# Is this correct?

Options
Solar Expert Posts: 201 ✭✭✭✭✭
I decided to give myself a rough idea of what I'd be looking at for an investment in solar power. I used my worst case scenario power bill, 1815KWH, rounded to 2000KWH for the month of September last year. This was right before I added a foot of attic insulation too. This is also with all my tired old pre-Energy Star appliances, which are getting close to replacement time.

Using a worksheet I found online, also using Mitsubishi 180V panels and guesstimating 2 hours of sun available (insolation page for my site shows 3 to 3.5 hours, worst case in the winter)

Here's the worksheet I used:

1. Enter your daily amp-hour requirement (From the Load Sizing Worksheet, line 4)
AH/Day_66___

2. Enter the sun-hours per day for your area. Refer to chart.
H/Day__2___

3. Divide line 1 by line 2. This is the total amperage required from your solar array.
___16.5_______

4. Enter the peak amperage of the solar module you have selected.

Peak A__7__

5. Divide line 3 by line 4. This is the number of solar modules needed in parallel.
____5______

6. Select the required modules in series from the following chart.
_____2_____

Battery Bank Voltage

Number of Modules in Series
12V 1
24V 2
48V 4

7. Multiply line 5 by line 6 to find the total number of modules needed in your array.
Total___10____

8. Enter the nominal power rating (in watts) of the module you have chosen.
W___170______

9. Multiply line 7 by line 8. This is the nominal power output of your system.
W__1700_______

=======================================================

Now if I understand this correctly, and I'm looking for a math check here, does this mean that 10 180W solar panels (shows 170 due to the spec sheet lists 170W as the lowest nominal value) would be enough to run my existing house, as is, with no changes?

Now I do still intend to cut my used power by quite a bit, seeing as I do need all new appliances, currently live in a poorly insulated house (interior perimeter walls are warm in summer, cold in winter) and will be unloading two teenagers in the next few years, but seriously. Just 10 panels? Or is this worksheet way off? Because if this is correct I could almost have a 100 percent electric house, including heating with a heat pump, with 10 panels. Thanks.

• Solar Expert Posts: 639 ✭✭✭✭
Options
Re: Is this correct?

Sorry you seem to be off a bit, your 2000kwh per months turns in to 66kwh per day, not amphr/day. 66kwh or 66,000 wh on a 48v bank is "66,000/48" = 1375 ah/day so that might change your numbers a bit...
3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
• Super Moderators, Administrators Posts: 33,477 admin
Options
Re: Is this correct?

Telco,

Where I think you went wrong is the 66 amp/hours per day... We need to know the voltage, but assuming even that this is 48 VDC, you get:

66AH*48V*(1/1,000wperkW)*30days/month= 95kWhrs per month...

If you want 2,000kWhrs/month:

2,000/95 * 10 modules (per 95kW/month)=210 panels (assuming the rest of your calculations are OK)

From the Government website, using Tulsa, 4kW Grid Tied system with defaults for September (499 kWhrs per month):

4kW * 2,000/499 = 16kW worth of panels (~\$160,000 Grid Tied system retail installed)

If you want this to be an off-grid system, you will need at least 40% more solar panels than Grid Tied because of battery/inverter/charge controller inefficiencies).

Your current cost for electricity is ~\$0.077 per kWhr.

Cost for Grid Tied power (based on your September numbers), 20 years, no interest charges, assume you have net metering available, and no rebates, no tax breaks:

\$160,000 / ((2,000/499 scaling from 4kW to 16kW) 5,354 kWhrs/year * 25 years) = \$0.37 per kWhr for Solar Electric Power on your site...

You are not going to save money in the near term... Go down your road to save power first... Massive conservation, insulation, energy star heating/cooling/ground sourced heat pump/Solar Thermal, etc. first...

Obviously, the above numbers are probably +/-20% and don't account for rebates and tax credits, and don't account for the cost of money--but it is a start.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 114 ✭✭✭✭
Options
Re: Is this correct?

2000kWh per onth is a LOT. That's over 4 times what I'm consuming. Spend some serious effort conserving first. Get a T.E.D. and look at your energy pigs...
• Solar Expert Posts: 639 ✭✭✭✭
Options
Re: Is this correct?

Another quick and dirty way to figure out your array size is if you know your kwh consumed per day you need, then divided by your sun hours in solar panel watts. So in your case of 66kw per day and assuming 2 hours of sun you need a 33kw array or 33,000 watt array, that’s pretty large...

We also used about 1/4 that in our last house.
3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
• Solar Expert Posts: 201 ✭✭✭✭✭
Options
Re: Is this correct?

It does indeed make a difference. Thanks for the correction. It did seem like it was skewed mightily. BB, the voltage for the 66 amp hours was at 120V, this was derived by dividing my monthly power company bill, rounded up to 2000KWH, divided by 30 days.

1. Enter your daily amp-hour requirement (From the Load Sizing Worksheet, line 4)
AH/Day_550___

2. Enter the sun-hours per day for your area. Refer to chart.
H/Day__2___

3. Divide line 1 by line 2. This is the total amperage required from your solar array.
___275_______

4. Enter the peak amperage of the solar module you have selected.

Peak A__7__

5. Divide line 3 by line 4. This is the number of solar modules needed in parallel.
____40______

6. Select the required modules in series from the following chart.
_____2_____

Battery Bank Voltage

Number of Mudules in Series
12V 1
24V 2
48V 4

7. Multiply line 5 by line 6 to find the total number of modules needed in your array.
Total___80____

8. Enter the nominal power rating (in watts) of the module you have chosen.
W___170______

9. Multiply line 7 by line 8. This is the nominal power output of your system.
W__13600_______

Youch. 80 panels, and the batteries needed to hold the power, and this doesn't include the inefficiencies pointed out by BB. Suddenly not so cheap!!! :D;) Thanks for the correction.

Autoxsteve - Yes, I know. Way more power than I want to be using, but that's also the worst of it. For the last year I used 12194KWH, or an average of about 1016 KWH per month. This is the difference between air conditioning being on and being off, when the AC's not being run my house uses 700 - 800 KWH/month. The HVAC is a contractor's special, ie cheapest they could get away with installing. I'm hoping it'll cost less to cool this summer, as I had another foot of insulation blown into the roof last fall. Could be a helluva lot worse though, our neighbors turn on their HVAC in April and don't shut it off until November, have many lights on all hours of the day and night, run little reflection waterfalls and stuff outside their homes, put on a Christmas display that can be seen from Mars (my house is conspicuously dark around Christmas), and humongous plasma screens can be seen from the outside, sometimes on with nobody watching them. I imagine their weekly power usage is more than my monthly.

Really looking forward to getting out of this dump and into something more efficient. In the meantime, thinking that a new Energy Star fridge might be just the thing to blow Uncle Sam's deficit check on.
• Super Moderators, Administrators Posts: 33,477 admin
Options
Re: Is this correct?

Do you have ye' ol' kill-a-watt meter yet? Pay attention to what is old and using the most power first... Fridge/heater/AC/single pane windows/bad-no insulation/stuff attic space/etc...

Learn to love your old house... Mine is certainly not the best on the block--but I guarantee that it uses the least amount of electricity and is draft free.

Less house means that I had more "free cash" to spend on energy reduction. At your usage levels in California, you would be paying over \$0.30 per kWhr for anything higher than ~1,000 kWhrs per month.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 114 ✭✭✭✭
Options
Re: Is this correct?

I understand.

One thing that I had mentioned that you might not have picked up on was T.E.D. It is a device that monitors your entire home's electrical consumption. By installing that device (part of it goes in your electrical service panel and the other part is placed near a pc - it has a usb cable so the pc can collect, store and present usage info to you!) I have increased my entire families electrical usage awareness! You also can see where your electrical energy hogs are. The device costs about \$210.00 with the software and will ensure you know where the 'high nails' are.

KIill-a-watt meter is similar in approach but does one 110vac item (or items) at a time.
• Super Moderators, Administrators Posts: 33,477 admin
Options
Re: Is this correct?

One issue with the TED's is that they don't play well with RE Generation systems... A TED can't tell which direction the power is flowing so for those with Grid Tied systems that "turn the meter backwards" the current TEDs will not work for us (TEDs can't turn backwards)...

A new one is supposed to be coming out "soon"... But it has been several years that they have been promising the new product...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 114 ✭✭✭✭
Options
Re: Is this correct?

agreed. B ut, this gent is not generating power yet. My point was he needs to spend a considerable amount of time identifying his high energy devices and reducing his consumption prior to diving into a solar system....

If we get more TED's sold then we can increase the pressure to get them to change them so they do read reverse!
• Solar Expert Posts: 201 ✭✭✭✭✭
Options
Re: Is this correct?

I just ordered the Killawatt, been meaning to for a while, so now I'm a NAWS customer, not just a board abuser!!! Heh heh... And, it'll be the fridge that goes first as it has other issues, not just it isn't an Energy Star model.

On the TED, looks like a nice little toy but I'm going to pass on that one. Really I already know what my big loads are and how much they suck by checking the power meter before and after turning them on.

Wish I could love the house, but there's just too much wrong with the place, not all of it due to the house itself. I could go into it all, but I'd just be whining. The short list is lack of insulation (ceiling only had 6 inches of blown fiberglass, for example), CATHEDRAL CEILINGS (geez, never again!), the HOA Nazis, the neighbor's houses being only 10 feet away, ect.
• Super Moderators, Administrators Posts: 33,477 admin
Options
Re: Is this correct?

I have no problem with the TED and was very interested in purchasing one myself a few years ago.... Until I realized that once I put in Grid Tied power, the TED would become pretty close to useless in my home (because of where the grid tie inverter power is tied in).

One of the advantages I had when going to grid tied/net metered was that my new meter is full electronic and it displays the current kW usage (or generation). So, I can do a quick check of the home every once in a while to make sure that I don't have any unexplained loads (like lights left, sump pump stuck, etc.) and that my solar grid tie inverter is pumping out power in the middle of the day.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 29
Options
Re: Is this correct?

BB,
The website link is useful. I wasn't aware that off grid was that inefficient. I thought inverters were greater than 95%. Are batteries the weakest link?

Telco,
Say your bill uses 500 KWH's per month. Divide by 30 for KWH's per day. Divide by 24 for your average hourly usage. More interesting would be guessing the average during high usage periods. This would probably be in the morning when heating the house and evening when heating again and cooking.
If work and travel take 12 hrs, then you might be able to try dividing the daily by 8 for these hours. An a more accurate number would be determined by checking your meter before and after breakfast and dinner.
• Solar Expert Posts: 29
Options
Re: Is this correct?

BB,
I haven't had time to use it much yet, but this is an alternative to the Kill A Watt. The display seemed a little easier to use for me, but the plug is on the front.

http://www.smarthome.com/1139.html

Dale
• Solar Expert Posts: 10,300 ✭✭✭✭
Options
Re: Is this correct?

i think the kill-a-watt does more for the money and it too has the receptical on the front. what ever floats your boat though as it's the results you see that help you save money that count.
• Solar Expert Posts: 201 ✭✭✭✭✭
Options
Re: Is this correct?

Since the Kill-a-watt seems to be the de-facto standard (I see it mentioned almost everywhere power savings are discussed) this is what I went with. Thanks folks, as soon as it arrives it'll be time to start identifying and eliminating the power sucking crap. :cool: