Solar questions

[phuz]

I am looking at doing a new system and have some questions.
If irradiance is 1000W/m^2, then how is a panel 60x40 only putting out 255W?

[J.P.M.]

I am looking at doing a new system and have some questions.
If irradiance is 1000W/m^2, then how is a panel 60x40 only putting out 255W?

1.) Panels themselves are something like ~ 15 to 20% efficient.

2.) The irradiance in the plane of the panel is probably < 1,000 W/m^2 and changing constantly. How do you know it's 1,000 W/m^2 ?

3.) Lots of other things influence panel output, many of which are changing constantly. The output can be assumed non steady state.

4.) Other system losses will reduce output further.

Buy a solar for dummies book for explanations of the above.

[phuz]

1.) Panels themselves are something like ~ 15 to 20% efficient.

2.) The irradiance in the plane of the panel is probably < 1,000 W/m^2 and changing constantly. How do you know it's 1,000 W/m^2 ?

3.) Lots of other things influence panel output, many of which are changing constantly. The output can be assumed non steady state.

4.) Other system losses will reduce output further.

Buy a solar for dummies book for explanations of the above.

OK, with the low efficiencies, it makes sense now.
Thanks. I am in the very early stages of planning and deciding how big I want to go. I am an electro-mechanical engineer and limit myself to automation control systems, but this is not out of my realm. I am planning on a grid-tie system, but would also like to be able to use the solar when the grid is down rather than run my 15kw propane generator which gets rather costly, so I know that is a decision I have to make.

[SunEagle]

OK, with the low efficiencies, it makes sense now.
Thanks. I am in the very early stages of planning and deciding how big I want to go. I am an electro-mechanical engineer and limit myself to automation control systems, but this is not out of my realm. I am planning on a grid-tie system, but would also like to be able to use the solar when the grid is down rather than run my 15kw propane generator which gets rather costly, so I know that is a decision I have to make.

Have you gone to PVwatts.com yet. While not perfect it provides some pretty good data on cost and sizing of a PV system for your house. Also knowing what you use per year in kWh and the cost structure the Utility charges you will help you size your system.

As for not wanting to run that propane generator during a power outage. Going with a battery system will be much more costly then using a generator. If you don't need the whole 15kw then find a smaller and more efficient one for those outages.

[phuz]

Have you gone to PVwatts.com yet. While not perfect it provides some pretty good data on cost and sizing of a PV system for your house. Also knowing what you use per year in kWh and the cost structure the Utility charges you will help you size your system.

As for not wanting to run that propane generator during a power outage. Going with a battery system will be much more costly then using a generator. If you don't need the whole 15kw then find a smaller and more efficient one for those outages.

It's tied into the house, so not worth changing it at this point. I'll check out that site.
Thanks.

[SunEagle]

It's tied into the house, so not worth changing it at this point. I'll check out that site.
Thanks.

Since just about all normal "grid tie" solar pv systems will not work when the grid is down you will still need another power source as a backup.

You can install a Hybrid solar pv system which includes batteries (and generator) but they are very expensive and usually not cost effective for places that only go without power for short periods of time.

No question that Solar Grid Tie systems can pay for themselves and can be worth installing.

Having a solar battery system as backup power is not worth it because even that noisy generator will give you more power for a lot less money.

[Sunking]

use the solar when the grid is down rather than run my 15kw propane generator which gets rather costly, so I know that is a decision I have to make.

Generators and fuel are a lot less expensive than batteries. Very foolish to have battery backup. Unless you have an iron lung, but even then you only need a few minutes of batteries to get a generator started up to take over. There is not one high tech company that relies on batteries for back up, all use generators for a very good reason. Solar is unreliable, power limited, and way to expensive for back up power.

[phuz]

Generators and fuel are a lot less expensive than batteries. Very foolish to have battery backup. Unless you have an iron lung, but even then you only need a few minutes of batteries to get a generator started up to take over. There is not one high tech company that relies on batteries for back up, all use generators for a very good reason. Solar is unreliable, power limited, and way to expensive for back up power.

Yes, not sure where the inference came from, but i would never consider adding batteries.

[Wy_White_Wolf]

Yes, not sure where the inference came from, but i would never consider adding batteries.

Thats the only way you could use solar instead of the genny.

WWW

[SunEagle]

Yes, not sure where the inference came from, but i would never consider adding batteries.

A "grid tie" system will shut down when the Utility grid goes down because of the safeties built into the inverter. So no grid power and no solar pv power.

There are some grid tie inverters that have a small emergency connection that allows the pv panels to provide power when the grid goes down but it is a fraction of the full power of the panels and it is subject to only working when the sun is shining. If you get a cloud you will not even have that little power.

The only way to have backup power system with solar is to add batteries (very expensive) or have a generator like you have.

[HOASUNBuster]

Yes, not sure where the inference came from, but i would never consider adding batteries.

I have a small system that encompasses all three. I started with a solar battery system to run lights and tv/stereo in case of a power outage. I then added a 7kw generator to that mix to be able to run my house in the event of a larger local disaster. After that, I decided to build the panel system and go from off grid to grid tie. Your neighbors would appreciate the silence of batteries depending on what you actually want to run with them for short periods. They will also appreciate your generator in the off case that infrastructure gets damaged and you are one of 3 homes in the neighborhood prepared for such a rare occurrence but they will be at your door step none the less. My generator is set up to use natural gas but is not attached so for short outages, I just flip over to battery. If I need air conditioning, I have no option but the generator as my off grid inverter is a sine wave 1kw Xantrex and was not sized to do more than lights and some entertainment on TV/DVD waiting for the restoration of power. The batteries are no more than a convenience for short outages. As the rest have said, a large battery bank is very expensive.

[Sunking]

Yes, not sure where the inference came from, but i would never consider adding batteries.

Well yes you did infer to using batteries as that is the only way you can use solar when commercial power is out.

[phuz]

Well yes you did infer to using batteries as that is the only way you can use solar when commercial power is out.

I apologize if I confused you. I was implying my two options were grid-tie (no solar power when grid down) or off-grid (grid status irrelevant).

[SunEagle]

I apologize if I confused you. I was implying my two options were grid-tie (no solar power when grid down) or off-grid (grid status irrelevant).

Grid tie with a backup generator is your best option. Stay away from batteries especially if you have grid power available.

[phuz]

Grid tie with a backup generator is your best option. Stay away from batteries especially if you have grid power available.

I do like having the 15kw generator, although it runs on propane (natural gas capable). We have natural gas 600' from our house and the utility wanted $20k to run a line to my house. Unbelievable.

So grid-tie seems to be the way to go and I may get an inverter that has a designated output regardless of grid status.
Thanks!