New guy saying hello!

Re: New guy saying hello!

Welcome to the Forum Jamie,

First answer--For solar PV systems, the excess power is simply not used. Yes, you can use GT inverters but that add cost and complexity. We can ignore that for now. Not really practical for a small system (especially once you add permitting/inspection/utility approval costs.

Regarding your questions... Generally we like to suggest that people learn about their loads--Peak Watts and Watts*Hours of use per day. We then size the system to support those loads.

Another way is to "pick" one element, and design the system around that. Number of panels that will fit in a sunny spot on the roof, amount of batteries that will fit in an RV, etc.

Since you talked about a 4x 6 volt (golf cart I assume) batteries... We can start there using basic rules of thumb that usually get "close enough" for solar work (I will carry out more digits in the math so you can follow my equations and mistakes :roll:. In general, +/- 10% is about as accurate as you can get).

First, the battery bank, usually we recommend around 5% to 13% rate of charge for the battery bank. Too little, it will take forever to recharge the battery bank and possibly sulfate faster (and die). More than 13% rate of charge, can be pretty expensive for solar panels and charge controllers (and much of the sun is wasted as the battery bank quickly fills). For deep cycle fork lift batteries, about 13% (or C/8.) is the maximum continuous charge/discharge current you can do for hours without overheating).

A typical "golf cart" battery is 6 volts @ 220 AH. 4x of them in a 12 volt bank will be two in series/parallel configuration with 12 volts @ 440 AH. The recommended amount of solar array wattage would be:

• 14.5 volts charging * 440 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 414 Watts minimum
• 14.5 volts charging * 440 AH * 1/0.77 panel+controller derating * 0.10 rate of charge = 829 watts "nominal"
• 14.5 volts charging * 440 AH * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,077 watts "max cost effective"

Now, how much power would this produce... First, the battery bank. Normally, you don't want to cycle below 50% for maximum battery life:

• 12 volts * 440 AH * 0.50 max discharge * 0.85 inverter eff = 1,320 Watt*Hours from battery bank (no sun)

And average/maximum surge AC power:

• 12 volts * 440 AH * 0.85 inverter eff * 1/8 rate of discharge = 561 watts max continuous (for ~4 hours of use)
• 12 volts * 440 AH * 0.85 inverter eff * 1/2.5 max surge = 1,795 watts max surge (for starting motors, pumps, etc.)

The recommended size of inverter would be around 500-900 watts of 120 VAC (typically a good inverter will surge about 2x its rated load--1/2*1,800 watt surge = 900 watts).

A really great small True Sine Wave inverter is the MorningStar 300 watt TSW 12 volt inverter with search/remote on-off switching.
And how much power can you get from your solar array for your area. Using PV Watts for Philadelphia Pennsylvania, the hours of sun we see are:

Month Solar Radiation (kWh/m 2/day)
1 3.30
2 4.16
3 4.74
4 5.06
5 5.20
6 5.43
7 5.51
8 5.67
9 5.07
10 4.59
11 3.37
12 2.67
Year 4.57

So, for February you get around 4.16 hours of average sun per day. Using the suggested array numbers we would get (again, February):

• 414 Watts * 0.52 over all system eff * 4.16 hours = 896 WH per day
• 829 watts * 0.52 over all system eff * 4.16 hours = 1,791 WH per day
• 1,077 watts * 0.52 over all system eff * 4.16 hours = 2,328 WH per day

So, how much AC power do you use in a day? I would suggest you get a Kill-a-Watt or similar meter. Very handy to measure the loads of your 120 VAC 15 amp plug-in appliances (and very handy to use around the home for conservation--How much power does the fridge or desktop server use, etc.).

I will stop here. Enough data thrown at you for the moment--Questions?

-Bill