By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
You keep talking about energy storage for batteries, and energy charging from solar panels, but you’ve never written about how much energy is in a gallon of gasoline for my generator. Any idea? —Karla P.
Well, that depends on a lot of things, mostly the efficiency of the particular generator you’re running, and how much wattage you’re asking it to produce. But I can pick a few numbers out of the air to get a general idea for comparison.
Now, I’ve not run this test in my lab, so I can only use manufacturer specs. But Honda claims that their EU2200is inverter generator will run for around 4 hours at 100% output on 1 gallon of gasoline. So that’s a pretty good start.
A few calculations
Let’s assume 2,000 watts of continuous power for 4 hours of run time. To calculate the watt-hrs of energy, all we have to do is multiply the watts times the hours. So 2,000 watts x 4 hrs = 8,000 watt-hrs or 8 kilowatt-hours (kWh).
A 100 amp-hr battery (12 volts) is able to store around 1,200 watt-hrs (1.2 kWh) of energy. Again, we just have to do a little multiplication to find that 100 amp-hrs x 12 volts = 1,200 watt-hrs (1.2 kWh).
Your results will vary depending on the time of year, angle of your panels to the sun, and degrees of latitude, but industry numbers work out to around 300 watt-hrs of energy per day for each 100-watt solar panel on the roof of your RV.
Charging with an inverter generator
We can simply do a little arithmetic to calculate the approximate equivalencies. If you have a 1.2 kWh battery (that’s a standard 100 amp-hr lithium battery that can be discharged down to 0% State of Charge [SOC]), we just divide the available energy from a Honda generator using 1 gallon of gasoline (8 kWh) by the energy needed to recharge the battery (1.2 kWh).
So 8 kWh / 1.2 kWh = 6.6 refills. That suggests 1 gallon of gasoline in a Honda EU2200is inverter generator would be able to completely recharge your 100 amp-hr battery from 0% to 100% SOC about 6 times. Of course, there can be other inefficiencies that may reduce this number, but it should be reasonably close (what I call a SWAG for Scientific Wild A** Guess).
Charging with solar panels
So, to find out how long a 100-watt solar panel (which can produce 300 watt-hrs per day) would take to recharge a 100 amp-hr battery (which is 1,200 watt-hrs), we simply divide 1,200 watt-hrs of storage by 300 watt-hrs of solar energy per day and see that 1,200 watt-hrs / 300 watt-hrs = 4 days.
Yup, it would take a single 100-watt solar panel approximately 4 days to completely recharge a 100 amp-hr battery from 0% to 100% SOC. And if you put 400 watts of solar panels on your RV, that would cut the recharge time from 4 days down to 1 day. Now do you see why RVers are cramming as many solar panels on their rooftops as possible?
But the calculations suggest that running a 2kW generator for 30 minutes per day (which would use around 1 pint of gasoline) would provide as much energy (1,000 watt-hrs) as 3 or 4 solar panels rated for 100 watts per panel. (Each 100 watts of solar panels can generate around 300 watt-hrs of energy per day.)
Also be aware that a single 100 amp-hr battery in your RV will have a hard time keeping up with any high-energy appliances such as hair dryers, microwave ovens and even Instant Pots. I’ll do a few more experiments this summer to determine just how much stored battery energy is needed to run a variety of RV appliances. So, stand by for science….
Back to the lab…
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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