Panels in series add their voltage and panels in parallel add their current, and the array must be chosen against the battery bank in both volts and amp-hours. A daily load of 190 Wh at 12 V needs about 63.2 Ah of bank if it is drained no more than half and must ride through two days, and about 59.4 W of panel at four hours of full sun with the 1.25 margin. An array too small starves the bank, and a bank too small is cycled deep and dies young.
Part of Solar Panel Construction
Choosing panel strings for the right voltage and sizing the array against the battery bank, the arithmetic behind most solar system failures.
Series Raises Voltage, Parallel Raises Current
Joining panels positive to negative adds voltage and keeps current the same; joining positive to positive and negative to negative keeps voltage and adds current (Series & Parallel Analysis, Cell Voltage). The carrier’s table: 36 cells in series give 18 V for charging a 12 V battery; 72 cells in series give 36 V for 24 V systems; two strings in parallel give the same voltage and twice the current.
The array voltage must clear the battery’s charging ceiling plus losses (Cell Interconnection and Panel Wiring). Higher system voltage also cuts wire loss: for the same 500 W load on the same wire and distance, the voltage-drop article shows 46% loss at 12 V and 2.9% at 48 V (Voltage Drop). So a long run from array to bank favours 24 or 48 V.
The Bank And The Array Must Be Sized Together
A panel makes power only while the sun shines, and the load wants power all day and night. The bank carries the difference. The corpus supplies the pieces:
- Daily energy: list the loads in watts and hours, and sum watt-hours (Energy Storage & Batteries).
- Ampere-hours: Ah = Wh ÷ V.
- Depth of discharge: the energy-storage article applies a 50% rule for lead-acid, so the nominal capacity is double the usable.
- Autonomy: it adds a margin of days with no charging; its example is two days.
- Safety margin: Load Calculation multiplies by 1.25.
- Sun: the same article says to plan on 4–5 peak sun hours a day in temperate climates, meaning hours of full-strength sun (Solar Energy Fundamentals).
- Charge rate: C/10 is a safe maximum (Rechargeable Batteries).
What You Need
- A load list in watts and hours per day
- The system voltage (12, 24 or 48 V) and the battery type
- Panel data: rated watts or measured voltage and current in full sun
- A calculator and paper
Method: Size The System
Step 1: Daily energy E = sum of watts × hours.
Step 2: Bank, usable: E ÷ V amp-hours per day. Nominal = usable × 2 (50% rule) × days of autonomy.
Step 3: Array watts = E × 1.25 ÷ peak sun hours.
Step 4: Check the charge current. Array watts ÷ charging voltage must not exceed C/10 of the bank, or the excess is dumped by the controller.
Step 5: Choose strings for the voltage, then parallel strings for the current.
Worked Example: A 190 Wh Daily Load On A 12 V System
The load is the energy-storage article’s: 190 Wh a day.
- Ampere-hours per day: 190 ÷ 12 = 15.8 Ah.
- At the 50% rule: 15.8 × 2 = 31.6 Ah nominal for one day.
- Two days of autonomy: 31.6 × 2 = 63.2 Ah nominal. The usable half is 31.6 Ah.
- Array energy with the 1.25 margin: 190 × 1.25 = 237.5 Wh/day. At 4 sun hours the array is 237.5 ÷ 4 = 59.4 W; at 5 hours, 237.5 ÷ 5 = 47.5 W.
- Charge current at the ceiling: 59.4 ÷ 14.4 = 4.1 A, against C/10 of 63.2 Ah = 6.3 A. Within the limit.
- The 12 V panel: 36 cells at 0.5 V give 18 V and the carrier’s 0.5 A, 9 W, so 59.4 ÷ 9 = 6.6, about seven panels in parallel (or a single larger string).
Array too small. A 20 W array at 4 sun hours makes 80 Wh a day against a 190 Wh load: a deficit of 110 Wh, or 110 ÷ 12 = 9.2 Ah a day. The usable 31.6 Ah is then gone in 31.6 ÷ 9.2 = 3.4 days, and the bank sulfates thereafter (Desulfation Techniques).
Bank too small. A 20 Ah bank facing 15.8 Ah a day is drained 15.8 ÷ 20 = 79% every day, well past the 50% rule. The rechargeable-batteries article gives lead-acid a life of 200–500 cycles, without stating the depth; at one cycle a day that is 200 ÷ 365 = 0.55 to 500 ÷ 365 = 1.4 years.
Array too big. A 300 W array at 14.4 V pushes 300 ÷ 14.4 = 20.8 A, which is 20.8 ÷ 63.2 = 0.33 C against the safe C/10, so the controller has to shed most of it (Charge Controllers).
Common Mistakes
- Sizing the array for peak watts, not daily energy — power and energy are different (the solar fundamentals article)
- Treating nominal capacity as usable — a lead-acid bank is only half usable
- Ignoring wire loss at 12 V — the voltage-drop article’s 46% case
- Joining parallel strings of different voltage without a diode or fuse on each — general knowledge, not from the other articles: a shaded or weaker string can be driven backwards by the others
- Mixing panels of different current in one series string — the weakest sets the current
Safety
A series string reaches dangerous voltage, and it cannot be switched off
Panels of 18 V in series make 36 V for two, 54 V for three and 108 V for six, in the 50 to 120 V range the lethal-current article calls potentially serious for wet skin, and a panel is live in any light, rain included. Work with the array covered or disconnected at the array, never rely on a battery-side switch, and treat the string as live regardless. A parallel bank can deliver hundreds of amps into a short, so fuse each string and the bank (Fuse Design).
What’s Next
- Learn next: Bypass Diodes — protecting a series string from shade
- Combine with: Charge Controllers — handling the surplus