How many panels do I need?

[Sergeant]

Is there a way I can determine how much power (wattage) I will need to
have to run my home by looking at my current electric bill? I have space on
my roof to put 24 panels, with an output of approximately 1,800 watts, but how am I to know if this will be enough, or if it will be overkill?

[Mike90250]

Does your electric bill have the KWh used for the last year? or just the last month, we need the # to figure out how many panels you need, and your location.

[denisl]

In most cases when going off grid for the first time, there will be scope for reducing your electricity consumption first. So hopefully, using your electricity bill will be overkill.

[Mike90250]

have space on my roof to put 24 panels, with an output of approximately 1,800 watts

Wow, those are only 75W panels. Generally, you can get a much better price on panels over 100W.

There are also "ground mount" racks, and "pole top" mounts too. They dont have to go on the roof.

[pvpbryan]

If you could post the total energy used per month from you electric bill (in kW-hrs) that would be very helpful in determining the system size you need. Generally, most people need much larger systems than 1.8 kW to accommodate their current electricity needs. There is a great deal on Kyocera Panels at the moment and these panels crank out more energy using less space compared to other brands. Only downside is that Kyocera has a 20 year warranty instead of a 25 year warranty.

[Sergeant]

The electric company doesn't list the yearly use, just monthly. I added the last year together from Dec. 08 to Dec. 09 and the total KWH used was 21,483 KWH. I live in Northeast Texas, bordering Arkansas & Louisiana. On the south roof I am planning on putting them, I have unobstructed sunlight for a full 9 hours.

[Sergeant]

How many panels do I need?

Does your electric bill have the KWh used for the last year? or just the last month, we need the # to figure out how many panels you need, and your location.

The electric company doesn't list the yearly use, just monthly. I added the last year together from Dec. 08 to Dec. 09 and the total KWH used was 21,483 KWH. I live in Northeast Texas, bordering Arkansas & Louisiana. On the south roof I am planning on putting them, I have unobstructed sunlight for a full 9 hours.

[Mike90250]

The electric company doesn't list the yearly use, just monthly. I added the last year together from Dec. 08 to Dec. 09 and the total KWH used was 21,483 KWH. I live in Northeast Texas, bordering Arkansas & Louisiana. On the south roof I am planning on putting them, I have unobstructed sunlight for a full 9 hours.

Can you post the KWh for Dec and for your hottest month?

[Sergeant]

Can you post the KWh for Dec and for your hottest month?

My hottest month, I used 1786 and in December I used 2986. Thanks for your help.

[Mike90250]

I have space on my roof to put 24 panels,
with an output of approximately 1,800 watts

My hottest month, I used
1786 and in December I used
2986. Thanks for your help.

1800 x .8 (a 20% loss in power from being heated by the sun) = 1440

1440 x 5 hours = 7,200Wh per day

7.2 KWh x 30 days = 216KWh per month expected harvest, with perfect sun angle


So, installing a Grid Tie (no batteries, just PV and Inverter) system, looks like it could make a big dent in your electric bill, with a yearly average net meter program. If you add batteries, you increase the cost a fair amount, increase maintainaince, and decrease efficiency a bit.

If you can take and input your roof direction and pitch (angle) into the PV watts website, for your area, you will get a much better, climate adjusted (cloud cover) estimate for your area.

http://www.pvwatts.org/

[Mike90250]

I have unobstructed sunlight for a full 9 hours.

The hours before 9am and after 3pm, have little usable energy, so they have to be ignored. In winter, that changes to 10am - 2pm, so with good sun view, 5 hours is a good rule of thumb to start with.

[Sergeant]

1800 x .8 (a 20% loss in power from being heated by the sun) = 1440

1440 x 5 hours = 7,200Wh per day

7.2 KWh x 30 days = 216KWh per month expected harvest, with perfect sun angle


So, installing a Grid Tie (no batteries, just PV and Inverter) system, looks like it could make a big dent in your electric bill, with a yearly average net meter program. If you add batteries, you increase the cost a fair amount, increase maintainaince, and decrease efficiency a bit.

If you can take and input your roof direction and pitch (angle) into the PV watts website, for your area, you will get a much better, climate adjusted (cloud cover) estimate for your area.

http://www.pvwatts.org/

Thanks Mike, This is what I was looking for but I hope I'm understanding it correctly. As a beginner, I was under the impression I had to have batteries...sounds like I don"t. I was also under the impression I have to be off grid, but you referenced a grid tie system????? I live in a very rural area. Should I not be running an off grid system? Some of the research I've done suggests you have to jump through a bunch of hoops to be in a grid tie system. 2nd thing. How would batteries decrease my efficiency? Last thing, your computations reflected a 216kwh production each month. 24 panels would not be near enough since I am using 2986kwh in my peak months, would it??? Am I understanding this wrong?? Thanks.

[Mike90250]

Grid-Tie vs Off-Grid

If you only have a few, spotty outages from your electric company, you really want to go Grid Tie. You need a building permit anyway, and just need Utility to sign off. Most utilities in larger cities have established plans. More Rural areas are sometimes more difficult, but generally, not impossible. You use the electric company as your nighttime battery. If a power failure happens, ride it out with a cheap $500 generator you bought by not buying $5,000 worth of batteries.

If you annually, have regional ice storms that take out power for days/weeks at a time, then it's worth a battery backup/grid tie. Your house keeps running if the neighborhood shuts down. 1 backup generator, 2KW. Costs more, and less efficient, but useful.

If you cannot get power to your property, and have to go off grid (like me) electric will cost you 5x as much as buying it from the electric company. And battery replacement costs every 7 years. And 2 backup generators. And meticulous maintainance to keep it reliable 24/7

I may have mis-read your actual monthly usage. Wow! You need to look into conserving a lot of energy. I think you can see that there is a big difference, and conserving is less expensive than installing a lot of PV. 2,986KWh is a LOT of power.
When your utility switches to a 3 tier plan, where you get billed in higher tiers
.07
.14
.28 c per KWh as you exceed each tier, your solar is shaving off the monthly usage, and bringing you into the lower tiers.

[Sergeant]

In most cases when going off grid for the first time, there will be scope for reducing your electricity consumption first. So hopefully, using your electricity bill will be overkill.

"when going off grid for the first time, there will be scope for reducing your electricity consumption first" Denisl, can you elaborate on this statement please? Can you explain "off grid" and "scope for reducing electricity consumption"? This sounds like something that can help me, if you can please explain...Thanks.

[Sunking]

Sergent first thing is if you have grid power available, you do not even want to consider going off-grid with a battery system. The reason is simple, if you go in with a battery system you will be paying 5 to 30 times more for electricity than you do now buying it from the electric company. There are a few rare cases where going off grid can ever pay off as it takes some very special circumstances like living like a hermit and a recluse.

Now before you even consider going grid tied or off-grid, the very first step is a complete remodel of your home, upgrading all appliances, lighting, and all mechanical systems in addition to life style changes. For every dollar you spend on upgrades will save you 2 dollars on the solar PV install

Upgrades mean replace all windows and doors, add insulated siding, insulation in attic and crawl spaces. Replacing Air Conditioning with highly efficient dual fuel heat pumps. Getting rid of all electric hot water heaters and cooking and use either LP or NG, replace all fixed lighting fixtures with fluorescent and all bulbs with CFL. Finally upgrade your refrigerator.

Once you get all that done first, you may find out you do not need solar PV, and if you do, well you just cut the cost down by 50% or more. The upgrades to your home are equity positive meaning the value of your home goes up more than what you put into it. Solar PV in all states except states like CA and AZ is net negative.