Designing a small 12V solar power system

I often camp at one site for several days and use solar panels to recharge my auxiliary battery. This article describes how to design a small 12 V solar power system with examples.

Wiring diagram for solar power system. “A” is an ammeter and very helpful for monitoring performance.

Wiring diagram for solar power system. “A” is an ammeter and very helpful for monitoring performance.

Daily energy consumption

Electrical power is typically measured in watts (W) and energy in watt-hours or kilowatt-hours (Wh or kWh). Voltage varies over a narrow range and for 12 V systems power (equals voltage times current) is usually measured in amps (A) and energy in amp-hours (Ah).

The first design step is to estimate the typical daily energy consumption. Last year I bought a 12 V fridge-freezer which massively increased my energy consumption. I will only consider the fridge in this example because I have no other 24-hour loads.

Fridge consumption = 2 A (average) × 24 h = 48 Ah.

Simulations for solar panel and battery sizing

There are many variables to consider in sizing a solar power system: solar panel size (A), battery capacity (Ah), the weather and time (days). Simulations are most helpful in considering these factors together.

First, estimate the minimum solar panel size based on daily energy consumption. On a perfectly fine day, there is about 6 hours of maximum solar irradiance, from around 0900h to 1500h.

Minimum solar panel current = 48 Ah ÷ 6 h = 8 A.

Next, assume a larger solar array to allow for some cloudy days. About 1.5 to 2 times the minimum current is a reasonable starting point. For my design, I selected two 120 panels at 6.7 A each. Here are my simulation parameters:

  • Daily consumption = 48 Ah.
  • Charging current in fine weather = 13.4 A = 2 × 6.7 A.
  • Charging current in cloudy weather = 10 per cent of fine weather current.
  • Good charging hours per day = 6 h.
  • Initial charge deficit = 0 (aux battery fully charged).
  • Battery charging efficiency = 85 per cent (flooded battery, or 95 per cent for AGM batteries).

Simulations were calculated in a spreadsheet. Each simulation started with cloudy weather and the frequencies of fine and cloudy days were varied. The spreadsheet calculated the charge deficit at the end of successive 24-hour periods (= the difference between cumulative amp-hours used and cumulative amp-hours charged).

3/4 fine days 1/2 fine days 0 fine days
Day Charge Deficit Charge Deficit Charge Deficit
1 7 -41 7 -41 7 -41
2 68 -21 68 -21 7 -82
3 68 0 7 -62 7 -123
4 68 0 68 -42 7 -165
5 7 -41 7 -83 7 -206
6 68 -21 68 -62 7 -247
7 68 0 7 -104 7 -288
Solar charging simulation example. Daily consumption is 48 Ah. Charging in fine weather = 13.4 A × 6 h × 0.85 = 68 Ah. Solar input in cloudy weather = 68 × 0.10 = 7 Ah. For example, after the first day the charge deficit is -48 + 7 = -41 Ah. If the second day is fine then the charge deficit is -41 – 48 + 68 = -21 Ah. If the fine weather continues, on day 3 the maximum charge deficit will be zero (the battery can not store any surplus).
Solar simulation example including data from the previous table and additional scenarios. Daily consumption = 48 Ah. Charging in fine weather = 68 Ah. Charging in cloudy weather = 7 Ah.

Solar simulation example including data from the previous table and additional scenarios. Daily consumption = 48 Ah. Charging in fine weather = 68 Ah. Charging in cloudy weather = 7 Ah.

Batteries can be sized from the charge deficit results and respecting for the rule that deep-cycle batteries should never be discharged below 20% state of charge:

Required battery capacity = Charge deficit ÷ 0.8 = Charge deficit × 1.25.

For example, if I want to camp for maximum 7 days with ½ fine days the required battery size from the above simulations is 104 ÷ 0.8 = 130 Ah. The simulations can be run again with different panel sizes as required.

Keep in mind that continuous rainy days are just bad luck and any practical solar array will not be able to keep up with consumption. The batteries will be drained and you will have to charge the batteries by other means.

A second consideration for battery sizing is that maximum charging current should not exceed 25 per cent of battery capacity:

Minimum battery capacity = Maximum current ÷ 0.25 = Maximum current × 4.

For example, the minimum battery size is 13.4 × 4 = 54 Ah, which is anyhow a small capacity. The maximum charging current is more relevant for alternator charging in a dual battery system.

Solar charge controller sizing

Solar panel output can exceed rated maximum current by about 25% in very bright sun and cold weather. You will notice this if you have an ammeter in the charging circuit.

For this example, the minimum charge controller rating = 13.4 × 1.25 = 17 A and a 20 A rated controller will be fine.

Then you will have to choose between Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) charge controllers. MPPT is great but too expensive for small, 12 V solar power systems and not necessary in warm, sunny climates.

Do not buy cheap Chinese solar charge controllers. An internet search will find reports and discussions of bargain MPPT controllers that did not perform as claimed. Deep cycle batteries are expensive and incorrect charging will reduce battery life. For small, 12 V solar power systems, I recommend PWM controllers from established, reputable manufacturers.

Solar cable sizing

It is more comfortable to park in the shade and more effective to place the solar panels in direct sunlight at the end of a long cable. In my experience, 10 metres is a useful solar panel cable length.

12 V solar panels typically operate at 15 to 18 V. 12 V batteries require 13 to 15 V to charge. 12 V solar panels can overcome 5% voltage losses in the solar panel cable. The following table can assist cable sizing:

5 A 10 A
15 A  20 A 25 A Max.
2 m 4 4 4 4 4
5 m 4 4 4 6 6
10 m 4 6 6
15 m 4 6
20 m 6
Recommended solar cable sizes assuming solar panel operating at 15 V and maximum 5% voltage losses. Read the cable size in mm2 at the intersection of maximum solar panel current along the top row and cable length on the left column. Voltage losses were calculated for twin-core solar cables: 4 mm2 and 6 mm2 conductor area.

For this example (13.4 A), I selected 15 m of 6 mm2 cable. The voltage drop would be a bit more than 5% with this long cable.

Fat cables are expensive, bulky and difficult to manage. High power systems (>240 W = 20 A at 12 V) with long cables will perform better using high voltage panels (same power, lower current, smaller voltage losses). MPPT controllers can convert high voltage, low current power to low voltage, high current.

For compatibility with a dual battery system, the solar power voltage must follow the vehicle charging system (usually 12 V). Consider yourself lucky if you have a larger vehicle with 24 V electrics and alternator.

Many panels today come with MC4 connectors rather than traditional junction boxes. Solar cables can be purchased with MC4 connectors although they’re not real cheap. For small 12 V solar arrays, low voltage cables may be more economical (e.g. 4 mm2 “landscaping” twin core cable).

Battery cable sizing

The charge controller needs accurate battery voltages for good performance. Voltage drops in the controller to battery cable should be less than 2%. The following tables can be used to size the solar controller to battery cable. I used some spare 6 AWG battery cable and the distance from battery to charge controller was 2 m.

AWG 5 A 10 A 15 A 20 A 25 A Max.
1 m 18 14 12 12 10
2 m 14 12 10 8 8
5 m 10 8 6
10 m 8
American Wire Gauge (AWG) cable sizes for 2% (0.24 V) voltage drop, twin-core cable. Read the cable size at the intersection of maximum solar panel current along the top row and cable length on the left column.

These tables can also be used for sizing power cables. Load current can then be divided by two and entered in the tables because a 4% voltage drop is allowable for most loads. I have about 2 m of 2.5 mm2 cable taking power off the battery.

ISO 5 A 10 A 15 A 20 A 25 A Max.
1 m 1.0 2.5 4 4 6
2 m 2.5 4 6 10 10
5 m 6 10 16
10 m 10
Metric (mm2) cable sizes for 2% (0.24 V) voltage drop, twin-core cable. Read the cable size at the intersection of maximum solar panel current along the top row and cable length on the left column.

For Australia, beware that some cables are confusingly specified by total diameter, which includes insulation! For example, 6 mm auto cable is about 4.58 mm2 and 4 mm auto cable is about 1.85 mm2. To avoid mistakes, always check the conductor area (mm2).

Fuses and connections

For 12 V systems, automotive fuses are the obvious choice. Between the battery and controller I installed a 15 A blade fuse. For the load, I installed a 10 A fuse. Fuses should be rated close to the maximum expected current.

There is no fuse between the panel and charge controller because solar panel short-circuit currents are about the same as maximum power currents. Secondly, the battery to controller cable is fused.

Since I installed a fat cable between the battery and controller, I soldered the smaller gauge blade fuse holder wire on to the fat wire. I recommend soldered connections and terminals for corrosion resistance, especially when solar power systems are planned to operate for many years.

Bigger batteries or larger panels?

For a mobile solar power system, cost, size, weight and safety are all import considerations:

  • Solar panels are cheap today and deep cycle batteries are still expensive.
  • Solar panels last for decades and deep cycle batteries last for only a few years.
  • Solar panels are useless in cloudy weather and battery storage will save the day.
  • Solar panels are bulky for transport, batteries are heavy.
  • Wet batteries produce explosive hydrogen gas during charging.

I recommend investing in the largest solar panels that will fit inside or on top of your vehicle, perhaps two times the minimum required charging current (refer to the simulations above). A generous excess of solar power will prevent deep discharges and extend battery life.

The problem with big panels is finding somewhere to store them for transport. I now store my panels in a padded box, made of thin plywood, in the back of my ute (pick-up). The box now stands vertically although I might also be able to put it on top of my cargo (camping boxes, water containers, jerrycans etc.).

Solar panel box for transport. Made from cheap plywood and scrap pine, with recyled high density foam padding inside.

Solar panel box for transport. Made from cheap plywood and scrap pine, with recycled high-density foam padding inside.

Some more ideas

Hopefully this post gives some confidence in designing a small 12 V solar power system without having to blindly follow rules of thumb and friendly advice. Here are a few rules of thumb, with my comments:

Suggestion Comments
A 100 W solar panel is adequate for a compressor fridge-freezer. False. Ipmax < 8 A will not cover daily power consumption.
MPPT controllers give 20-40% more power. In warm, sunny weather PWM and MPPT performance is similar.
Battery capacity should be 3 times daily consumption. Others have recommended 4 times and then why not 5 times?
Battery capacity in Ah should equal solar panel power in Watts. Again, one size does not fit all.
Cable voltage losses should be less than 0.2 V. Could be impractical for long solar panel cables.

5 Responses to Designing a small 12V solar power system

  1. Anonymous says:

    very good and precise explanation of a 12 volt system, I found it very informative.
    I am in the middle of wiring up my new camper trailer, so i found t interesting.
    Biggest problem I find with information is they all assume one is familair with terminology, and trade talk (Bla Bla) …But in efect they simply make it more complex..

    Thank u for the article…very good…in fact best explanation i have seen or read!

  2. Anonymous says:

    I enjoyed the 12v section very much. I had a little trouble following until I realized you weren’t using American English. I had the same trouble putting together a green house kit from England. Hard living has left my brain a little addled. I appreciate the effort you made.

  3. Anonymous says:

    Excellent, excellent article!

  4. Anonymous says:

    Hi just trying to find out if charging an agm 120amp battery with 2 solar panels at the same time will over charge. waeco 120w and waeco 80w both having own dedicated 10amp regulators.


    • bulumakao says:

      The solar regulator prevents overcharging. The battery will charge faster with larger or multiple panels. Just make sure the voltage and current ratings for the regulator are greater than the solar panel (array) output

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