A rooftop system is only as good as the numbers that prove it works. Meters decide what the customer pays; monitoring tells you, and them, whether the system is producing what it should. Both cause confusion if nobody explains them at handover.
The meters
Net meter (bidirectional meter)
The DISCOM replaces the existing meter with a bidirectional (net) meter. It keeps separate registers for:
- Import: energy taken from the grid (kWh)
- Export: energy sent to the grid (kWh)
The bill uses these to work out net units under the state's rules (net metering, net billing or gross metering; module m08). The display usually scrolls through several parameters. On many meters the standard identification codes (OBIS codes) appear next to each value; 1.8.0 is total imported energy and 2.8.0 is total exported energy. Teach the customer which screens to read.
Points to know:
- The meter must be of a type approved by the DISCOM and tested in its laboratory (or an accredited one) before installation; the DISCOM seals it.
- Many DISCOMs are rolling out smart meters, often prepaid, under the national distribution reform programme. Smart meters record import and export in time blocks, which time-of-day tariffs use.
- Larger connections use CT-operated meters, where readings are multiplied by a factor printed on the bill.
Generation (solar) meter
A unidirectional meter at the inverter output that records total solar generation. Some DISCOMs, state schemes and commercial arrangements require it (for example where charges are levied on gross generation, or for renewable purchase obligation accounting). Where it is not required, the inverter's own reading serves for performance checks, although it is less accurate than a meter.
How the numbers fit together
Worked example: reading a March bill in Pune
A 3.3 kWp system. The generation meter shows 520 units for the month; the net meter shows import 210 and export 330.
- Solar used directly in the house: 520 − 330 = 190 units
- Total home consumption: 190 + 210 = 400 units
- Net at the meter: 210 − 330 = −120 units, banked for the monsoon under annual settlement
The customer sees "import 210" and asks why they still take power from the grid. The answer: solar makes nothing at night, so evening and night use always come from the grid. What matters is the net figure over the year.
Monitoring options
| Setup | Typical use | Notes |
|---|---|---|
| Inverter display only | Very small or budget systems | Customer must walk to the inverter; faults go unnoticed |
| Wi-Fi dongle and app | Most homes | Needs a steady router signal at the inverter; changing Wi-Fi password breaks it |
| 4G/LTE dongle | Sites with weak Wi-Fi, farms | Needs a data plan |
| Ethernet/LAN | Offices, schools | Most reliable |
| Data logger (RS485/Modbus) | Several inverters, C&I | Combines inverters, meters, weather sensors |
| Irradiance sensor and module temperature probe | Larger commercial systems | Enables true performance-ratio tracking |
| Zero-export controller with CT or meter | Where export is not permitted | Limits inverter output to on-site load |
What to watch
- Daily units per kWp against the expected value for the month.
- String voltages and currents on inverters that show them: one string reading lower than its twin at the same moment points to shade, a failed module or a bad connection.
- Fault and event logs: grid over-voltage trips, insulation (ISO) faults, over-temperature.
- Gaps in data: often just Wi-Fi, but sometimes the inverter is off.
Worked example: is the system healthy?
Pune specific yield in March is about 158 units per kWp for the month, or 158 ÷ 31 = 5.1 units per kWp per day. A 3.3 kWp system should average about 3.3 × 5.1 = 16.8 units a day in a clear March, allowing for some variation day to day.
If the app shows 12–13 units a day for two clear weeks, about 25% low, check in this order: soiling, new shade (a neighbour's construction?), one string down (compare string currents), inverter derating from heat, frequent grid trips in the log.
Data, privacy and handover
- Create the monitoring account in the customer's name and email, and add the installer as a second user for service.
- Explain what the app shows, how to read daily and monthly totals, and what alerts mean.
- Note the Wi-Fi network and dongle serial number in the handover file; most "monitoring stopped" calls follow a router change.
Key points
- The net meter records import and export separately; the bill uses them under the state's net-metering rules.
- Home consumption = generation − export + import; self-consumption = generation − export.
- A generation meter records total solar output and is required by some DISCOMs and schemes.
- Wi-Fi monitoring suits most homes; data loggers, sensors and zero-export controllers suit larger sites.
- Compare daily units per kWp with the expected monthly value and check string data and fault logs when output drops.
Check yourself
- Generation 450, export 260, import 300 for a month. What was the home's consumption?
- Why does a solar home still import power every month?
- Monitoring stopped updating the day after the customer got a new router. What is the likely fix?
Answers
- 450 − 260 + 300 = 490 units.
- Solar produces nothing at night and little in early morning or evening, so those loads are supplied by the grid.
- Reconnect the inverter's Wi-Fi dongle to the new network (new name or password) using the maker's app.