Yes, you can charge a solar battery with a generator, and it’s a genuinely common and sensible backup strategy for off-grid systems, particularly through a UK winter when solar generation alone can’t reliably keep a battery topped up. The key is using the right type of charger between the generator and battery – you generally can’t just wire a generator directly to a battery the way you might occasionally get away with for a very small solar panel.
Why a Generator Needs a Proper Charger, Not a Direct Connection
Generators typically output AC (alternating current) electricity, similar to mains power, while batteries store DC (direct current) – so a battery charger (effectively a rectifier plus charge controller) is needed to convert and regulate the generator’s output into a safe charging profile for the battery. This is a similar principle to why you can’t wire a solar panel’s DC output directly to appliances designed for AC mains without an inverter doing the reverse conversion.
How Generator Charging Fits Into an Off-Grid Solar Setup
Most off-grid systems designed with a generator in mind include a combined inverter-charger unit that can draw from solar during the day and automatically switch to generator charging when the battery drops below a set threshold, typically during an extended cloudy spell. This automatic switching removes the need to manually monitor battery levels and start a generator yourself every time, which matters considerably for unattended or remote installations.
Sizing a Generator for Battery Charging
| Battery bank size | Suggested generator output | Notes |
|---|---|---|
| 5-10kWh | 2-3kW | Adequate for a moderate recharge over a few hours |
| 10-20kWh | 3-5kW | Faster recharge, useful if generator run-time is limited |
| 20kWh+ | 5-8kW+ | Common for larger off-grid homes needing substantial backup capacity |
Oversizing the generator relative to your charger’s maximum input rate doesn’t speed up charging beyond what the charger itself can accept, so it’s worth matching generator output to your charger’s actual specification rather than assuming a bigger generator always charges faster.
Petrol, Diesel or Inverter Generator: Does It Matter for Charging?
For battery charging specifically, an inverter generator (which produces cleaner, more stable AC output than a basic conventional generator) is generally the better match for sensitive battery charging electronics, reducing the risk of power quality issues affecting the charge process. Diesel generators tend to be more fuel-efficient for longer, sustained charging sessions, while petrol inverter generators are often preferred for smaller, occasional backup use given their quieter running and cleaner output.
When Generator Charging Becomes Genuinely Necessary
For a fully off-grid UK property, an extended overcast winter spell can leave solar generation insufficient to keep even a well-sized battery bank topped up, which is exactly the scenario a backup generator is designed for – not a sign the solar and battery sizing was done wrong, but a deliberate, sensible insurance measure given how variable UK weather genuinely is. See our guide on off-grid solar system costs for how generator backup factors into overall system budgeting.
Frequently Asked Questions About Charging Solar Batteries With a Generator
Can any generator charge a solar battery, or do I need a specific type?
Most AC generators can charge a battery through the right charger, but inverter generators produce cleaner, more stable output better suited to sensitive charging electronics. Check your battery charger’s specification for compatible input voltage and frequency rather than assuming any generator will work interchangeably.
Is charging from a generator different from charging from mains electricity?
The battery charger performs a similar conversion role either way, but generator output can be less stable in voltage and frequency than mains electricity, particularly for cheaper conventional (non-inverter) generators, which is why matching charger and generator specifications carefully matters more here than with a mains connection.
How long does it typically take to charge a battery from a generator?
This depends on your charger’s output rate relative to the battery’s capacity and depth of discharge – broadly similar principles to solar charging time calculations, just typically faster since a generator can usually deliver more consistent, higher current than variable solar generation.
