How to Calcluate Requirement

[Anjen]

When getting estimates for a solar installation how can I determine if the calculation from the salesperson is correct? For example, my "yearly" total of usage is 15,590 KW for the 12 months with varying usage each month (1k to 2k kWh range over the period). On the billing it seems that the KWH charges are .1245 @ 1% to 30% and .1689 @ 31% to 100% and .228 when over the 100% baseline. Of course I have no idea what this means or if it is even important in the determination (this information was from one months billing).

One of the bids I had was a system that was 10.7kw but it seems to me if I am using an average of 15.5 per year and I want to increase my usage by 15% then I should be looking for a system that is 17.8kw (current usage plus 15%). Also not taking consideration the panel will not produce 100% through the life of the panel.

Help please.

[sensij]

Check out PVWatts, read the documentation. It will help you understand what kind of production to expect. "Premium" panels and losses of 8-10% are appropriate for most.

[ButchDeal]

When getting estimates for a solar installation how can I determine if the calculation from the salesperson is correct? For example, my "yearly" total of usage is 15,590 KW for the 12 months with varying usage each month (1k to 2k kWh range over the period). On the billing it seems that the KWH charges are .1245 @ 1% to 30% and .1689 @ 31% to 100% and .228 when over the 100% baseline. Of course I have no idea what this means or if it is even important in the determination (this information was from one months billing).

One of the bids I had was a system that was 10.7kw but it seems to me if I am using an average of 15.5 per year and I want to increase my usage by 15% then I should be looking for a system that is 17.8kw (current usage plus 15%). Also not taking consideration the panel will not produce 100% through the life of the panel.

Help please.

first 15.590kWh annual usage is 15.59MWh annual. A 15% increase in consumption does not equate to a 17.8kw system.

a 10.7kW system seems excessivly big for you, about 150% of consumption. 7kw should be pretty close to net zero annual for your usage. 17.8kw is WAY over the top. I am not sure about the arrangement, maybe you have heavy shadows.

If you want to increase your usage by 15% you should have a reason for that as part of the interconnect application. Such as purchase of electric vehicle etc.

[Anjen]

ButchDeal, the reason for the increase (or expected increase) in usage is because I am adding on to my house and making the estimation based upon added square footage. But I still don't understand how to determine that the 15 in yearly usage equates to a 7kw system, how is that calculation figured (feel like I should be back in math class, possibly over complicating this).

As your comments suggest the oversized system is what I don't want to happen to I want to in essence police the salesman myself to make sure I don't over purchase.

[Anjen]

Also I do not have heavy (or much shadowing) at all. We are installing on three roofs, two are East & West facing and one is South Facing

[ButchDeal]

Also I do not have heavy (or much shadowing) at all. We are installing on three roofs, two are East & West facing and one is South Facing

well you do have heavy shadows then. The East is very shadowed when the west is producing.
same for the west.

[ButchDeal]

ButchDeal, the reason for the increase (or expected increase) in usage is because I am adding on to my house and making the estimation based upon added square footage. But I still don't understand how to determine that the 15 in yearly usage equates to a 7kw system, how is that calculation figured (feel like I should be back in math class, possibly over complicating this).

As your comments suggest the oversized system is what I don't want to happen to I want to in essence police the salesman myself to make sure I don't over purchase.

7kw is to meet current load but that is with a south facing. with a mix of east/west/and south it would be a little more depending on the mix up.
15% increase would simply be the size X 1.15.
You had just in your comment set the annual size to equal the annual consumption divided by 1000. That doesn't make since, a 17.8kw system is WAY over sized.

[SunEagle]

ButchDeal, the reason for the increase (or expected increase) in usage is because I am adding on to my house and making the estimation based upon added square footage. But I still don't understand how to determine that the 15 in yearly usage equates to a 7kw system, how is that calculation figured (feel like I should be back in math class, possibly over complicating this).

As your comments suggest the oversized system is what I don't want to happen to I want to in essence police the salesman myself to make sure I don't over purchase.

First off your annual usage is 15,590 k watt hours or 15.59 MWh. And you are confused because 1 watt of solar panel produces more than 1 kWh a year.

Now depending on your location you will get a value of 1800 hours times the wattage . So if you install 10.7kw system it might generate ~ 10.7kW x 1800 hr ~ 19.26MWh which is much more than what you currently use or about 23% more. This will vary from year to year and by location but it should clear up the "math" a little.

Best way to understand is to go to pvwatt.com and enter some basic information like system wattage and your location. That will provide you with an estimated kWh production per month and year.

[ButchDeal]

First off your annual usage is 15,590 watt hours or 15.59 kWh.

Anjen stated before annual was 15,590 KW so pretty sure that is 15,590kWh or 15.59MWh
15.59kWh annual is pretty low.

[J.P.M.]

Anjen:

The salesperson is not correct, or you misunderstood the person, or both. Your solar ignorance is costing you money. That's not a sin, but do yourself a favor and download "Solar Power Your Home or Dummies", a free net download. Knowledge is power. Read it and get some of both. Do so before, or concurrent with, getting familiar w/ PVWatts as suggested in another post. I'd suggest you not buy anything until you complete those tasks.

Then, understand how you are charged for electricity.

Then get more quotes and use your newly acquired knowledge to lower your elec. bill in the most efficient ways possible consistent with your goals and lifestyle choices.

[randomuser]

Anjen, as everyone has said, use PVWatts as a guide. If that's too technical, I suggest getting 5-6 quotes and listen to what the sales people say in their sizing and your requirements. I'm hoping out of those 5-6 quotes, you'll get an idea what you need from what they're telling you.

If you don't understand how to read your electric bill, that's the first step I think. Know what you're paying for today. You can see how much you consumed in the past and how much you paid. The solar companies will give you a report of how much you used in the last year and how much you'll generate in the future with the system size they design. Good luck with your research into solar.

If all this info is too hard to digest, find someone you can trust to help you wade through all this info. This forum provides great assistance but people here expect you to have some knowledge of the terms they're using in their replies to your questions. I think most people here wouldn't recommend the big solar installers in places like Home Depot, Costco, etc but there's a place for companies selling turnkey systems to buyers who don't want to learn about all this stuff.

[SunEagle]

Anjen stated before annual was 15,590 KW so pretty sure that is 15,590kWh or 15.59MWh
15.59kWh annual is pretty low.

Yeah. Sometimes I can't do math without my third cup of coffee. I was also having trouble getting my calculation to work so I was off by 1000. Stupid mistake.

[Anjen]

Okay so after doing more research (my homework from these posts) I have figured out that if I want to 100% offset and my current yearly usage is 15,590 and I want a 15% increase PLUS add for future pool equipment (estimated at 325 per month) then I need 17,928 plus 3900 for pool = 21,828 per year. Which converts to a 10,870 KW system. That will produce 17,893 kWh per year (at 82% usage). If my price is 60k for example, do I divide by the KWH per year or the KW system size to get the per kilowatt price?

[DanKegel]

Are you trying to compare with your utility bill, which is probably denominated in dollars per kWh? Then divide by kWh.

$60K is a lot of money. What's your total electricity bill for one year?

[SunEagle]

Okay so after doing more research (my homework from these posts) I have figured out that if I want to 100% offset and my current yearly usage is 15,590 and I want a 15% increase PLUS add for future pool equipment (estimated at 325 per month) then I need 17,928 plus 3900 for pool = 21,828 per year. Which converts to a 10,870 KW system. That will produce 17,893 kWh per year (at 82% usage). If my price is 60k for example, do I divide by the KWH per year or the KW system size to get the per kilowatt price?

The price per Watt would be $60,000 / 10,870 watts = $5.52 / watt. The is a very high cost for most standard pv systems. Especially when you install one bigger than 10,000 watts.