Off-grid, RV & cabin
Enter your load, panel, and generator charging rate. Find how many hours a day the generator needs to run, and what that costs in fuel.
How it works
Step 1
Use a lower sun hours figure to model a genuinely cloudy day scenario.
Step 2
The effective wattage the generator delivers into your battery through its charger.
Step 3
Often a surprisingly short runtime for a modest shortfall.
For a 3,000Wh daily load against an 800-watt panel producing 2,560Wh during a cloudy 4-hour sun day, the shortfall is 440Wh. At a 1,500-watt generator charger with 85% efficiency, that shortfall takes only about 0.35 hours, roughly 21 minutes, to fill, a genuinely short runtime for closing the gap.
For a 0.35-hour daily runtime at 0.4 gallons per hour and $3.50 a gallon, daily fuel cost comes to about $0.48. If that shortfall happened every single day for a month, monthly fuel cost would total around $14.49, a modest ongoing cost for closing a modest gap, though real-world use is typically far less frequent than every day. Fuel price assumptions here draw on EIA's data on what drives energy prices. Even off-grid, the money side matters. The 25-year lifetime savings calculator in our Savings & Payback section can help you compare the system cost with the cost of extending the grid.
A generator charging into a battery bank typically delivers far more instantaneous power than the actual daily energy shortfall requires, since generators are sized for peak load capability rather than gentle trickle charging. This means even a meaningful energy gap often closes in well under an hour of generator runtime, which surprises people who picture running the generator for hours to fully make up a deficit. The hybrid inverter sizing calculator in our inverters and equipment tools covers the equipment side of pairing a generator charger with an existing inverter setup. Your panels can only charge what your battery can hold. The backup runtime estimator in our Battery & Storage section can help you size the bank for your actual daily load.
Only running the generator when needed is almost always the better approach, since running it daily for a small shortfall wastes fuel on unnecessary trips to the fuel tank and adds wear and maintenance burden to the generator over time. Many hybrid systems only need generator backup during genuinely cloudy stretches lasting several days, not as a routine daily habit built into normal operation. EPA's overview of green power markets is worth a look if you're weighing how heavily to lean on the generator versus expanding the solar side instead.
Check the generator backup hybrid sizing tool for a full system-level sizing view, and see the cloudy day reserve calculator to plan for multi-day cloudy stretches beyond a single day's shortfall. The annual maintenance cost estimator in our maintenance and lifespan tools is worth budgeting for too, since a generator that runs even occasionally still needs regular service.
Questions
For a 3,000Wh daily load against an 800-watt panel producing 2,560Wh during a cloudy 4-hour sun day, the shortfall is 440Wh. At a 1,500-watt generator charger with 85% efficiency, that shortfall takes only about 0.35 hours, roughly 21 minutes, to fill.
For a 0.35-hour daily runtime at 0.4 gallons per hour and $3.50 a gallon, daily fuel cost comes to about $0.48. If that shortfall happened every single day for a month, monthly fuel cost would total around $14.49, a modest ongoing cost for closing a modest gap.
A generator charging into a battery bank typically delivers far more instantaneous power than the actual daily energy shortfall requires, since generators are sized for peak load capability rather than gentle trickle charging. This means even a meaningful energy gap often closes in well under an hour of generator runtime.
Only when needed is almost always the better approach, since running a generator daily for a small shortfall wastes fuel on unnecessary trips to the fuel tank and adds wear to the generator. Many systems only need generator backup during genuinely cloudy stretches, not as a routine daily habit.