# String calculations

Solar Expert Posts: 205 ✭✭
what I have derived by using Morningstar String calculator,

the smallest of the Panels give maximum possible wattage in any configuration e.g with Tristar MPPT 45

1. 32x CS5C-90M maximum wattage is 2880 watts.
2. 15x CS6P-255M maximum possible wattage is 2550 watts.

• Solar Expert Posts: 71 ✭✭
Re: String calculations

I am a big fan of the Midnite Solar calculator.

http://www.midnitesolar.com/sizingTool/displaySizing.php
• Banned Posts: 17,615 ✭✭
Re: String calculations

Often "smaller" panels come out ahead on string calculators because you can squeeze a few extra Watts into the string without exceeding maximum Voltage:

140 Watt panel with 17.5 Vmp and 21.5 Voc * 6 = 840 Watt string with Vmp of 105 and Voc of 129
240 Watt panel with 30.5 Vmp and 38.5 Voc * 3 = 720 Watt string with Vmp of 91.5 and Voc of 115.5

If you try to up the latter to four in series you get: 960 Watt string with Vmp of 122 and Voc of 154, which is over most MPPT controllers' 150 Volt max input.

Disadvantages to lots of smaller panels: higher cost per Watt, more connections, more mounting.

Ideal situation: make the bulk of the string with higher Wattage panels then add one on the end of lesser Wattage, lower Voltage but same Imp to bring the power up without going over the input max for the controller. Problem: spending forever hunting through panel specs to find a combination that works.

If only companies sold compatible "half panels". But it wouldn't be profitable for them to do so.