Battery Equivalents and Replacements

Off-the-Grid: How Much Electricity Does a Space Heater Use?

Space heaters convert electric energy directly into heat, making them rather inefficient but simple and affordable units, commonly used for warming up tents, RV, and similar objects when being off-the-grid, but also for quick warming up of rooms and other areas at homes and apartments.

Since they entirely rely on electric energy, many people wonder how much electricity a space heater use and how they can power such devices while being outdoors.

Updated: May 2, 2023.

space heater 1

Electric Space Heaters Features and Specifications

First of all, electric space heaters are not the only way one can warm up a tent, RV, boat cabin, and similar living/sleeping areas while being off-the-grid.

Propane, natural gas, gasoline/petrol, and even diesel-powered heaters are often used for warming up areas, and they do it well.

However, such space heaters burn fossil fuels and deplete oxygen from the air in a warmed area that has to be ventilated often. While such devices often come with low oxygen meters/alarms, carbon monoxide poisoning is also possible while oxygen levels are still 'safe.'

For short: I would never use a space heater powered by fossil fuel indoors, regardless of how cold it is. IMHO, of course. And now, back to electric space heaters.

Electric space heaters directly convert 1Wh of electric energy into 1Wh of thermal energy - 1Wh of electric energy converted into heat equals ~3.412 BTU of heat. Similarly, 1 kWh (kilowatt) or electric energy equals ~3412 BTU of heat.

Note: modern Air Conditioning units feature heat pumps (inverters), allowing them to be much more efficient than ordinary electric heaters. If You are in an RV with an AC unit and you also have an electric space heater, keep yourself warm using an AC unit - your batteries/fuel will last longer.

Electric space heaters are commonly rated between 500 and 2000 watts, although there are many models outside of this range.

The following chart lists the required current (in Amps) for the most popular 120V AC space heaters:

Power 500W 750W 1000W 1250W 1500W 1750W 2000W
Current ~4.2A 6.25A ~8.4A ~10.4A 12.5A 14.6A ~16.7A

For example, if one has a 1500W electric space heater and wants to power it for 8 hours (for example, during the night), then this space heater will convert 12kWh (1500W * 8 h = 12000 kWh) of electric energy into heat energy.

Also, it is good to know that 1500W equals ~5100 BTU/h, and 12 kWh equals ~41000 BTU.

So, if You want to know how much will your electric bill increase for running a 1500W electric heater for 8 hours while being at home and using mains power, just multiply 12 kWh by your local price for electric energy, which is usually given in (euro)cents per kWh.

$$$ = 1500W * 8h * 0.15 $/kWh = 1.8 US$

Note: we assumed an average price of 0.15 US$/kWh of electric energy.

This doesn't sound much, but if you run a 1.5kW space heater for 8 hours, 30 days in a row, that is a ~54 US$ increase in your electric bill.

Similarly, to find out how much electricity does a space heater use in 24 hours, we will use the following formula:

$$$ = 1500W * 24h * 0.15 $/kWh = 5.4 US$/24h

Again, we assumed the average cost of electricity to be 0.15 US$/kWh.

So, if you run your 1500W space heater using mains power at a rate of 0.15 US$/kWh, that will cost you 5.4 US$/24h. IMHO, this is not cheap...

How to Power Electric Space Heater While Being Off-the-Grid?

There are several options for powering electric space heaters while being off-the-grid, but the most reliable method is producing the electricity using a small power generator powered by gas (gasoline, petrol) or propane/LPG gas.

Battery Pack/Power Inverter vs. Portable Inverter Generator

In order to power a 1500W electric space heater for a period of 8 hours, one needs a good 2000W power inverter and a large enough battery pack.

Our electric heater requires the following:

E(heater) = 1500W * 8h = 12000 Wh

Power inverters are not 100% efficient units - most popular units feature energy efficiency of 80-85%, thus requiring the battery pack to be able to store at least:

E(battery pack) = E(heater) / Power Inverter Efficiency = 12000 Wh/0.85 = ~14100 Wh = ~14100 VAh

Since most battery packs are made using 12V batteries, a battery pack with an energy capacity of 14100 VAh, would need a capacity of 1175 Ah.

In order to achieve such capacity, one has to combine large batteries in the battery pack.

mighty max ml4d agm tableFor example, when being discharged for 8 hours, the Mighty Max ML4D AGM Deep Cycle Battery features an actual capacity of ~165 Ah.

Thus, in order to create a battery pack with an energy capacity of 14100 VAh, or 1175 Ah at 12 volts, one would need 8 such batteries.

A battery pack consisting of eight Mighty Max ML4D batteries would be able to provide the following:

E (battery pack) = 8 * 12V * 165Ah = ~15800 VAh

which is more than the required 14100 VAh.

So, such a battery pack can power a 1500W space heater for 8 hours using an 85% efficient power inverter.

Issues? Well, eight such batteries weigh ~917 pounds (~415 kg) and that is a lot to pull around when going camping, for example.

The weight of the battery pack can be decreased using deep-cycle lithium batteries, but the price of such a battery pack would be very high.

But, what about portable inverter power generators?

honda eu2000i eu2000ic mPortable power inverter generators must operate outside since they burn fuel in order to produce electricity. Such units are very popular and commonly used when camping, in emergencies, and whenever one requires electricity while being off-the-grid.

For example, one of the very popular 2000W units is the Honda EU2200i which features 1800 running watts with a maximum output of 2200 watts.

Tank capacity of 0.95 gallons (~3.6 liters) ensures a running time of 3.2 hours @100% load (1800W) and ~8h @25% load (450W). Also, the unit is very quiet, with noise levels of 49 dB @25% load and 57 dB @100% load.

Honda EU2200i weighs 46.5 pounds (~21 kg) without fuel. To power a 1500W strong electric space heater for at least 8 hours, Honda EU2200i requires ~2 tanks of fuel, perhaps slightly more - let's say 2.5 gallons (~9.5 liters) which is ~16-17 pounds of fuel.

So, the combined weight of Honda EU2200i and fuel required to power a 1500W electric space heater for 8 hours is at most 64 pounds (~29 kg) which is far less than ~917 pounds (~415 kg) of the battery pack consisting of 8 Mighty Max ML4D batteries.

How Strong Space Heater Is Needed?

On average, 10W is enough for 1 square foot, although this depends on many details, like thermal insulation, draft, outdoor/indoor temperature difference, and similar.

So, on average, a 1500W space heater is strong enough for a 150 sq. ft. (~14 m2) room.

Again, it is not the same to try to warm up a house, RV, or tent. But, if you are in an RV with a very efficient Air Conditioner, use it, and keep the space heater as a backup.

Long Story Short: when You are connected to mains power, a 1500W electric space heater doesn't use much electricity, just 1500Wh (1.5 kWh) per hour.

However, when being off-the-grid, it is not an easy task to provide such power for a longer period of time - the best possible option is using a portable inverter generator that directly powers a 1500W (or similar) electric space heater.

Again, never, but really never operate power generators indoors - carbon monoxide is a toxic, tasteless gas that is very poisonous to people and animals.