My solar inverter installation

A solar power installation costing less than ₹30,000.

When online classes started because of the lockdown, I had to quickly install a solar inverter. I thought I had done enough research and made several mistakes. I bought a Luminous 750 solar inverter. It can handle only 320 watts of AC loads. I also bought an ordinary inverter battery because solar batteries were expensive and not easily available. I thought I could run two fans for an entire night with a 100-AH lead-acid battery. It was too late when I realised that a 100-AH battery can only be discharged up to 50 AH.

I bought the solar panels (₹13,000) and inverter (₹4000) online at really cheap rates. The battery was ₹10,000. The solar cables, DC SPD and other materials brought the total to around ₹30,000.

I have 375 watts of solar panels. They charge the battery in about two hours. For the rest of the day, the panel output is an excess and would have been wasted as heat by the inverter. I run some AC loads on the inverter during this time and this (solar) power is free. (The AC input cord of the inverter is plugged in to an extension box that is in turn plugged in to to a wall socket. It is not switched on. It is used only to provide the mains earth connection for the inverter and its AC output.) At night, I would consume battery power for one hour around dinner time. After this, I would switch off the inverter for the night. The inverter battery is a tubular battery (used for ordinary inverters) but I do not deep-discharge it as it were a solar tubular battery.

I installed three-pin sockets in several rooms and connected them with dedicated cable conduits from the AC output of the inverter. When I wanted to switch off the solar inverter (because of the limited load and time limits), I had to manually unplug the devices from the new inverter-powered sockets and plug them to old mains-powered sockets. This was a hassle I wanted to avoid.

Belatedly, I added a 1000-watt inverter with a built-in 100-AH lithium battery to my solar power installation. I bought the lithium-battery-based inverter form Stalwart Solar for ₹25,000. This is a one-box system that includes the inverter, charger, UPS and lithium battery. It does not have a solar charger input. I charge it using the solar inverter. (If I had known better, I would have bought Stalwart Solar's 1000-watt solar inverter with built-in 100-AH lithium battery for only ₹30,000 and reduced my system size.) Now, I am able to use all 100 AHs of the lithium battery instead of only 50 AHs of the lead-acid battery.

Omega Digital UPS with solar inverter

The AC input cord of the solar inverter is plugged in to an extension box (red) for the mains connection. The lithium inverter is connected to the AC output of the solar inverter. I crafted a 16-amps two-gang electrical box (white power outlets) to connect to the AC output of the lithium inverter. The AC loads are now connected to this extension box. When the solar inverter is switched on, I am limited by its 400-VA load limit so I use it only for charging the lithium inverter. At night, I switch off the solar inverter and run the AC loads using the lithium inverter. It can handle 1000 watts but I do not push it beyond 200 watts. On a bright sunny day, the lead-acid battery gets charged in two or three hours. After that, all solar power is used to charge the lithium battery and power my AC loads. The lead-acid battery is unlikely to get cycled much now. Its life will increase but I will have to deep-discharge it once a month. I will also have to temporarily remove the lithium inverter during rainy season and use it sparingly during that time.

Video of Omega Digital UPS with solar inverter

UPDATE (November): As the lithium inverter's AC power cord was plugged into the AC output of the solar inverter, I could not draw AC power directly from the solar inverter. If I connected AC loads to the lithium inverter's output, it would draw power from solar inverter, as the AC input cord and AC output socket are directly connected. The lithium inverter draws a lot of power during charging, almost equal to what my solar panels were outputting. Any loads connected to the lithium inverter would deplete the lead-acid battery of the solar inverter. After that, the overload beeping noise would start to occur.

To solve this problem, I added another 16-amps two-gang electrical box. It has two input cords — one for each inverter's output. However, the second cord is connected to one of the sockets of the old gang box. (The lithium inverter is plugged in to first socket.) Now, I can draw power either from the solar inverter or the lithium inverter. Using the old box, I can turn off power to the lithium inverter when I consume power from the solar inverter.

The daily routine is:

  1. From before sunrise to when the solar panels start outputting power, I use the lithium inverter.
  2. During most of the day, I use the solar inverter while it also charges its lead-acid battery.
  3. Around noon, I switch to mains power for a few hours while the solar inverter tops up the lithium inverter's battery.
  4. In the evening, I switch off the solar inverter and turn on the lithium inverter. The latter remains on but no load is connected to it unless there is a power cut.

Neither the lithium battery or the lead-acid battery gets fully cycled every day, prolonging their life. They are topped up to full every day using only solar power. Excess solar power during the day is fully consumed. Mains power consumption is minimized.

  • The content of this post has been directly lifted from my book ‘How To Install Solar’. It is available on Amazon Kindle for ₹100. The full-colour paperback can only be ordered from Amazon UK.

    Cover of the book

  • Because of the lockdown, the Salem, TN-based Stalwart Solar is not selling the inverter in Kerala. I went to the Kerala-Tamil-Nadu border and bought my unit from a Coimbatore-based dealer. If you want to buy a lithium inverter, you can contact Stalwart Solar at ☎ 9944706699 or ☏ 7708855311. It is best if you buy their solar inverter with built-in lithium battery. You will not need to buy a separate lead-acid battery. I bought this ordinary inverter because I already have a solar inverter.

  • Stalwart Solar has published a few videos on the Net. If you know Tamizh, you will understand what they are saying.
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