☀️ Solar Desk
How to Convert a Normal Inverter to Solar Using an SMU: Complete Retrofit Guide
Retrofit your existing inverter-battery backup system with solar panels using a Solar Management Unit (SMU) — cost, wiring overview and safety rules explained in simple terms.
Almost every Bengali home has an inverter-battery backup — running fans and tube lights during load shedding. But that battery charges from grid electricity, which means the bill keeps coming. With an SMU (Solar Management Unit), that same inverter-battery can be charged by the sun — on a small budget, without throwing anything away.
1. What exactly is an SMU?
An SMU is a solar charge controller that sits between the inverter and the solar panels:
Solar panels (DC) ──▶ SMU (MPPT charge controller) ──▶ Battery (12V/24V) ──▶ Your old inverter ──▶ Home load- When the sun is out, the SMU charges the battery from the panels (MPPT technology, ~95% efficiency).
- Once the battery is full, charging is cut off — no overcharging.
- On cloudy days or at night, the inverter automatically charges from the grid.
2. What you’ll need and what it costs
| Component | Specification | Approximate price |
|---|---|---|
| SMU | MPPT, 24V, 30–50A (panels up to 2kW) | ₹3,000–6,000 |
| Solar panels | 2 × 330W (total ~660W, in a 24V system) | ₹12,000–16,000 |
| Mounting stand | Adjustable structure on a GI/tiled roof | ₹3,000–5,000 |
| DC cable, MC4 connectors, fuse | 4–6 sq mm copper | ₹2,000–3,500 |
| Installation (licensed electrician) | — | ₹2,000–4,000 |
| Total | ~₹22,000–34,500 |
Assumes an existing 150Ah battery and inverter. If you need a new battery, see our 150Ah solar battery guide.
3. The core connection concept (summary)
- Connect the panels in series — add the Voc of the two 330W panels and keep it within the SMU’s maximum DC input voltage.
- Panels → SMU solar input — wrong polarity (+/–) destroys the SMU; verify with MC4 connectors before connecting.
- SMU battery output → battery terminals — a fuse/MCB in between is mandatory.
- The inverter stays connected to the battery as before — no changes inside the inverter are needed.
- Earthing and lightning protection — the roof structure and the SMU must be earthed.
⚠️ The steps above are for understanding — in practice, cable routing and termination must be done by an experienced technician. Wrong polarity or exposed DC terminals create a fire risk.
4. How much will you save?
A 660W panel array generates ~2.5 units a day on average (in West Bengal’s sunlight). That’s ~75 units a month, i.e. ₹500–700 saved per month depending on tariff. On that basis:
- Total cost ~₹25,000 → payback of 3.5–4.5 years
- Backup quality and reliability remain the same as before
5. Who an SMU retrofit is for — and who it isn’t
A good choice if:
- Your existing inverter-battery is new and in good condition
- Budget is small and you only want to run backup on solar
- You don’t need net metering/subsidy
Not a good choice if:
- The old inverter is already 5+ years old — better to buy a solar PCU outright
- You want to cut 300+ units of electricity a month → then a PM Surya Ghar on-grid system is the answer (see the guide)
- The roof is shaded or rented
Final word
An SMU retrofit is the cheapest entry into solar — for less than the price of a motorcycle, you cut the electricity bill and gain load-shedding-proof backup. But don’t start the work without correct sizing and a licensed technician.
❓ Frequently asked questions
What is an SMU and how does it work?
An SMU (Solar Management Unit) is a small controller device that charges your existing inverter's battery with DC power from the solar panels using MPPT. When the sun is out the battery charges from solar; otherwise normal grid charging takes over.
What does converting an old inverter to solar cost in total?
Approximately: SMU ₹3,000–6,000 + two 330W panels ₹12,000–16,000 + stand-cable-fitting ₹4,000–8,000. In total, roughly ₹20,000–30,000 gets you a solar backup of around 1 kW.
Is an SMU retrofit better than buying a new solar inverter?
On a tight budget with a working inverter-battery, an SMU retrofit is the cheaper solution. But a dedicated solar PCU/hybrid inverter wins on efficiency and long-term reliability — for heavy loads and net metering you'll need a new inverter.