# Formula for PV Panels and Inverter

Discussion in 'Photovoltaics' started by sunnybouy60, Jan 12, 2006.

1. ### sunnybouy60Guest

Hi guys, I was wondering if someone could explain to me (in simple terms)
the forumla to determine how many PV panels are needed in a PV system?

I understand that Total Watts x Hours Usage Per Day = Total Watt Hours

and that

Total Watt Hours / Voltage System ie 12v = Total Amp Hours

and that

A Deep Cycle battery should only be discharged 50% so double the battery
capacity to cover the Amp Hours

now.... how do I determine how many PV panels I would need???

I know it is dependant on the amount of sun per day and my geographical
location. Also what is the forumla for determining the size of the Inventer
that would be needed? Can someone help me out here?

Thanks!!

2. ### S.C.F.Guest

want the anecdote?

3. ### SJCGuest

That is a bit like asking "how big is a fish". You make the system the
size you want to make it. I have seen grid tie inverters as small as 1KW
and they may make some smaller. If you buy a 1kw sine wave grid tie
inverter and use less panels, it will only supply to the grid what the
panels can provide. If you have more panels, then the inverter will
supply to the grid only what it can invert from DC to AC.
If you are not going on the grid you can just charge batteries and
use a square wave inverter to power small tools and those inverters
start at 100w or less.

4. ### S.C.F.Guest

hello,SJC!!!!
want the anecdote?

5. ### Chuck OlsonGuest

There's a website that you can use to calculate what will approximately
cancel your electric bill, averaged over a year's time. It's
http://www.findsolar.com/ and it is useful to find a solar installer.

Generally, there are two different types of PV you can plan on - - grid tied
or stand-alone. Sounds to me like you want a grid-tie system, so you can
forget about batteries. The grid is your energy storage facility with your
electric meter keeping track of what you put into storage versus what you
take out.

Now that we have that out of the way, you might want to think about the
future and your growing needs, and couple that with the fact that all that
energy derived from sunshine is free (the cost of the equipment adds to the
value of the home, so arguably that cancels out). Once you reach that
mindset, all you need to figure is how much roof area you have to play with,
and if it's facing a good direction. I have a 560 sq ft garage roof facing
south so I'm going to load that roof up with 36 panels, 170 watt each for a
little over 6 KW peak output, and roughly 25 KW-h per day storage. (With
that amount of energy, I can probably try to get my Prius converted to
plug-in and charge up an extra large battery in it overnight to go 30 miles
the next day on electric power only.) The state will contribute a little
over \$14,000 to help me pay for the PV system, and federal adds another
\$2000.

As the utility keeps track of things, you only have to "pay the piper" once
a year if your usage exceeds your generation. Unfortunately, if your
generation exceeds your usage, the piper says "forget about it" and you get
to think of new ways to use your excess energy, like an electrically-heated
hot-tub, or whatever improves your outlook on life.

Have fun,

Chuck

6. ### Douglas SiebertGuest

I don't think that's really accurate. If you put \$30K of panels on your
roof, that only adds \$30K to the value of your home for someone who wants
to have that system. I could put a \$30K gold plated custom bathtub in my
home, but it won't have a whole lot of value to someone who only takes
showers or doesn't need any "bling" with their bath, so I shouldn't expect
to realize anything like \$30K additional resale value on my home because
of it unless I sold my home to a pop star.

Realistically, all you can expect to get back in additional value of your
home is the production capacity of the PV array * utility rate * years of
expected lifetime - maintenance/repair costs. If you spent \$30K on a PV
system that generates 5000 khw per year and your utility rate is 10c/kwh,
that's \$500 a year. Using those numbers you could expect your home's
value to increase by at most \$5-10K (and many buyers might even consider
it a negative just because they won't want to hassle with something they
don't understand) Now if you find someone who is interested in PV they
might buy your home just because of that system, but even then I doubt
you could get back what you put into it. After all, for someone who is
into PV they'd probably rather buy a home without it and add it themselves
after a little research to find out what they wanted from it versus buying
it "used" and built to someone else's specifications.

I'm not saying it is dumb to do PV, you just shouldn't fool yourself that
it is a good investment today, unless you live in a place like CA with
very large rebates and high utility rates, or you believe utility rates
are going to go up way way up in the next few years. You definitely
shouldn't fool yourself into thinking it is going pay for itself through
increase in the resale value of your home!

I am interested in doing PV on my new home, but since it is at least 4-5
years from being break even (or close enough that I'm willing to do it
anyway) I'm going to build it "PV ready". (Which to me basically consists
of making sure the roof could take the weight of being fully covered by
panels on the south side, and running some metal conduit up to the roof
from the basement to allow me to run some extra thick copper wire up
there later)

7. ### sunnybouy60Guest

Too funny SolarFlaire...

Ok I must not have made my question clear here, I appreciate all of your
input. Could someone just give me a breakdown for the following?

5000 Watt Hours per day total
416.6 amp hours
4 - 210 Amp Hour Deep Cycle Batteries

How many PV panels and what Wattage do I need to support the above
configuration and how do you calculate the number/wattage of the panels
required?

What size Inverter would be best suited?

Thanks

8. ### Guest

*Appraisal Journal, October 1999.
"All solar systems are exempt from property taxes. The National Appraisal
Institute states that the value added to your home is 20 times the value of
electricity saved in the first year. A solar installation is one of the few
ways that you can increase the value of your home without increasing your
property taxes."

9. ### Douglas SiebertGuest

I'm aware that many states (including mine) exempt solar from property
taxes. I was talking about actual resale value of the home, which in
many places has only a very loose relationship to the taxable value of
the home.

Given that I'm paying about 9c/kwh and there are no rebates in my state,
if I installed a system capable of generating 5000 kwh per year in my
locale (i.e. a bit over 3000 "peak" watts) that's worth about \$450, so
multiplying it by 20 that's \$9000. Unfortunately we're about a factor
of 3 from a fully installed grid-tie system for \$3 per peak watt. So
even at that (IMHO somewhat optimistic but I guess NAI knows better
than me) 20x generation valuation its only a 33% payback. Stuff like
remodeling of older kitchens and bathrooms is supposedly a 90%+ payback,
and even stuff that's not a very good investment like adding a pool in
more northerly climates is still a lot better than 33%. So again my
point is that you have to do solar because YOU want to do solar, not
because you figure even if you are only in the house for a few years
you'll get most of it back when you sell your house, because you won't.

10. ### Guest

My \$500 12'x32'x16' tall plastic film sunspace was exempted as "HVAC space."
Let's not tar and feather sunspaces with PVs The economics are different,
adding valuable floorspace to a house while collecting heat energy at 5 cents
vs \$5 per peak watt.

Nick

11. ### Steve SpenceGuest

Your inverter needs to be sized to the loads you are trying to power.

12. ### George GhioGuest

Who says. 70% would be safe, 80% pushing it, Depends on haw soon you
cane replace the charge.
ROT - 60% of your load i.e. 1000Wh = 600 W inverter. Unlikely that you
will run more than 60% of your full load at once. Decide with care as
inverters cost money

13. ### Steve SpenceGuest

70% discharge really shortens battery life. Shame on you george for

so does everything but not a whole lot. 2500 watt MSW's are only \$269

14. ### wmbjkGuest

Don't forget that a deep discharge combined with a relatively large
load may cause the inverter to trip off due to temporary low voltage.
On paper that next 20% of battery capacity below 50% discharge might
be plenty to vacuum up some cat litter, but in reality one may have to
reset the inverter before breaking out a broom and dustpan.
No no no. Inverters should be sized for power. Imagine a system
supplying say, a standard submersible well pump for 10 minutes each
day. Call it 150Whrs per day. Your RoT 90W inverter owner will have to
use the bathroom at the filling station. And in the example you used,
that 600W inverter wouldn't be able to support some common household
appliances, even though 1kWhr per day might otherwise be sufficient.
In the OP's example of 5kWhrs per day, your RoT would recommend a 3kW
inverter. Which would crash if say, a water pressure booster pump
started while he was nuking a sandwich. Or would be far too large if
the system was say, running a couple hundred Watts of lighting 24-7.
In fact, I can't see any reason for this RoT to exist. If one knows
the daily consumption, then he already knows which loads and
combinations will be running when, and can choose the inverter
accordingly.

Wayne

15. ### George GhioGuest

Read the post. Clearly says (with typos) Depends on how soon you can
replace the charge. 70% discharge is common for max DOD for X number of
days autonomy.

It does not say "Run your batteries down to 30% every day".

70% does not "Really shorten battery life" unless you do it every day.

My daily DOD is currently 12%. Still it would not worry me to pull the
batteries down to 70% after ~6 days of no sun.

Shame on you steve for misleading him.
We know you buy second best to cut costs but even at that price why buy
an inverter just to buy another one due to poor choice in the first place.

16. ### George GhioGuest

This from Wayne who has admitted that he can chew through 10kWh in four
hours.

If you want to be taken seriously you can start by documenting your system.

You can't though can you?

17. ### George GhioGuest

The point is that 50% is bunk.

18. ### George GhioGuest

Oh dear. Truth, Wayne has not got a clue.

19. ### George GhioGuest

Let's see:

George - posted total energy use for all fuels + solar -

Wayne - posted weight of batteries and inverters -

Conclusion Wayne knows next to nothing.

Looks good to me

20. ### George GhioGuest

Ah yes, Windsun is a battery retailer. The reason for 50% max DOD is
that if you swallow that line you end up buying 30 to 50 percent more
batteries than you need and the battery retailer makes 30 to 50 percent
more money.