# how do you size a system?

#### andyselec@aol.com

##### Member
I would like to install a 150 watt LED lighting sytem to operate 8 hrs a day figure the most would be 3 day without sun (Raleigh, NC).
So is it as simple as 150w X 24 hrs= 3600 watts/ 12v= 300amp Batterie So what size panel would I need to recharge it? Or am I wrong I my original calculations. FYI Electrician 30+ yrs just trying to understand solar.

Thanks
Andy the Electrician

#### BillK-AZ

##### Senior Member
Twenty years ago, when the PV business was mostly standalone, I developed and sold a PV sizing program (PVCAD).

I ran the program for your description and it calculates a need for at least 682 watts of PV modules installed at a 60? tilt facing South and 600 AH of battery at 12 volts (3.7 days at 80% depth of discharge and winter temperature) but may not ride out the worst ever sequence of poor weather. If the load was a communication system that had to be guaranteed to always work, double the battery. With 600 AH of battery and assuming that the LEDs operate at night, December would result in an average daily depth of discharge of 22% that has to be considered in selecting a battery and its expected service life.

If your LEDs can operate from 24 volts, this would be more practical considering charge regulator and wiring costs.

#### ggunn

##### PE (Electrical), NABCEP certified
I would like to install a 150 watt LED lighting sytem to operate 8 hrs a day figure the most would be 3 day without sun (Raleigh, NC).
So is it as simple as 150w X 24 hrs= 3600 watts/ 12v= 300amp Batterie So what size panel would I need to recharge it? Or am I wrong I my original calculations. FYI Electrician 30+ yrs just trying to understand solar.

Thanks
Andy the Electrician
Your usage is 150 Watts times the number of hours you want to run the lights in Watt-hours. That number divided by your battery voltage is your raw Amp hours of battery. Multiply that times three for three days of autonomy and multiply it times two for 50% maximum depth of discharge (for daily use you never want to discharge your batteries below 50%). Battery efficiency depends on the type of battery, and you didn't say whether the LED's are AC or DC, so there may be inverter efficiency to consider as well. Those will add to the amount of battery capacity you will need.

#### jaggedben

##### Senior Member
If your LEDs can operate from 24 volts, this would be more practical considering charge regulator and wiring costs.
Perhaps a very stupid question, but here goes, at the risk of thread hijack...

Is it not possible to set up a 24 volt battery system with (sets of) two 12 volt batteries in series, and then run a 'neutral' from the middle of the two batteries to get two sources of 12V for the LEDs? Would an approach like this be 'practical' if his LEDs can't run off 24V?

#### electrimate

##### Member
To unhighjack this thread... Once you figure out how much battery power you need, you need to figure out how much power you need in order to keep them charged with your solar system. Then you can size the solar system. For a rough estimate of what the solar system will provide, take the watts of the system times 77% times 5 hours of sunlight per day.

#### ggunn

##### PE (Electrical), NABCEP certified
Perhaps a very stupid question, but here goes, at the risk of thread hijack...

Is it not possible to set up a 24 volt battery system with (sets of) two 12 volt batteries in series, and then run a 'neutral' from the middle of the two batteries to get two sources of 12V for the LEDs? Would an approach like this be 'practical' if his LEDs can't run off 24V?
You don't want to do this. No matter how you try to balance the loads, the batteries will discharge at different rates which will detrimentally affect the charging cycles.

#### tallgirl

##### Senior Member
You don't want to do this. No matter how you try to balance the loads, the batteries will discharge at different rates which will detrimentally affect the charging cycles.
It will detrimentally affect the life of the batteries, as well. The better solution would be a switching DC-to-DC converter to run the LEDs at whatever voltage they want. Conversion efficiencies are plenty high enough that it won't cause problems with the additional (conversion) load.

And where do you live in Austin?!?

#### ggunn

##### PE (Electrical), NABCEP certified
And where do you live in Austin?!?
Southeast, near the AMD/Spansion site.

#### tallgirl

##### Senior Member
Southeast, near the AMD/Spansion site.
Cool. I'm up north, near P-ville. I work near Burnet and US-183. If you look around there on Google Maps these days you can probably find my office. It's a building with solar panels on the roof I'll post piccies the next time I'm on the roof.

#### ggunn

##### PE (Electrical), NABCEP certified
Cool. I'm up north, near P-ville. I work near Burnet and US-183. If you look around there on Google Maps these days you can probably find my office. It's a building with solar panels on the roof I'll post piccies the next time I'm on the roof.
At work we are practically neighbors. Our office is at Cameron Road and Rundberg.