• 1. What's with all these different watts?
  • 2. How much solar does a typical house need to cancel out the electric bill from the utility company?
  • 3. What is net metering? or Your meter runs backwards!
  • 4. What is time-of-use billing?
  • 5. Did you get a battery storage system?
  • 6. How did you choose your generator?
  • 7. What? Your PV doesn't work during a blackout?
  • 8. What about wind power?
  • 9. Why didn't you do anything to conserve energy?
  • 10. Where is all your power going?
  • 11. What's the most efficient way to store excess power?
  • 12. You said there was a $4/watt rebate and 15% tax credit. Where'd it go?
  • 13. But I don't live in California.
  • 14. How long will it take for your PV system to pay for itself? long answer
  • Back to Going Solar page

    1. What's with all these different watts?

    A PV system converts the sun's energy into electric energy in the form of direct current (DC). An inverter converts the DC into alternating current (AC) which is the form in which we get our electricity from the wall socket. At each step of the way, some energy is lost, so you have a different number of watts at each step of the way.

    Solar Radiation is the energy we receive from the sun. It comes in many forms including visible light and heat (infrared). The amount of energy that hits an area, such as your roof varies with time of day, season, and weather. It is, of course, 0 watts per square meter a night. It can reach as high as 1415 W/m2 at noon in San Francsico. You can get data for the US city nearest you at the Renewable Resource Data Center

    Solar panel Manufacturer's Maximum Power ratings are given for Standard Test Conditions (STC), since Solar Radiation varies a lot. Standard Test Conditions are irradiance at 1000 W/m2, cell junction temperature of 25C and reference air mass of 1.5 solar spectral irradiance distribution. (illustrated details)(gory details)

    The CEC uses PTC or "PVUSA Test Conditions". The PTC watt rating for panels is based on 1000 W/m2 solar irradiance, 20C ambient temperature and 1 m/s wind speed. This gives a lower watt rating than the STC used by manufacturers. The CEC says our Sharp 185W panels have a PTC rating of 163.3W

    Solar panels can only convert a portion of the sun's energy to electricity. Our panels are only 14.2% efficient, so if you divide its STC rating of 185W by the area of the panel 1.3m2, you get 142w/m2. Solar panels have been getting more efficient over the years.

    The manufacturer's STC ratings and the PTC ratings are sometimes referred to as DC rating because the panels put out electricity as direct current (DC). Our homes are wired for alternating current (AC), so most of our lights and appliances can not use power directly from the solar panels. It must be converted from DC to AC in a device called an inverter. Some of the energy is lost in the inverter during the conversion. So an inverter has an efficiency rating. Our Sunnyboy 2500U has an efficiency of 94.4%. This means that 94.4% of the DC watts from the panels comes out as usable AC watts.

    State rebates for solar PV systems vary. In California currently it's $4/watt AC PTC, and in New York it is $4/watt DC STC. Our system has 48 Sharp 185W panels, which is 8880 watts DC STC, so if we were in NY we could get a $35,520 rebate. In California we multiply the CEC PTC panel rating of 163.3 watts by 48 panels and the 94.4% inverter efficiency to get 7399 watts AC PTC, which gives us a rebate of $29,596. That's quite a difference.

    The actual generated watts you'll get out of the system is totally different since you do not get 1000W/m2 shining on your solar panels all day and night. Solar Insolation is the daily amount of solar energy hitting the earth typically given in KWH/m2/day. It is also called Sunhours/day. The numbers are usually given for a surface, or a plate that is horizontal. You get higher numbers for a panel facing south (assuming you're in the northern hemisphere) and tilted at an angle equal to your latitude.

    So here, near San Francisco, we're at 37N latitude. Or average annual Solar Insolation for a horizontal plate is 4.7, but for a plate facing south, tilted up 37 degrees, the average annual Solar Insolation is 5.4. So we'd get more energy if we tilt our panels up 37 degrees facing south. We'd get even more energy (7.1) if we put our panels on a rotating mount that tracks the sun, moving to face the sun from sunrise in the east to sunset in the west. We opted not to do this because of the extra cost and complications, and the fact that moving mechanical systems tend to break down and require more maintenance.

    Our roof pitch is 5/12 (which means for every 12 inches in you go, the roof rises 5 inches) or 22 degrees, so the panels on that roof are actually tilted at Latitude - 15 degrees, which gives those panels an average annual solar insolation of 5.3. We have 16 panels facing that way. 16 x 163.6 x 94.4%= 2466 AC rated watts PTC, or 2.466kW. If I multiply this by the solar insolation value of 5.3, it says on average we should get 13KWH each day from those south-facing panels. Multiply this by 365 days, would give us 4745 KWH/year. I get roughly the same number if I enter our infomation into the RREDC Performance Calculator which lets you enter any orientation and tilt, or the Clean Power Estimator which is the easiest to use and gives you a lot of other useful information.

    Your numbers will be lower if you have shade trees or other things that cast a shadow on your solar panels. Your numbers will also vary from day to day depending on weather conditions, temperture, wind, cloud cover, smog.

    2. How much solar does a typical house need to cancel out the electric bill from the utility company?

    A typical house only needs a 2.5kW system. Every situation is different. If your monthly electicity bill is less than $50/mo, solar PV is not considered economically worthwhile. If you live off the grid, then that's another matter. For the do-it-yourselfers Cooperative Community Energy gives an equipment list with price estimates for a 2.4kW system and a 10kW system. I notice their 10kW system price is cheaper than our 7.5kW system. I think it's because they buy the equipment at a coop group discount rate and don't face the same mark up I get through a contractor. I found even better prices at GotSolar? and Taos Green Solar or in San Diego, AAPS. These might cost less because they are less inclusive of equipment than Cooperative Community Energy.

    3. What is net metering? or Your meter runs backwards!

    Net metering means that the electric utility keeps track of how much power it sells to you and how much power you sell to them. You get charged for the net amount. For example, if you use 500KWH of power from the grid in the evenings when your solar panels are not generating any power, and you sell into the grid 300KWH surplus energy that your solar panels generated during the day which you couldn't use immediately, then you get charged by the utility for 500-300 = 200KWHs. Another way to put it, is that you sell your solar power to the utility at the same rate (cents per KWH) as they sell it to you.

    Under California's net metering law, you are billed once a year for this net electric usage. This is because most customers size their solar array to roughly cancel out their annual electric bill. If they were billed monthly, they'd have to send the utility company money each month in the winter and the utility company would have to send them a check each month in the summer, which seems a bit silly. But in our extreme case where we use much more energy than the average household and our solar array can only offset half our annual bill, we will, at the end of 12 months get a bill for roughly $3,000. More details can be found in the actual California Net Metering Law and a summary of it. Some other states have similar laws.

    Actually, you still get a monthly electric bill for the billing component related to the number of days in the billing cycle and not related to the amount of power used. It's $.45733/day on the E-8 schedule (plus surcharges and taxes and legislated reductions). On the E-7 schedule it's called a Meter Charge and is $.12813/day. E-7 also has a Minimum Energy Charge of $.13020/day. I suspect this is also part of the monthly bill.

    The monthly bill lists your net KWH usage for the month and how much credit or debit you have based on that. This is for your information and helps you go budget for when you have to pay the accumulated amount at the end of 12 months. Or, if you choose, you can go ahead and pay the amount owed on a monthly basis rather than waiting for the end of 12 months.

    In 2002 a bill was passed such that any credits remaining at the end of the 12 months is given to the utility. They don't pay you for the excess amount you generated.

    In 2009 The California Solar Surplus Act of 2009 reversed that and said the utilities will have to pay the homeowner for the excess power they generated net over the 12 months. The CPUC will have to set the rate for that payment.

    4. What is time-of-use billing?

    Power companies often have rate schedules that charge more for electricity when usage is high and less when usage is low. This is to encourage people to shift their electric usage away from peak-usage times. This enables the utilities to make more efficient use of their power plants and can help them to keep from having to build new plants or to fire up the older more polluting plants to meet demand. Before we went solar, and for the first year on solar, we were on an E-8 seasonal schedule where we get charged 5 cents more per KWH in the summer than in the winter. For a few years, this rate schedule was no longer available to those who were not already on it.

    People with solar panels often prefer the E-7 time-of-use schedule where they get charged 9 cents per KWH during weekdays from 6pm to noon and on weekends, 12 cents/KWH noon to 6pm on Mondays through Fridays in the winter, and 31.5 cents/KWH noon-6pm Mon-Fri in the summer. For solar generators, this means they can buy power at night at 9 cents and sell excess power during the day for up to 31.5 cents. This way, you don't need as many panels to eliminate your power bill.

    The E-7 schedule is no longer available to those who are not already on it. Instead there is an E-6 time-of-use schedule with peak hours and partial-peak hours

    When the California Solar Initiative was passed in 2006, there was a requirement that incentive recipients be billed using Time-of-Use (TOU) rates. This requirement was removed in June 2007 with the implication that it would be reinstated at a later date. The problem with the TOU requirement was that in places that require a lot of air conditioning in the summer, such as the Central Valley, one could end up paying a larger power bill with a PV system that generates fewer than 50% of the KWH the home or business needs, than with no PV system.

    5. Did you get a battery storage system?

    No. We thought about it, but since there is a net metering law in California, it didn't seem necessary. We could sell to PG&E during the day, and buy energy from them at night. House battery systems cost money and need to be replaced every 5 to 10 years. Sun Power's quotation to include two battery banks of four 225Ah batteries would have added $12K to the project. Also, batteries are not 100% efficient, so you get less out of them than you put in. Even worse, they self-discharge at a rate of 6% a week.

    You'd need batteries if you're going to be off-grid. For extended blackouts, we thought a generator running off our natural gas line would be more reliable, particularly since we will not be generating enough PV power to meet our total electric needs.

    More on batteries at Home Power. It recommends sizing the battery pack with enough capacity to last between 4 to 21 days.

    6. How did you choose your generator?

    We wanted a generator for blackouts to keep our computer network running. So we needed a system that would automatically switch on even if we weren't home. This is sometimes called "autostart". We live in a house with natural gas, so that was the most logical fuel to use. We wouldn't have to worry about refueling during extended outages. It is good to choose a generator that has a rated load of 2x what you will need. We don't know exactly how much we need, peak usage, so we decided we needed a system at least 3x our average load of about 3kW. So we wanted a generator that was rated at least 9kW. We'll put the whole house on it except the A/C and the future hot tub.

    We thought we'd go with Generac since it is a well-known trusted brand. The Generac Guardian line is meant for residential backup power. It has the automatic transfer and weekly self test and can run off of propane or natural gas. It also comes in an enclosure to reduce noise. We decided on the Generac Guardian 04456 which is rated at 12kW. It's even available at the local Home Depot.

    When the contractor started working on this, they found that our house has gas coming in at a pressure of 7 column inches of water, while the Generac requires 11 inches. The utility said it is unable to increase the pressure to our house. So if we want to stick with natural gas instead of switching to propane, we need a generator that can operate at 7 inches pressure. So far I've found the Kohler 8RMY and 11RMY generators do.

    More on choosing generators at Home Power

    Update 8/22/2005: We had a 5-6 hour planned power outage two weeks ago which made us look at generators again. There appears to be new models including the Guardian QuietSource Home Standby - 11 KW Model #04916. Which claims "Sound level ratings are lower than any local municipality's definition of noise pollution. In fact, when standing in its proximity, you can carry on a normal conversation". Though 62.2 dB(A) at 23 ft doesn't impress me. According the the specifications, when it runs on natural gas, it requires only 5 to 7 inches of pressure. Looking at the Model 04456 we originally wanted, the specifications now say that it can run on 5 to 7" of pressure. This appears to be new since June 2004.

    7. What? Your PV doesn't work during a blackout?

    The solar panels do continue generating electricity even during a blackout, as long as the sun is shining on them. However, our utility company, and probably all grids in the US require that when there is no line power, all intertie systems fully shutdown. This means during a blackout, our inverters keep the solar panels from feeding power to our house circuit and so our lights would be off.

    This is a safety issue. The utility doesn't want our house feeding power to the grid endangering workers doing repair work.

    Solving this is not as easy as just putting a shut-off switch between the grid and our house/solar system. The panels generate a constantly varying amount of energy depending on sunshine, clouds passing over, wind changes. They almost never generate exactly the amount of power we need in the house. Having too much or too little power would damage equipment.

    Getting around this would involves batteries, which still might not be allowed on an intertie system. But even if it were, it's still more expense and hassle than getting a backup generator.

    8. What about wind power?

    We don't get much wind at our house, and it's unlikely one can get a permit to put up a tall wind turbine on a plot smaller than one acre. HomePower has an article on wind site analysis to help you determine if your site is a good place for wind power generation. As they say in the article, "Vegetables don't lie!" So pay attention to what your evergreens are saying.

    Or check out the American Wind Energy Association. Also Small Wind Electric Systems: A U.S. Consumer's Guide

    Capstone is working on a propless wind turbine to put on tall buildings. Maybe it will be able to avoid the one acre rule. Maybe not. Still, our house doesn't count as a tall building.

    9. Why didn't you do anything to conserve energy?

    Well we did. From the day we moved into the house, every time a light bulb burnt out, we replaced it, if we could, with a compact fluorescent bulb, with the exception of one bulb in the kitchen and one in over the bathroom mirror, to retain good color rendition. We keep our programmable thermostat at 85F in the summer and 62F during the day and 55F at night in the winter. Since weather is so nice, here, that means the heater and A/C almost never turn on except on the hottest days and coldest nights, so putting in insulation doesn't seem worth much. We use fans in the windows on summer nights to bring cool air in. We have an evaporative cooler next to the computer rack. We changed our computer screens from CRT to LCD flat panels. We bought an energy efficient refrigerator and got a Equator EZ 3600 CEE washer/dryer that claims to use only $16/year to operate. We replaced our shower head with a low-flow model. We raised the thermostat on the wine cellar a couple of degrees which made it stop cycling on and off all night in the winter. We're also planning to get as our next car the hybrid Toyota Prius, a Partial Zero Emission Vehicle (PZEV).

    One thing we haven't done, which we should, is eliminate phantom loads in our house.

    Update: Now the kitchen light bulbs are also compact fluorescent as well as many of the outdoor lights. CFLs have gotten better over the years with handling the colder temperatures outdoors. Instead of fans in the windows, in 2005 we installed a set of Quiet Cool QC-1500 whole house fans which we use instead of the air conditioner whenever it is cooler outside. They are more effective than the window fans, so they can keep it cool enough indoors during the day that we don't have to turn on the AC as often. In 2009 we replaced our incandescent christmas lights with energy efficient LED christmas lights. Philips now has some nice sets, though we didn't get those. In 2010 we got an LG WM3987HW which is even more energy efficient than our old clothes washer/dryer. We added a 2.5 GPM low-flow aerator to our kitchen faucet.

    If and when the air conditioner finally fails, we are considering getting a Coolerado M30 with an energy efficiency ratio (EER) of 40. (Currently ACs more typically are EER 14.) It would use the same amount of power as our whole house fans, but works even when it's hotter outside than inside. But the unit is still larger than our house needs. I'm hoping that by waiting until our air conditioner fails, maybe a smaller more efficient model will come out in a few years.

    When our water heater dies, we are considering getting a Rheem 95 tankless water heater. It is much more efficient than our current hot water heater. The reason why I like this model is because the minimum flow rate is 0.26 gallons per minute, while many other models have a 0.75 GPM minimum. Considering our low-flow faucets that we often don't run at full blast, having a low minimum flow rate is important. I wouldn't want to waste water just to be able to get any hot water. I suspect the Rheem 95 can handle both low and high flow rates because it has multiple burners that can be turned on and off individually.

    In 2010 I called in a contractor to give us a quotation on a solar hot water (SHW) heater, but the prime space on our roof is already covered in photovoltaic panels. For whatever reason, the structural engineers didn't want to put the SHW tank on the SW facing roof over where our current hot water tank is. They wanted to either put it on the front roof of our garage where they'd have to add a 30 foot pipe to get the water to the back up heater, or put it next to our PV panels shading some of them for half of the day. Either way, it was going to cost over $8000 before rebates and over $4,000 after the 30% federal investment tax credit (ITC) and the California solar hot water rebate. Considering our PV panels are in series, where shading a few cells of one panel can reduce the output of all the panels on the string, I did not want a SHW panel shading our prime PV panels at all. We saved $4800 on our electricity bill last year while SHW could save us about $100 per year. I wouldn't want to save $100 per year on a natural gas bill if the shading cost us more than $100 per year in electric bill savings.

    With the new California Air Resources Board Zero Emissions Vehicle mandate (CARB ZEV mandate) going into effect in 2012, there will be more choices of energy efficient cars available. I keep hoping for a plug-in Prius, but that seems unlikely at the moment. Also the more promising compact hybrids being planned are not scheduled to be available in the US. So far the straight electric vehicles seem to be very expensive.

    10. Where is all your power going?

    How do we use so much more power than the average household? Over half of it is going to computers. I know this because for the first month we lived here, our internet connection was not set up. Once it was connected, our power bill practically doubled. Then there's the wine cellar which is a refrigerator of sorts, the regular refrigator, the coke fridge, the A/V system, and all the phantom loads associated with all the other electronic equipment in the house. As we have gas heat, gas hot water and gas range/oven, those are not adding to our electric bill.

    11. What is the most efficient way to store excess power?

    Q. We now generate a fair amount of excess electricity during the day. What is the most efficient way of storing power generated by electricity when you wish to draw it back out as electricity later in the day or the week when you need it?

    A. The best method will depend on how much you want to store, how rapidly (i.e. at what rate of power delivery) you want it back, and how much you care about size, weight, lifetime, standby loss, reliability, and cost.

    For bulk storage, at utility scale, compressed-air storage in solution-mined underground caverns can cost less than $0.01/kWh and and is typically the cheapest option, followed by hydraulic pumped storage ("things that go pump in the night") if you have the right topography. For small-scale storage, chemical batteries are much more expensive; in some circumstances, you may do better with a flow battery (a sort of cross between a battery and a fuel cell, storing chemical energy in tanks) or an electrolyzer/fuel cell/hydrogen tank (or combining the first two into a reversible fuel cell). If you want very high power out, you may like a superflywheel, a conventional low-speed iron flywheel with a clutch, or an ultracapacitor. There are also some other methods...but you don't provide enough information to give a helpful answer. In gneeral, if you can avoid storing electricity, e.g. by storing heat or changing usage patterns or generation technologies, it'll be easier and cheaper than storing kWh. --A.B. Lovins

    More info at http://www.renewableenergyaccess.com/rea/news/qa/story?id=45893

    12. You said there was a $4/watt rebate and 15% credit. Where did it go?

    In California, the $4.50/watt rebate went down to $4/watt on January 1, 2003, and drops another $.20/watt every 6 months. They cut the rebate an additional $.40/watt on January 1, 2004. As of January 1, 2005, the rebate is only $2.80/watt and not $3.20/watt. The rebate will continue to drop $.20/watt every 6 months. I suspect because demand for the rebates has exceeded the funding, they are trying to give more people smaller rebates with the same total annual funding. The 15% credit drops down to 7.5% in 2004 and goes away completely after 2006.

    In 2006-2008 there is a Federal tax credit of 30% of the system cost up to a maximum $2000 credit. The $2,000 limit for homeowners to the 30% Federal Residential Renewable Energy Tax Credit has been lifted as of January 1, 2009.

    These laws change fairly often, so even this FAQ may not be up to date when you read it.

    13. But I don't live in California.

    It's true, the solar cells generate more power in sunny places, but they also work more efficiently at cool temperatures, so don't immediately rule out colder climates.

    It's also true that solar panels make more economic sense where electricity prices are high and there are rebate incentives. Most states don't have great rebate programs, but New Jersey's Renewable Energy Incentives are quite nice with a sales tax exemption on the equipment and a $5.50/watt rebate on the equipment and installation. New York's Incentives aren't bad either, if you were planning to have a professional install your solar system. There's a $4-$5/watt incentive and a 25% personal tax credit on the equipment up to $3,750, a 15 year property tax exemption on the solar system. PV power generators in both states, as well as in almost all the states except California are eligible for the Mainstay Energy Rewards Program - Green Tag Purchase Program that pays 2-5 cents/KWH of electricity generated,even if you use all the power yourself.

    For incentives for other states, go to http://www.dsireusa.org and click on your state.

    The Clean Power Estimators for California, New Jersey, Florida, Hawaii, New York, Long Island and other states are very helpful. They will estimate how much electricity you can generate where you live, and the costs vs savings.

    How long will it take for your PV system to pay for itself?

    Currently I estimate it will take 10 to 15 years, depending on how you want to count things. I created a separate page explaining how I reached those numbers and what assumptions I used. Some of the assumptions are wrong, but are used to make the calculations bearable.

    Carolyn Luce