# PV sizing



## MarkW (Jan 29, 2014)

When selecting one's array, is it better to oversize somewhat or try to stay within the usage you have now? 
I suppose there's the initial cost factor and I'm not sure what the buyback rates are for my area.  More considerations than those?


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## peakbagger (Jan 29, 2014)

The most important question is On grid or off grid? Its a totally different and more complex approach for off gird. I will assume on grid.

This link may help you get started as the incentives and utility tariffs make major difference http://www.dsireusa.org/incentives/index.cfm?re=0&ee=0&spv=0&st=0&srp=1&state=OH

The utility interconnect is another big one. If they allow net metering that's good but some contracts are for fixed periods like 12 months so you cant carry forward credit across seasons as well. If they do a cash out after 12 months they usually pay you wholesale. If you have a generous Feed in Tariff option, then the bigger the better.

Some states like NH have a limit on their PV rebate, thus many systems are designed around the incentives to minimize payback.

If aesthetics are important, plan on an all at once install as panels change frequently so getting more of the same panels a few years from now will be difficult.

Most folks go somewhat larger and use a bit more power as it is "free". I added a small freezer and added more PV to run a mini split to cut back on wood use and to handle shoulder seasons. So far it has worked well and cut my wood usage.

One of these years the 30% fed rebate goes away so don't wait to long.


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## MarkW (Jan 29, 2014)

I'm looking to build new this year with generous roof space angled for good production. I see the rebates are currently good til 2016 so I have a bit of time.  I'd just like to be sure to integrate PV into the build in the most efficient way possible to avoid costly changes later on.
As we all know, Murphy says all those costly changes will come after the rebate expires.


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## peakbagger (Jan 29, 2014)

If this is new construction do yourself a favor and run a 2" metal conduit from the attic to the wherever the panel will be. Ultimately you have to run PV cables in metal conduit and its a heck of a lot easier to run it now than later.


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## MarkW (Jan 29, 2014)

That's exactly the sort of thing I want to do.  I'd like to decide on exactly what to install in the near future so that I may place the rest of my build apropriately.


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## Former Farmer (Jan 29, 2014)

If you size it that you anticipate producing more that you use, then the price paid by the utility is the biggest factor in calculating payback period.


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## MarkW (Jan 29, 2014)

Consequently if the utility doesn't pay squat then I want to size for useage? Ill have to see what Ohio has in their regs.


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## jebatty (Jan 30, 2014)

MN has net metering at the retail rate. Solar production is an offset to each month's bill with annual settlement of excess. My annual usage was about 12,000 kwh, including about 1,200 kwh for electric hot water (off peak reduced rate), 5,000 kwh for electric heat (interruptible reduced rate) and 6,000 kwh for general service (full retail rate). I sized the system at estimated annual production of 9,000 kwh with the intention of additional conservation measures to reduce usage and then coming close to fully meeting all usage needs. The amount of buy-back at the retail rate also now will cover the base charge of $14/month and charges I have on a separate meter for my shop. Without net metering at the retail rate I would have sized the system at about the 5-6,000 watt level.


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## peakbagger (Jan 30, 2014)

Another thing to build into the new house may be the provision for a line sized tap between the meter and the main panel. The PV system can only backfeed a panel up to 20% of the buss bar rating. Most but not all panels have a buss rating equal to main breaker. If you are going with a 200 amp panel with 200 amp bus then you are limited to 40 amps of backfeed. Unless you play games with  a subpanel, that could limit you on how big of a system you can install. It would require knowing what you ultimately plan to install as the backfeed amperage is based on the backfeed breaker size of the inverter(s). If you install provisions for a line size tap between the main panel and the utility meter then this is not an issue. Alternatively you could buy a panel with a buss rated for higher amperage. Generally standard residential panels are far lower cost than higher rated panels so its all a cost issue. This is probably not an issue for most home systems with a 100 amp panel unless you plan to charge an electric vehicle and build  system to match it.

I think the easiest way to go with a line sized tap is to install a meter base setup for mobile homes which have a main breaker in a separate compartment. The line size tap would be spliced  into the cable between the meter and the main panel in that compartment. The utility really doesn't want any splices in their side of the box so you need a meter box with separate compartments. 

I would discuss this with an electrician familiar with solar as the devil is in the details.


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## jebatty (Jan 30, 2014)

> If you are going with a 200 amp panel with 200 amp bus then you are limited to 40 amps of backfeed.


Explains how the electrician installed my system. 200 amp main panel, 200 amp main backfeed panel with a 40 amp backfeed breaker.


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## MarkW (Jan 30, 2014)

peakbagger said:


> I would discuss this with an electrician familiar with solar as the devil is in the details.


certianly sound advise

So, I see OH has provisions for net metering but rates seem undefined by the law.  Additionally, it seems the regulations are written such that the customer-generator system is not intended to exceed usage greatly.  While there seems to be some ambiguity regarding that excess production amounts, I've no interest in getting into a pissing contest with the utility about what exactly constitutes "excessive" production.
Thanks OH for clearly spelling it out


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## Where2 (Jan 30, 2014)

The monopoly doesn't like competition. 

Size the system to meet your current needs plus around 10-15% additional. As the panels degrade by 10-20% over 20-25 years you will stay ahead of the game. Conservation is cheaper than a PV system, but it only goes so far.

My initial system at 4400W covers ~55% of my needs. It was all I could fit on my second floor roof. My second phase is already in planning and will be another 4600W-5000W ground mount system.


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## MarkW (Jan 30, 2014)

Where2 said:


> Conservation is cheaper than a PV system, but it only goes so far


 I'll be loosing my 2 biggest energy consumers in the next few years(daughters).  My needs should drop drastically then.  So, sizing for current consumption should take into account panel degredation.


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## GaryGary (Jan 30, 2014)

Hi,
I think that if you are building new, you are in a really good position to work the conservation and efficiency angle carefully as the house is built.  Review every device that goes in the house that uses electricity -- lights, appliances, furnace (eg ECM blower uses about 1/3rd the power),  motion activated switches, ... lots of opportunities and all cheaper than adding PV to cover "normal" use.  We cut our electricity use in half just doing simple things and then added PV -- I would think that if you are building new you should be able to cut your usage by at least half from where you are now (assuming you have not taken a lot of steps already to reduce usage).

One thing to consider is if you think you might want to go to an electric car at some point -- this is a pretty big increase in electricity use.  Maybe have a plan for expanding the size of the PV array in the future if you go to an electric car.

This site gives a pretty good rundown for every state on the incentives and net metering rules: http://www.dsireusa.org/solar/index.cfm?ee=0&RE=0&spf=1&st=1 


Gary


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