How to Calculate and Connect Solar Panels and Inverters for Home Batteries

Steps

1. 1

   Step 1: Understanding electricity basics for solar panels

   Before working with solar panels, you need basic knowledge of electricity. Specifically, you need to understand what a series circuit and a parallel circuit are. Just like batteries, solar panels can be connected in series or in parallel.

2. 2

   Step 2: Buying cheap used solar panels on Marketplace

   People often want modern 450W and 500W panels, but older panels still work 100 percent perfectly. I bought 18 solar panels and a matching 4000W inverter via Marketplace. With some extra cables and a small fuse box adjustment, I built a 3 kW solar set for about 160 euros total.

   This investment pays for itself extremely fast, in about one month during the summer months.

   What I used

   Item	Amount	Notes
   Solar panels	18	Bought used via Marketplace for 100 euro
   Inverter	1	Bought used via Marketplace for 40 euro
   Cables and fuse box parts	some	Cost about 20 euro

3. 3

   Step 3: Reading the solar panel sticker specifications

   The sticker on the back of a solar panel contains very important information. For our calculation example, the panel has an open circuit voltage of 36.9 volts and a current at Pmax of 8.16 amperes.

   Specification	Value
   Open circuit voltage (Voc)	36.9 V
   Current at Pmax (Imp)	8.16 A

   What I used

   Item	Amount	Notes
   Solar panel	1	Reference panel from Marketplace

4. 4

   Step 4: Wiring panels in parallel

   As an example, if we place four panels on the roof and connect them in parallel, the voltage stays the same at 36.9 volts. Because they are parallel, the amperes add up, giving a total of 32.64 amperes. This requires very heavy cables, making this setup inefficient.

   What I used

   Item	Amount	Notes
   Solar panels	4	Connected in parallel for example

5. 5

   Step 5: Wiring panels in series

   In a series connection, the positive of the first panel connects to the negative of the second panel. If we chain six panels together, the voltage adds up to 221.4 volts while the amperage stays at 8.16 amperes. If we chain 12 panels together, the voltage doubles again to 442.8 volts while keeping the same amperes.

   High voltage is not harmful to the cables because solar cables are double insulated and specifically made to handle higher voltages safely.

   Configuration	Total Voltage	Total Amperage
   6 panels in series	221.4 V	8.16 A
   12 panels in series	442.8 V	8.16 A

   Tip

   Solar panel cables are double insulated and designed to handle higher voltages.

   What I used

   Item	Amount	Notes
   Solar panels	6 or 12	Connected in series

6. 6

   Step 6: Series connection example with 12 panels

   If we chain 12 panels together, the voltage adds up to 442.8 volts. Meanwhile, the amperage stays at 8.16 amperes.

   What I used

   Item	Amount	Notes
   Solar panels	12	Connected in series

7. 7

   Step 7: Calculating total solar power capacity

   To calculate the total power of your solar setup, use a simple calculation. I have 12 solar panels with a maximum power of 250 watts each. Multiplying 12 by 250 watts gives 3,000 watts of maximum power.

   What I used

   Item	Amount	Notes
   Solar panels	12	250W max power each

8. 8

   Step 8: Choosing the right inverter

   We must choose an inverter that can handle both the voltages and the total power of our panels. For example, the GoodWe GW2000 has a max string voltage of 500V, an MPPT voltage range of 40 to 450V, and a max input of 12.5A, which are all within specs. However, it can only convert a maximum of 2,000 watts, while our system produces 3,000 watts, making it unsuitable.

   Instead, the GoodWe 3000 has a max input voltage of 500V, an MPPT range of 80 to 450V, a max input current of 18A, and a nominal output of 3,000 watts, making it a proper match.

   Inverter Model	Max String Voltage	MPPT Voltage Range	Max Input Current	Nominal Output
   GoodWe GW2000	500V	40-450V	12.5A	2000W
   GoodWe 3000	500V	80-450V	18A	3000W

   What I used

   Item	Amount	Notes
   GoodWe GW2000 inverter	1	Too small for a 3000W setup
   GoodWe 3000 inverter	1	Matches the 3000W setup

9. 9

   Step 9: Creating a series and parallel combination circuit

   In some situations, making a combined series and parallel circuit is useful. In a small example with three sets of two panels in series connected in parallel, each pair gives 73.8 volts, and the three parallel strings give a total output current of 24.48 amperes. The total wattage remains the same regardless of whether you wire them in series, parallel, or a combination.

   Higher amperage requires thicker cables, while higher voltage allows thinner, more efficient cables.

   What I used

   Item	Amount	Notes
   Solar panels	6	Wired in a series-parallel combination

10. 10

    Step 10: Selecting between AC inverters and MPPT charge controllers

    A standard AC inverter takes about 400 volts from 10 to 12 panels in series and converts it to grid voltage. Alternatively, an MPPT charge controller converts DC solar voltage directly to charge a battery pack instead of the grid. MPPTs generally work with lower voltages, so a high 400V series string will not work with many MPPTs.

    My own setup uses 12 panels on the back roof, 12 on the veranda, and 6 on the house roof all connected to normal AC inverters. For my shed, I use 6 panels connected via an MPPT directly to my battery pack, which is more efficient for smaller strings.

    What I used

    Item	Amount	Notes
    AC inverters	multiple	Used for roof and veranda strings
    MPPT charge controller	1	Used for the shed string

11. 11

    Step 11: Matching panels to an MPPT charge controller

    As an example for an MPPT, the Victron Energy Solar charge controller MPPT 100/30 handles up to 100 volts on the input and 30 amperes of current. Our combination circuit of six panels gives 73.8 volts and 24.48 amperes, which stays safely below the 100V and 30A limits. While this series-parallel setup was not ideal for AC inverters, it fits this lower-voltage MPPT charger perfectly.

    What I used

    Item	Amount	Notes
    Victron Energy Solar charge controller MPPT 100/30	1	Handles up to 100V and 30A

12. 12

    Step 12: Adding disconnect switches and safe wiring

    Some inverters have a large built-in switch on the bottom to disconnect the solar panels. If your inverter lacks this, you must build an extra switch in between to safely disconnect the panels. You must never connect the inverter output directly to a normal wall socket in your house.

    It must connect to a separate fuse in your fuse box to avoid dangerous overloads and fire hazards.

    Warning

    Never connect your inverter output directly to a wall socket; it must go to a separate fuse in your meter box.

    What I used

    Item	Amount	Notes
    Disconnect switch	1	Installed between panels and inverter if needed

13. 13

    Step 13: Connecting MPPT controllers to battery systems

    With an MPPT, solar power enters the PV input and the output connects directly to a battery instead of the grid. You configure the inverter software for your specific battery voltage, such as 12V, 24V, or 48V. The MPPT converts the solar voltage into a charging voltage to fill the battery and stops when it is full.

    If you want to feed power from your battery back to the grid, you need a grid-tied inverter like the Victron MultiPlus.

    What I used

    Item	Amount	Notes
    Victron MultiPlus	1	Used to feed stored battery energy back to the grid

14. 14

    Step 14: Takeaways

    • You can build a budget solar set using cheap second-hand panels and inverters.

    • Series connections add up voltage while parallel connections add up amperage.

    • Higher voltage allows the use of thinner and more efficient cables.

    • Always match your total panel wattage and voltage to your inverter specifications.

    • MPPT charge controllers work well with lower-voltage combination circuits to charge batteries directly.