1. Types of Solar Inverters
String Inverter
A string inverter connects several solar panels in series to form a “string.” Multiple strings can then be connected to the inverter.
Advantages:
- Lower cost than microinverters
- Simple installation and maintenance
- Good efficiency
- Suitable for residential and commercial systems
- MPPT can optimize different strings
Disadvantages:
- If one panel/string has a problem, that string’s output can be affected
- Shading can reduce production
- Usually installed in a location away from individual panels
Central Inverter
A central inverter is a large inverter that receives power from many PV strings and converts the combined DC power into AC.
Advantages:
- Suitable for large-scale solar plants
- Lower cost per watt for large installations
- High power capacity
- Easier centralized maintenance
Disadvantages:
- A failure can affect a large portion of the plant
- Requires higher-voltage DC cabling
- Shading/mismatch can affect groups of panels
- More difficult to install and replace because of its size
Microinverter
A microinverter is installed with an individual solar panel, converting that panel’s DC electricity into AC.
Advantages:
- Each panel works independently
- Better performance with partial shading
- Panel-level monitoring
- No high-voltage DC running through the system after each panel
- Good for roofs with different orientations
Disadvantages:
- Higher cost
- More electronic devices on the roof
- Maintenance/replacement can be more difficult because equipment is installed near the panels
2. On-Grid vs Off-Grid Inverter
On-Grid Inverter
An on-grid (grid-tied) inverter connects the solar PV system to the utility grid.
Solar panel → DC → Inverter → AC → Home/Grid
Normally, it does not provide backup power during a grid outage unless the system has special backup/islanding functionality and appropriate equipment.
Off-Grid Inverter
An off-grid inverter is used in a system that operates independently of the utility grid.
Typical system:
Solar panel → Charge controller/inverter → Battery → Loads
A battery is generally required to supply energy when solar power is unavailable.
Hybrid Inverter
A hybrid inverter can combine solar, battery and grid operation.
Solar + Grid + Battery → Inverter → Loads
It can be useful when you want both grid connection and battery backup.
3. Important Solar PV Terms & Full Forms
| Abbreviation | Full form | Meaning |
|---|---|---|
| PV | Photovoltaic | Converts sunlight into electrical energy |
| DC | Direct Current | Current produced by solar panels |
| AC | Alternating Current | Current used by most household/grid loads |
| Voc | Open-Circuit Voltage | Maximum voltage of a PV module/string when no load is connected |
| Isc | Short-Circuit Current | Current when the PV output is short-circuited |
| Vmp | Voltage at Maximum Power | Voltage at which the panel produces maximum power |
| Imp | Current at Maximum Power | Current at which the panel produces maximum power |
| Pmp | Power at Maximum Power | Maximum power produced at the MPP |
| MPP | Maximum Power Point | Operating point where PV output power is maximum |
| MPPT | Maximum Power Point Tracking | Inverter/controller function that keeps the PV system near its maximum-power operating point |
| MCB | Miniature Circuit Breaker | Protects circuits from overcurrent/short circuit |
| MCCB | Molded Case Circuit Breaker | Higher-capacity protective circuit breaker |
| SPD | Surge Protective Device | Protects equipment from transient voltage surges |
| PWM | Pulse Width Modulation | Method of controlling electrical power using pulses |
| STC | Standard Test Conditions | Standard conditions used for PV module ratings |
| BOS | Balance of System | All PV-system components other than the modules themselves |
| D.C. | Direct Current | Same as DC |
| A.C. | Alternating Current | Same as AC |
Important relationship
The basic power equation is:
P = V × I
At maximum power:
Pmp = Vmp × Imp
For example, if a panel has:
- Vmp = 40 V
- Imp = 12.5 A
Then:
Pmp = 40 × 12.5 = 500 W
4. Voc, Isc, Vmp, Imp — Easy Way to Remember
Think of a solar panel like this:
Voc → voltage with NO load
Isc → current with SHORT circuit
Vmp + Imp → normal operating point for maximum power
And:
MPP = Vmp × Imp = Pmp
A typical panel’s Voc and Isc are not the same as its operating voltage and current.
5. MPPT vs PWM
MPPT — Maximum Power Point Tracking
MPPT continuously adjusts the operating point so that the solar array can extract as much available power as possible.
MPPT = better utilization of available PV power
PWM — Pulse Width Modulation
PWM is a switching/control technique. In solar charge controllers, a PWM controller generally connects the panel more directly to the battery, so the panel voltage is pulled closer to battery voltage.
For many modern PV systems, MPPT charge controllers provide better energy harvesting than PWM controllers, especially when the PV voltage is significantly higher than battery voltage.
6. MCB vs SPD
MCB
MCB = Miniature Circuit Breaker
Main purpose: overcurrent and short-circuit protection.
SPD
SPD = Surge Protective Device
Main purpose: protection against voltage surges, such as transient surges caused by lightning or switching events.
So remember:
MCB → overcurrent/short circuit
SPD → surge/overvoltage protection
7. String vs Central vs Microinverter
| Feature | String | Central | Microinverter |
|---|---|---|---|
| Size | Medium | Large | Very small |
| Installation | String level | Plant level | Panel level |
| Typical application | Residential/commercial | Large solar plants | Residential/commercial |
| Panel-level MPPT | Usually no | No | Yes |
| Shading tolerance | Moderate | Lower | Excellent |
| Cost | Medium | Low per watt at large scale | Higher |
| Maintenance | Relatively easy | Centralized | Panel/roof level |
| Failure impact | String/inverter section | Large section | Usually one panel |
| Monitoring | String/system | Plant/system | Panel level |
Easy memory trick
Central inverter = MANY panels → ONE BIG inverter
String inverter = GROUP of panels → ONE inverter
Microinverter = ONE panel → ONE small inverter
8. Very Short Revision
Before an interview or exam, remember these:
- Voc = Open-circuit voltage
- Isc = Short-circuit current
- Vmp = Voltage at maximum power
- Imp = Current at maximum power
- Pmp = Maximum power
- MPP = Maximum Power Point
- MPPT = Maximum Power Point Tracking
- MCB = Miniature Circuit Breaker
- SPD = Surge Protective Device
- PWM = Pulse Width Modulation
- PV = Photovoltaic
- DC = Direct Current
- AC = Alternating Current
- On-grid = connected to utility grid
- Off-grid = operates independently of utility grid
- String inverter = several panels/strings connected to an inverter
- Central inverter = large inverter for many PV strings
- Microinverter = inverter at individual-panel level
