. In order to do that, the panel production during the hours of available sunlight has to exceed the home consumption for the full 24 hour period.

So if your household uses 16 kW hours per day you need roughly 5000 Watts of panel to make that up in four hours of good sun. If you use less, fewer panels. If you have more sun, fewer panels.

Four 250 Watt panels = 1000 Watts and in 5 hours or so could indeed make 4kW hours of electric. If everything in the house was shut off, all of this would be sold back to the utility.

Sixteen of those panels would be 4000 Watts and would in the same time produce 4X the amount of power: 16kW hours. Possibly enough to offset the home usage, and if that is small there would even be some to sell back.

Even at \$1 per Watt just for the panels it would be \$4000. Then there's the GTI, wiring, hardware, et cetera. Ain't no free lunch.

Lets just say I'm learning.
• Banned Posts: 17,615 ✭✭✭
Re: Newbie question about DIY solar panels on the rooftop.
C_Heath wrote: »
Lets just say I'm learning.

We all are. Continually!
• Solar Expert Posts: 2,397 ✭✭✭✭
Re: Newbie question about DIY solar panels on the rooftop.
C_Heath wrote: »
Lets just say I'm learning.

Well it is a good thing you are asking the questions first!

The panels are only the first step in an installation, you need racking for the panels, the GTI inverter, wiring and disconnects to the DC side and the AC side, perhaps a DC combiner with DC breakers, maybe a meter to show your total solar production for the utility. A design for the permitting people to approve, and interconnect agreement with your utility, and all the inspections of course.

So a DYI with the support of a licensed electrician is possible in most locales. The cost usually for DYI could be as low as \$2 a watt.