I'm Doug Mycko, owner of SolarMan Consulting. Generator problems are some of the most misunderstood problems I see on off-grid systems.
The customer hears the generator running and naturally expects the batteries to be charging hard. But generator power has to travel through the inverter/charger, compete with house loads and stay within the battery's charging limits.
How Generator Charging Actually Works
If the house is consuming a large amount of generator power, less may remain for charging. If the inverter charger is programmed conservatively, it may intentionally limit generator draw. If the lithium BMS requests less charging current, the inverter may reduce charging again.
Seven Reasons a Generator Runs but Battery Charging Is Weak
1. AC Input Limit
The inverter may be programmed not to draw more than a specified amount from the generator. Increasing the generator size does nothing if the programmed input limit remains low.
2. Charger Limit
Many inverter/chargers have a separate charging-current limit. The generator can have unused capacity while the charger intentionally stays below its maximum.
3. House Loads
Generator capacity used by pumps, appliances, water heating or other loads is not available for battery charging.
4. Battery BMS Limit
A communicating lithium battery can reduce permitted charge current because of SOC, temperature, cell conditions or its own programmed limits.
5. Generator Voltage or Frequency
If AC quality moves outside what the inverter accepts, charging can reduce or disconnect even while the engine continues running.
6. Charging Stage
A battery near full does not necessarily accept the same charging power as a depleted battery. Charging behavior changes with battery state and system programming.
7. Solar Interaction
On some systems, charging controls and battery limits determine the combined amount accepted from PV and the generator. More available sources do not always mean more total charging.
Why Does the Generator Work Harder When the Solar Is Turned Off?
This can look completely backwards: solar is producing, but the generator loafs. Turn the PV off and suddenly the generator loads up and begins charging harder.
That behavior can occur when the system is limiting total battery charging. Solar is already supplying part of the allowed charging power. When solar disappears, the inverter charger may be permitted to fill more of that charging allowance from the generator.
Blindly increasing every current setting is not the answer. We need to identify which limit is actually active.
How Big Should My Generator Be for Off-Grid Solar?
I size a generator around more than battery charging. It may have to power the house and charge the battery simultaneously.
A useful starting concept is:
But generator rating alone is not enough. The inverter's AC-input capacity, charger capacity, power factor, 120/240-volt configuration and battery charging limits all matter.
A Huge Generator Can Still Charge Slowly
Suppose a large generator is connected to an inverter/charger that can only use a fraction of the generator's output for charging. The unused generator capacity does not magically reach the battery.
This is why I don't recommend a generator simply by saying, “Buy the biggest one you can afford.” Oversizing can mean unnecessary fuel consumption and poor loading without solving the actual bottleneck.
A Generator Can Be Too Small, Too
If household loads consume nearly everything the generator can produce, battery charging may repeatedly fall away whenever a pump, microwave, water heater or other significant load comes on.
A marginal generator can also experience voltage or frequency instability under surge, causing the inverter to reject its AC input.
Lithium Batteries Changed Generator Charging
Older lead-acid systems often relied heavily on voltage-based charging stages. Modern lithium systems add BMS limits and, on compatible closed-loop systems, direct battery-to-inverter communication.
That can be extremely useful, but it also means troubleshooting requires looking at what the battery is requesting—not just what the inverter is capable of producing.
How Long Should the Generator Run?
The answer depends on how many kWh need to be replaced and how much real charging power is reaching the battery.
If 10 kWh needs to be replaced and the battery is actually receiving about 5 kW, the theoretical energy replacement time is roughly two hours. Real systems have changing loads and charging limits, so actual runtime can be longer.
Automatic Generator Start vs. Manual Start
Some off-grid owners want the inverter system to start and stop the generator automatically. Others deliberately prefer manual generator control.
Either approach can work. What matters is that the inverter's charging and AC-input settings match the generator and battery. A system does not need automatic generator start simply to use generator charging correctly.
Generator Troubleshooting Checklist
Measure Generator AC
Confirm voltage and frequency at the inverter input under load.
Check AC Input Settings
Determine how much generator current the inverter is actually allowed to use.
Check Charger Settings
Find the inverter charger's maximum setting and any battery-specific limits.
Watch House Loads
Compare charging behavior while major household loads turn on and off.
Check Battery Limits
On lithium systems, determine whether the BMS is restricting charge current.
Compare PV On vs. Off
If charging changes dramatically, investigate combined charging limits and source priority.
Generator Backup Is Part of Good Big Island Design
On a fully off-grid Big Island home, I consider the generator part of the power system. We may rarely need it during excellent weather, but extended clouds, unusual loads or equipment service can make backup generation extremely valuable.
The goal isn't to run the generator constantly. The goal is to have it correctly sized and correctly integrated so that when it is needed, it accomplishes something.
Generator Running but Your Batteries Aren't Charging Right?
Send me the generator make/model, inverter make/model, battery system and a description of what happens with the PV on and off. Those details can tell us much more than simply knowing that the generator is running.
Doug Mycko
SolarMan Consulting, LLC
Big Island, Hawaiʻi
808-313-0608
www.SolarManHawaii.com
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Frequently Asked Questions
Why is my generator running but not charging my solar batteries?
Possible causes include inverter AC-input limits, charger-current limits, household loads consuming generator capacity, battery BMS limits, AC voltage/frequency problems or charging-stage behavior.
Why does my generator charge harder when I turn my solar panels off?
One possibility is a total battery-charging limit. With PV active, solar may already occupy part of the allowed charging power. When PV disappears, the inverter charger may be allowed to supply more.
Can my generator power the house and charge batteries at the same time?
Usually yes when the system is designed and programmed for it, but generator capacity is divided between household loads and battery charging.
Does a bigger generator charge batteries faster?
Only if the inverter charger and battery can use the additional available power. A larger generator cannot overcome a charger or battery-current limit.
Do I need automatic generator start for off-grid solar?
No. Automatic start is an option, not a requirement for proper generator charging. Many systems use manually started generators successfully.
SolarMan Consulting provides off-grid solar design, equipment supply, troubleshooting and commissioning support. Electrical installation requiring a contractor license should be performed by appropriately licensed personnel.