Hybrid Systems With Generators: Our Engineering Approach

If you want the plain-language explanation of how a solar and generator hybrid works for a factory owner, read Solar + generator hybrid system for factories. For homes, see using a home generator with a hybrid inverter. This article goes one level deeper into how we engineer it.

Our starting point: the generator sets the rules

When the grid fails, the generator becomes the reference for voltage and frequency on site. Solar inverters can follow it, but they cannot replace it unless a battery system is designed to do so. So the generator's limits decide how much solar can run during an outage.

We are a solar company, but our roots are in power generation. Our power-generation experience goes back to the 1980s through our founder, Akram Ullah Khan, whose career includes WAPDA and Saudi Electricity Company, and the family business later imported and supplied generators before Solar Works Pakistan was founded in 2021. We no longer sell generators, but that background shapes how we treat them: as machines with real operating limits, not just a backup source.

Diagram

Generator and hybrid inverter

Grid

First source

Battery

Second source

Generator

Backup, sized for charging

Step 1: collect generator and load data

Good hybrid design starts with data. Before designing, we ask for or record:

For larger factories, an energy audit before solar gives much better data than estimates. Where the site runs captive gas generation, the same approach applies; see from captive gas power to solar.

Step 2: choose the architecture

There is no single "hybrid system". We choose between a few layouts depending on the site:

1. On-grid solar plus a generator controller. The most common factory layout. Three-phase on-grid inverters run freely on the grid and are limited by the controller on generator power. See our guide to on-grid inverters for factories.
2. On-grid solar, controller and BESS. A battery system takes on short outages, smooths motor starts and absorbs surplus solar. See BESS for factories.
3. Hybrid inverters with a generator input. Usually for smaller commercial sites, where the inverter itself manages the generator connection.
4. Solar, battery and generator with little or no grid. For sites with very weak supply, where the battery system may form the local grid and the generator becomes the backup.

The inverter, controller and battery are selected together, because they must communicate. For commercial inverters and BESS we work with brands including Huawei and Sungrow, chosen per project and named by model in the BOQ.

Step 3: set the control limits

The controller measures generator output continuously and adjusts the inverters. The engineering is in the setpoints.

• Minimum generator load. Taken from the manufacturer's figure, plus a margin. Running diesel engines at light load for long periods can cause incomplete combustion and fuel and soot build-up in the exhaust, often called wet stacking. As one reference point, the US standard NFPA 110 calls for emergency generators to be exercised at 30% or more of their nameplate rating during testing.
• No reverse power. Generator output must stay above zero at all times. Generator protection usually includes a reverse-power trip, and the control scheme is designed so it is never reached in normal operation.
• Spinning reserve for motors. When a large motor starts, the load jumps suddenly. If solar is carrying most of the load and a cloud passes at the same moment, the generator must pick up both. We keep enough headroom on the generator for the largest motor starts on that site.
• Ramp rates. How quickly solar output is allowed to rise or fall, so the generator's governor can keep up.

Step 4: check reactive power and power factor

This is the point that is often missed. Most solar inverters, by default, supply real power (kW) at or near unity power factor. Motors and many other factory loads also need reactive power (kVAr). When solar takes over part of the real power, the generator is still left supplying most of the reactive power. Its own power factor drops, and its current can stay high even though its kW output has fallen.

Generators are rated for a certain power factor range. If the generator is pushed outside it, its alternator can overheat or its voltage regulation can suffer. So we check:

• the factory's reactive power demand, by shift if possible;
• the generator's capability at low power factor;
• whether the inverters can supply some reactive power, and whether the controller can command it;
• whether existing capacitor banks behave correctly on generator power.

Power factor on grid supply is a related but separate topic; see power factor and solar. Variable-speed drives and other non-linear loads can also add harmonics, which matter more on a generator than on the grid; see harmonics and power quality.

Step 5: integrate the changeover and protection

The controller needs a reliable signal from the changeover or transfer arrangement, so it knows whether the grid or the generator is live. Nothing in the hybrid design may ever allow the generator or solar to feed back into the grid during an outage. Our guide to the changeover switch for solar, generator and WAPDA explains the safety rules.

With more than one generator, the design also accounts for sets starting and stopping, and the controller must know how many are online so the minimum-load floor follows the running capacity.

Step 6: test every scenario at commissioning

A hybrid system has to work in every combination of sources. During testing and commissioning, we check the system's response to:

• grid present, generator off: solar runs freely;
• grid failure and generator start: solar limits to the setpoints;
• load dropping suddenly on generator: solar backs off before the generator reaches its floor;
• large motor start on generator: reserve is enough;
• grid return: the controller releases the limit;
• battery behaviour, where fitted, in each case.

Settings are then fine-tuned with monitoring data once the factory runs normally. Our remote monitoring is subject to equipment capability.

Phasing a hybrid project

Many factories do not build everything at once. A common path is solar with a generator controller first, then battery storage once there is real data on outages and evening load. If you plan to add BESS later, the first phase should leave space, cable capacity and control compatibility for it. See planning a phased solar rollout.

Why in-house design matters for hybrids

Hybrid systems are where design shortcuts cause the most trouble: tripped generators, curtailed solar or worse. Our engineering and AutoCAD design are done in-house, and our own teams install and commission the system. Read why in-house engineering and design matter. We have served 300+ customers, including 50+ industrial clients, from about 6 kW to megawatt scale; see industrial and commercial solar.

After handover, our 1-year service warranty applies under the applicable project terms, structures carry a 3-year structure warranty, and our own engineers handle after-sales with a 48-hour target to send personnel after a complaint is registered. Manufacturer warranties for inverters, batteries and panels are.

Get today's price or a free site survey

Send us 12 months of bills, your generator ratings and running hours. Our engineers will tell you plainly what a hybrid design can do on your site.

• WhatsApp or call: 0336 666 4321
• Email: info@solarworks.pk
• Head office: Main Boulevard, Sargodha Rd, Block F FDA City, Faisalabad
• Lahore branch: 2nd Floor, Jeff Heights, Gulberg, Lahore
• Hours: 9:00 AM to 6:00 PM, daily

We serve all of Punjab, and site surveys are normally free. See our Faisalabad and Lahore pages, or contact us.

Frequently asked questions

How do you decide how much solar can run on generator power?

From the generator's own limits: its rating, the manufacturer's minimum recommended load, its power factor capability and the reserve needed for motor starts. The controller then holds solar within those limits in real time.

Why does power factor matter in a solar generator hybrid?

When solar supplies part of the real power, the generator still supplies most of the reactive power that motors need. Its power factor falls and its current can stay high. The design checks that the generator stays within its rated range.

Can solar run a factory with the generator switched off?

Not with on-grid inverters alone, because they need a grid or generator as a reference. A battery system designed to form the local grid can do this for a time, which is one of the jobs BESS can take on.

Can you add solar to a factory with several generators?

Yes. The controller has to know how many sets are running so the minimum-load limit follows the running capacity. The design and commissioning tests cover sets starting and stopping.

Do you supply the generator?

No. Solar Works Pakistan no longer sells generators. We design around your existing generator and integrate it where appropriate.

Does this apply to gas generators too?

The same principles apply: no reverse power, a safe minimum load and enough reserve. The exact limits come from the generator manufacturer.

How much fuel will a hybrid system save?

We do not quote a fixed figure, because it depends on outage hours, load and generator behaviour on your site. Our hybrid system guide shows an example method you can use with your own fuel logs.

Sources

1. Consulting-Specifying Engineer, "A close look at wet stacking" (cites the NFPA 110 test-loading requirement)