Friday, March 22, 2013

Some Solar Power Considerations

I am starting to work on getting solar power at our house, so the blog will probably have a lot of solar power calculations in coming weeks.  The above is a NREL map of the solar resource in the United States.  You can get the full (poster) size image here.  I think it's a gorgeous map.  Pretty much the whole US (with the partial exception of Alaska and parts of the Pacific northwest) has a solar resource ranging from good to great.  Where I am in upstate New York, there's round about 4 kWhr/day/mfor a flat panel tilted at latitude (around 42o in my case) and facing south.  Multiplying by 365, that gives 1460 kWhr/yr/m2.

A typical panel with peak output of 240W has an area of 1.64 m2 and an efficiency of 13.4%.  The efficiency would take my 1460 kWhr/yr/m2 down to 196 kWhr/yr/m2. Multiplying by the panel area, this implies an output of 320 kWhr/yr per panel.  Another way to think about this is 1330 kWhr per kW of peak power (320/0.24).  This is the theoretical value before we start taking account of dirt, imperfect alignment, shading, inverter losses, and so forth.  PVWatts (a calculator from the National Renewable Energy Labs) can do these calculations, and for my location and situation (in particular I have roofs that face south-east and south-west), I get about 1050 kWhr per kW.

It looks like the house currently is on track to consume about 10000 kWhr per year, suggesting a current need for about 9.5kW of panels to provide all our energy usage.  However, we also want to power electric cars in the next couple of years.  Between my wife and I, we drive about 30000 miles/year.  The Nissan leaf gets about 3 miles/kWhr and the Tesla Model S is similar but a little less efficient (38 kWhr/100 mi).  So for 30000 miles of electric vehicle usage, we will need about another 10kW of panels, making 20 kW total, in round numbers.  This corresponds to around 83 of the typical modules above, and will consume a roof area of 136m(or 163 sq yd, if you'd rather think of it that way).

Luckily, I have a barn as well as a house to put these things on.

Lastly, it may be of interest to see PVWatts estimate of monthly energy production from a 10kW installation at my location:

I'm a bit sceptical of the winter values - around here, everything is covered in snow most of the time in January and February, and I can't see the panels generating a lot of power then (and I certainly have no intention of going up on the barn roof to clear them off).  So maybe we should call our final need 22kW to be safe.


buck smith said...

I think you are missing the effects of seasonal length of the daylight. Unless you are planning some way to store extra energy generated in the summer to use in the winter, there is no way you are powering electric vehicles with your solar panels in the winter.

Phil Plasma said...

The cost of that many panels plus all of the components to make it work is likely going to be crazy. What about battery storage to level off the peaks and troughs? I guess, like you mention, you'll be posting about this over the coming weeks.

I don't know if you read Gail's Renewables post; it definitely gives food for thought.

Also, how is wind in your area and would you also consider that? Often it is windy when it isn't sunny, and sunny when it isn't windy.

Stuart Staniford said...

buck - we'll be grid tied with net metering. So we'll generate excess power in the summer and sell it to the utility, and buy it back in the winter.

Stuart Staniford said...


I think small wind doesn't generally work well, and certainly our location is not windy enough.

Jim McDonald said...

I like that you are including the car mileage. I took that thought a bit further and used the price of electricity from the aeso to convert what I spend on groceries (etc) into kWhr. Unsurprisingly I need quite a spread of solar to cover my energy inputs.

As an aside it would be neat to view all my personal economics in kWhr. What I make and what I spend. I get some kWhr from my work in the form of dollars and some from the solar on my roof.

Aaron said...

If at all possible I'd plan to clear the panels of snow if I were you. All it takes is standard long broom brush attached to a very long handle. It must be cleared before the snow melts and packs down into an icy popsicle. Clear the snow while it's still light and fluffy. I don't know how accessible your barn roof is but you might consider some scaffolding to reach it. Winter solar, although dismissed, does have it advantages - as temps drop into the teens (and lower) the air cannot hold as much moisture and the sun becomes very bright. My personal experience with solar is that some winter days are better than the best summer days (except right around the summer solstice).

ColdNorth said...

I'm in Ontario, which has a Feed In Tariff @ $.549/KWh. We are looking at putting a 10KW system on the roof. Prices for this size of installation (as a turnkey system) are in the scale of $38K (+/- $5K). Ontario has a requirement for made-in-Ontario content which probably pushes up prices.

I understand that an inch or two of snow will readily melt (the next day). Panels in the winter will still run at ~20C on sunny days due to their colour, and between 45C and 65C in the summer. The winter performance gets a bump in to the higher efficiency in the winter, due to the cooler operating temperatures. For thicker snow falls they are prone to slide off a roof (think avalanche) on warmer days. A $50 roof rake can help make the snow's departure from the roof more orderly and less of an exciting event.

Mr. Sunshine said...


As a solar power technology teacher and course developer who lives "off-grid", I'm glad to see you off to a good start in design. The NREL PVWATTS program and data are quite good and, I've found the predictions quite accurate over many years of comparison to installed systems.

Albany's 43 deg latitude would require your barn have a 12:12 pitch (45 deg) for flush mounting - and if this is the case, snow will melt off fairly quickly in winter. If your barn roof is 6:12, the array will only be tilted 27.5 degrees, which will not let snow fall off so quickly, but your summer power production will be enhanced a lot, which will offset the winter loss to a good degree.

I'd suggest you round up and look at a 10kW system, since typically, that';s as large as most local governments and utilities will consider for residential rebates, etc. Check for all of the incentives in your area.

10kW, grid tied, no battery, will probably run $56,000 in your area, fully installed. There is a 30% federal tax credit rebate on that amount, plus whatever local and state incentives you have available. I'm fairly sure solar leasing is legal in NY - you might look into no-money-down arrangements with firms like Solarcity, Sunrun or Suntergy.

Remember batteries have a 20% loss, so one must increase PV battery charger systems by a factor of 1.2 to make sure this is accounted for if you are charging batteries / running off grid.

Have fun!

sunbeam said...

Mr. Sunshine, I have a question for you if you lurk here a lot.

How does the economics change if you just look at the panels as a way to keep the electricity bill low.

By that I mean minimal batteries for an "accumlator," but basically you don't try for total independence, just to keep the meter from spinning for the power company as much as possible.

Unless you have dc appliances in your house (they do make these, truckers buy a lot of them, though capacities and whatnot may not fit your needs), you will need an inverter.

Heck I'm curious as to how it would work if you didn't have a single battery.

Basically you would just use the power company for any power that is over what your panels supply at any given time. Ignoring for the moment selling power back to the utility, has anyone ever tried this?

Anonymous said...

Do you have enough land for a ground mounted array? Much easier to clear snow. Also, consider selling annual SRECs produced by the system (though you can't claim the clean energy benefits as "yours", even though you'll be using most of the electrons..)

TomVee said...

We installed PV on a 6/12 pitch roof in Boulder Colorado, and the panels clear themselves pretty well after a sunny day or two.
But we were so intent on maximizing panel area that we took the panels close to the eave and the resulting avalanche when the panels unload is a bit much. So think about the snow unloading from panels and what is below during your design process. I have seen some installations here with metal hooks on the roof above the eave to catch and break up the sliding mass. Or just leaving some shingles exposed below the panels slows or stops the avalanche.
In our case the avalanche lands on a lower roof and so far has done no damage, but the big thump when the snow mass lands is kind of un-nerving.
So far our panel output is higher than predicted and we are banking kWh in our net-metered account to eventually power an electric vehicle (we don't own one yet).

John said...

10,000 kWh per year? How many people in your house? I live by myself in the UK and consume 824 kWh per year for cooking, heating the kitchen, lighting, computer, stereo, fridge etc. Hot water and living room heating is by oil.

Stuart Staniford said...

John - good for you. Average residential consumption of electricity for a single family home in the northeastern US is 9853kWhr/yr according to the EIA, so we are about average in our current usage (but no doubt starting with a bit of a disadvantage being in a 1850 house in a 8000 degree day climate with baseboard electric heat).

Angus Wallace said...

My family of four lives in Southern Australia and uses about 4 - 6 kWh/day. I plan to try and reduce this to 3 - 4 kWh/day with the help of a few retrofits. Granted, you would need much more heating than we do.

I'm not getting holier-than-thou, but I would suggest that you can make retrofits and minor changes to your lifestyle that would reduce your power consumption by more than 75%, which would allow you to use a much smaller system.

From reading your blog, you are clearly interested in sustainability, so I suggest you reflect on the sustainability of every house in the US installing a 22 kW PV system.

Again, this is not a dig, but a suggestion that your money and efforts may be better spent elsewhere.

Best wishes.

The Rational Pessimist said...

Jim McDonald

The exercise you crave for has been done by Cambridge (UK) Professor David MacKay in the e-book "Sustainable Energy - Without the Hot Air" and is available for free at

There you can compare energy consumption related to drinking a pint of milk, heating your home, driving a car and providing national defence and so on to your heart's content. It is a treasure trove of numbers (if like me you like this stuff) and absolutely fascinating.

BTW, MacKay is now Scientific Advisor to the UK Department of Energy and Climate Change.

John said...

>> in a 1850 house in a 8000 degree day climate with baseboard electric heat <<

Electric heat? That explains it :)

Wouldn't air source heat pumps be more efficient? HereInHalifax swears by them :)

PS. my place was built in the 1750's but is now double glazed and heavily insulated...

Susan Kraemer said...

I too wonder if you usage number is correct. After all the efficiency upgrades it should not be that high unless you are using electric for heating too?

Our very efficient house (to meet California title 24 building codes of the 1980s) resulted in 4,500 kwh per year energy consumption (+heat with gas) and 80% coverage with a 3.15 KW net metered system.

But after you did all that you should be doing a lot better than 10,000 kwh/yr.

Also, adding 2 EVs should not double that huge amount. Don't be afraid to get several solar estimates from the industry where they can show you electric (net metered) bills in your area before and after solar (free no obligation) but also in touch with real world situation.

Snow tends to melt faster on panels which are mounted a bit above the roof, so there is an air pocket. Also, once exposed, if it snows after sun has been on them, they tend melt snow as it comes down if its not too fast as they give off heat during operation.

Mr. Sunshine said...


A battery-free grid tied system design is quite simple... add a year's electric use in kWh, then average. Divide by 30 to get kWh per day. Divide by average daily insolation in kWh (from PVWATTS). If using .77 efficiency per NREL, multip0ly by 1.3 to get array size in your area for your average daily load. Pick modules and an inverter to deliver the energy (SMA Sunny Boy is my favorite grid tied inverter and, using their AC coupling and Sunny Island boxes you can add battery backup later with little array change). Use PVWATTS to size the array given your pitch and azimuth. email me at bill at 'dc power and light' [one word] dot com --- happy to help you.

Stuart Staniford said...

On all the various folks questioning our power usage - it's a busy family of four in a very old house in a cold climate in which the secondary heat is electric. The house started out quite inefficient, and we've so far cut the wintertime electric usage by a factor of three as discussed here:

The house still has mostly Victorian windows and high levels of infiltration (3100cfm @ 50Pa). In the next year or two, we plan to expand the house, and at the same time do a deep energy retrofit to hopefully lower the energy consumption further. So it's a work in progress.

No doubt along the way I'll continue to study the problem and look for further efficiencies. We've done the really obvious easy stuff (all the appliances are now fairly new/efficient, and we have CFLs most everywhere). I will say in general though, that my attitude is that the path to carbon neutrality cannot lead through asking people to give up massively on their standard of living - it just won't happen - and accordingly, we won't be taking measures like washing the dishes in cold water, having rooms at 50 F, etc. See eg

for my general long-held attitude on this. In particular, I just don't really see having a big solar array as a bad thing - we're creating demand for solar gear, being another visibly solar house in the neighborhood, and, in our small way, helping to transition society to carbon neutrality.

For those that choose to make much more drastic changes in lifestyle to conserve energy - awesome - more power to you - go for it. There's room for more than one approach.

On the EVs - I don't think there's much doubt on my math. EVs get around about 3 miles per kWHr, so 30000 miles requires 10000 kWHr.

Stuart Staniford said...

On the studying the problem part- I'm particularly interested to know of experiences anyone has with the various systems for figuring out which circuits in the house are using how much power?

ColdNorth said...

Tom Murphy has some interesting posts about details on an energy audit, and circuit-level energy consumption.

Nick G said...


I'm sure you've considered air-source heat pumps - seems like they'd be a natural for those rooms farthest from the wood stove.

I'd be very curious about your thoughts.

A thought about those Victorian windows: laminated glass is a very good insulator, and can be added to existing windows. We used 1/2" for stationary windows and 3/8" for moveable, and now we don't need to turn on our heat above freezing: body heat and lighting is enough.

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