Electric heater battery discharge rate…

A reader with 600Ah of Lithium batteries wonders why a 750-watt space heater is discharging his batteries so quickly (Rerun from Jan 2023)

Hey Mike,

I have 600 Amp-hours of lithium ion (actually you must mean Lithium Iron Phosphate - Mike) battery in my 5th wheel. In an attempt to save propane, I tried running a 750 Watt electric heater in the bedroom. Normally overnight the batteries drop from 95% to 82%. But by running the electric heater for just 4 hours they dropped to 41%. That seemed a little bit much. Does that amount seem correct? - Sonny

Dear Sonny,

It’s amazing just how much energy a space heater uses. And just as amazing is how much energy is in a 20 lb tank of propane. In fact, a single 20 lb tank of propane contains as much energy as 100 Lithium batteries that are 100 Ah each. Read my article on propane vs battery energy HERE.

Let’s crunch some numbers!

To figure this out we need to get everything into common energy units. While we think of battery storage in Ah (Amp Hours) of storage, that really doesn’t correlate very easily for 120-volt systems. So I’ll convert everything into Watt-hours (Wh) of energy. I’ll also covert everything to KiloWatt-Hrs (kWh) so we can use these calculations to compare costs to what your power company charges you.

600 Ah at 12.5 volts = 7,500 Wh (7.5 kWh)

That’s because Amps times Volts equals Watts. And a Watt-hour is 1 Watt of energy for 1 hour. So 100 Watts for an hour is 100 Watt-hours. And 100 Watts for 10 hours is 1,000 Watt-hours, which is the same a 1 kWh (Killowatt-hour).

750 watts x 4 hours = 3,000 Wh (3 kWh)

To find out how much energy your 750 watt space heater is using, simply multiply the Watts times how long it’s powered on. That works out to 3,000 Watt-hours of energy used from your batteries.

3,000 Wh / 7,500 Wh = 0.4 which is 40%

That shows us that running a 750 Watt space heater from a 600 Ah batter bank will use up 40% of the battery capacity. So instead of your battery SoC (State of Charge) being at 82% in the morning, we subtract an additional 40% of charge and the calculations predict your battery SoC will be at 42% because 82% - 40% = 42%. That’s within 1% of your actual discharge rate, so that’s a perfect prediction.

Electric heaters are power hogs

Depending on how much you pay per kWh for your residential power, a single electric space heater can easily add $40 to $80 to your electric bill per month. Yikes!

The same sort of BIG DRAIN happens to your RV batteries with Air Fryers, Toaster Ovens, Hair Dryers and even Crock Pots. While it seems like 600 Ah of batteries is a lot of storage (it really is), the amount of energy stored pales in comparison with the energy in a propane tank. Hope this helps you understand battery powering electric space heaters.

Let’s play safe out there - Mike