The first question a buyer in Pakistan asks about solar is not how many panels but which inverter: grid-tied, hybrid or off-grid. The answer is decided by one thing, how many hours the grid is away and what must keep running while it is. A grid-tied inverter is the cheapest way to turn sunlight into units and is dark the moment the grid is; a hybrid puts a battery and the essential loads behind the inverter and still uses the grid; an off-grid system carries everything itself and pays for it in storage. This guide explains the mechanism of each, what the market sells under each name, and how to size and approve them.
Grid-tied: cheapest per watt, dark when the grid is
A grid-tied inverter does one job. It takes the array's DC, tracks its maximum power point, and pushes the power into the house's AC wiring in step with the grid. The loads take what they need, the surplus flows out through the meter, and at night everything comes in from the grid as before. There is no battery, no charger and no transfer switch, so the inverter is simple, efficient (97–98 %) and the cheapest per watt of the three, and the array can be as large as the connection allows.
The price of that simplicity is anti-islanding. A grid-tied inverter is required to stop within two seconds of losing the grid, tested to IEC 62116 and the interconnection rules of IEEE 1547, because it must not energise a line that the utility believes is dead and the crews are working on, and because it cannot hold voltage and frequency for a neighbourhood on its own. So during load-shedding, at noon, with the array in full sun, the inverter switches off and the house is as dark as the street. Where the grid is away six hours a day that is not a small caveat; it is the reason grid-tied is the wrong answer for most Pakistani homes and the right answer for a daytime business with a reliable feeder or a generator for the outages.
The economics are the export economics. Under net metering the units exported are credited against the units imported; the credit per exported unit has been changed by the regulator and may change again, and where it is lower than the import tariff the value of a unit used at home is higher than the value of a unit sent out. That pushes a grid-tied design toward the size of the daytime load rather than the size of the roof.
Hybrid: solar plus battery plus grid, with the grid as one input
A hybrid inverter is a grid-tied inverter, a battery charger, an inverter-charger for the battery and a transfer switch in one box. It has an AC input from the grid, an AC output to the loads it protects, a DC input from the array through one or more maximum-power-point trackers, and a battery port. In the day the array feeds the loads; the surplus charges the battery and, if the system is on net metering, exports; in the evening the battery carries the loads; when it is low, the grid does, and the inverter can also be told to charge from the grid at night if the tariff makes that worthwhile. The priority, solar then battery then grid, or solar then grid then battery to spare the battery, is a setting.
When the grid fails the inverter opens its input relay, within 10–20 ms on a good unit, and carries the output from the battery and the array: anti-islanding is satisfied because the house is disconnected from the line, and the essential loads never notice. That output is the whole point. It carries what has to run during load-shedding, which for most homes and offices is lights, fans, the fridge, the router and the computers, and for a shop the tills and the signage. Air conditioners, geysers and motors are usually left on the grid side, or put on the output only if the inverter and the battery are sized for them. A separate generator input, or a generator mode on the AC input, lets a set take over for the long outage and charge the battery while it runs, at a controlled current.
This is why the hybrid has become the default answer in Pakistan. It uses the grid whenever it is there, which is still most of the time, so the battery is sized for an evening and not for days; and it keeps the array producing through the load-shedding hours that would switch a grid-tied inverter off. The battery is sized for the evening's essential load, four to six hours of a kilowatt or so, not for the whole house, and that keeps it affordable.
Off-grid: only where the grid is not
An off-grid system has no grid input at all. The array must produce the whole daily load plus the losses in the worst month of the year, the battery must carry the nights and a run of cloudy days, and a generator is the only backup. Sizing starts from the worst month's sun hours and works up: array to the daily energy with a margin, battery to two or three days of autonomy at a depth of discharge the chemistry can stand, inverter-charger to the peak load with its surges, generator to the load plus the charging current. Storage dominates the cost, because everything that the grid would have provided for a few rupees a unit, the night, the cloudy week, the summer peak, has to be built from batteries and panels.
Off-grid is right where the grid is not: a farm or tube well beyond the last pole, a telecom site, a village in the north, a plant whose feeder is too weak to be worth connecting. In a city it is a mistake. An "off-grid" design behind a working connection pays for autonomy the grid would give for PKR 45 a unit, cycles the battery every day of its life, throws away the summer surplus that could have been exported, and still goes dark after the third cloudy day. If a grid connection exists, wire it to the inverter's AC input, and the design becomes a hybrid.
The "solar UPS" and other hybrids in the market
Most of what is sold in Pakistan as a "solar UPS" or "solar inverter" is a hybrid of some grade, from 1 kW to 6 kW at 24 V or 48 V, and the differences that matter are on the datasheet rather than in the name. A low-frequency inverter builds its output through a 50 Hz transformer: it is heavy and a little less efficient, and it will supply two or three times its rating for a moment, which is what a fixed-speed air conditioner, a water pump or a motor needs to start. A high-frequency, transformerless inverter is light, cheaper and more efficient, and its surge capacity is smaller and shorter; it is fine for lights, fans and electronics and marginal for motors. The output waveform must be a pure sine wave: the modified-sine units at the bottom of the market make motors hum and heat, and some electronics misbehave on them.
On the solar side, check the MPPT window: the PV input voltage range, the start-up voltage, the maximum input current and how many trackers there are, which decide how many panels can go in a string and how many strings on different roof faces can be used. A unit with a single tracker and a 60–115 V window wants strings of two or three panels; a unit with a 120–500 V window wants strings of eight to twelve and is far more forgiving of long cable runs. Check how much the array may be oversized against the inverter (a DC-to-AC ratio of 1.2–1.3 is usual). Then the features to demand: certification to IEC 62109-1 and -2 for safety and, if it will export, anti-islanding tested to IEC 62116 and a place on the regulator's approved list; a battery interface that talks to a lithium battery's management system over CAN or RS485 if a lithium battery is planned; a generator input with a settable charging current; configurable priorities and a low-battery cut-off; a parallel capability if the system may grow; monitoring that works without the vendor's cloud; a realistic enclosure rating for a dusty, hot room; and spares and service in the country.
Batteries for each case
Two chemistries cover nearly every installation. Tubular flooded lead-acid, the 12 V 150–200 Ah blocks in every market, is cheap per ampere-hour, tolerant of abuse, and slow: it should be discharged no deeper than about 50 % for a life of the order of a thousand to fifteen hundred cycles, it needs its water topped up and a ventilated space, it loses capacity when cold and life when hot, and it takes six to eight hours to recharge properly. Lithium iron phosphate (LiFePO4), usually as a 48 V rack or wall pack of 5 kWh, can be discharged to 80–90 % for several thousand cycles, is sealed, light and fast to charge, costs several times as much per ampere-hour and, per cycle over its life, less. Its battery management system must talk to the inverter, which is why the interface is on the checklist above.
The arithmetic is the same for both, and the charging guide sets it out: a 200 Ah tubular battery taken to 50 % needs about 120 Ah put back, which at 20 A is six hours plus the slow tail of the absorption stage, and a hybrid inverter's maximum charge current, from the array or from the grid, is what decides whether the battery is full by evening. For a grid-tied system there is no battery. For a hybrid the battery is sized for the evening and the chemistry chosen on budget and on how many cycles a year it will see: daily load-shedding favours lithium. For off-grid the battery is the system, and the autonomy days and the depth of discharge decide a bank that is usually several times the hybrid's.
Sizing in one paragraph for each
Grid-tied. Size the array to the daytime load, extended toward the roof only as far as the export credit justifies, with the inverter at or a little below the array's kWp; check the sanctioned load and the net-metering limits before the array is fixed; the solar system sizing calculator turns a daily load into panels for the local sun hours, and a payback calculation against the tariff decides the size.
Hybrid. Size the inverter to the peak of the essential loads with the surge of the largest motor on it; the battery to the evening's essential energy divided by the depth of discharge and the inverter efficiency, which the battery backup runtime calculator does; and the array to the whole daily load plus what the battery takes to recharge, in the worst month that matters, following the solar sizing guide. The grid covers what is left.
Off-grid. Size the array to the worst month's sun hours with a margin of at least a fifth, the battery to two or three days of autonomy at the chemistry's depth of discharge, the inverter-charger to the peak with surge, and the generator to the peak plus the battery charging current; then check the summer, when the same array produces far more than the loads can use.
Approvals, net metering and the meter
Exporting to the grid is regulated. NEPRA's Distributed Generation and Net Metering Regulations of 2015, as amended, set out the mechanism: a consumer applies to their distribution company for a system within the regulations' capacity range and not larger than their sanctioned load; the DISCO inspects the site and the installation, which must use an inverter on the approved list and be carried out by a certified installer; a bidirectional meter replaces the ordinary one; and a net-metering agreement is signed. The meter records import and export separately, and the export is credited against the import on the bill according to the rules in force at the time.
The credit is where the rules have moved. The regulations began with export credited at the same rate as import, and the regulator has since changed the treatment of exported units and may do so again; the mechanism of separate metering and credit remains, the rate does not. A design that depends on the export credit for its payback is exposed to that; a design that sizes the array to what the site itself uses is not. For a hybrid on net metering, the inverter must be one of the approved grid-interactive models, and the battery does not change the process. For an off-grid system there is no application, and no export.
What we recommend, honestly
Count the hours the grid is away over a typical week, list what must keep running during them and what merely would be nice, and size storage for the first list only. If the feeder is reliable and the load is in the day, a grid-tied system sized to the daytime load is the cheapest electricity you will buy. If load-shedding is a fact of the site, and for most of Pakistan it is, a hybrid with an evening's battery behind the essential loads, the grid for the rest, and a generator input for the long outage, is the design that pays. Off-grid is for where there is no grid, and nowhere else.
Buy on the datasheet, not on the brand or the salesman's runtime figure: surge capacity for the motors on the output, the MPPT window against the strings on the roof, the certifications, the battery interface, and service in the country. Ahmedonics designs and integrates solar, battery, generator and control systems for homes, businesses and plants, and the reasoning above is the reasoning we use; the arithmetic is in the calculators linked from this page, and the design is the part worth paying for.
References
- IEC 62109-1:2010 and IEC 62109-2:2011, Safety of power converters for use in photovoltaic power systems — Part 1: General requirements; Part 2: Particular requirements for inverters
- IEC 62116:2014, Utility-interconnected photovoltaic inverters — Test procedure of islanding prevention measures
- IEEE Std 1547-2018, IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces — anti-islanding, voltage and frequency ride-through and the interconnection requirements
- NEPRA (Alternative & Renewable Energy) Distributed Generation and Net Metering Regulations, 2015 (as amended) — the application, inspection, bidirectional metering and credit mechanism for net metering in Pakistan; the rates in force change
- IEC 61427-1:2013, Secondary cells and batteries for renewable energy storage — General requirements and methods of test — Part 1: Photovoltaic off-grid application