Electricity Question for you experts: Which is better high watts at a short time, or low watts for a longer time?

[Santa]

I am considering my electric loads. I would like to know, IF one were to use an electrical heater, for instance, and there are 2 settings, either 800 watts or 1000 watts, and if you were to use the higher setting for say, 5 minutes, or use the lower setting for 7 minutes, which would be more energy efficient?

 

[Ped]

What kind of batteries?

 

[Santa]

What kind of batteries?

Well, at this time, AC (shore) power, but my system I am making is (2) 100Ah battleborns.

 

[Ped]

Then idk. Prolly be hard to theorize. I would do an actual test with a meter.

 

[Santa]

Then idk. Prolly be hard to theorize. I would do an actual test with a meter.

I need a killowatt meter then? OK, it was on my list, thank you!

 

[Mars]

Low wattage for longer time. System losses are based on I^2 x R. If you cut the current in half, the system losses are cut by a factor of four.

I need a killowatt meter then? OK, it was on my list, thank you!

Ask Santa for one for xmas.

Mars

 

[Santa]

Low wattage for longer time. System losses are based on I^2 x R. If you cut the current in half, the system losses are cut by a factor of four.



Ask Santa for one for xmas.

Mars

OK, thank you! lol

 

[krby]

Almost all batteries do better with a lower power draw over a longer period of time. So, using 1kWh over 1hr (literally 1kW for one hour) will use up more battery capacity than 1kWh for two hrs (0.5kW for 2 hours). This effect is pretty dramatic with lead acid batteries, less so for LFE batteries, but it's still there.

Your example is tougher because you're using more watt hours in the "800W for 7 min" scenario. You'll really have to test it.

 

[Ped]

Well theres also heat dissipation.

 

[SCClockDr]

Well theres also heat dissipation.

Provide all the system components are within the heated envelope heat losses in the circuitry actually help when heat is needed. When cool is needed not so much though.

 

[SolarRat]

In a properly designed system with quality components it shouldn't matter one bit. IOW, a system that needs to draw 800-1000w should be designed for 2000 watts. Half that load shouldn't even make it sweat. 200w difference would be peanuts. But if you undersize your system (to say 1000w max), then both are bad..

 

[SolarRat]

Anytime you are looking to heat something with electric, it's always more efficient to use a very low wattage device to heat a heavy block of metal or concrete slowly and have it radiate the heat. For many years we'd use a simple 60w incandescent bulb in our boat's engine room to keep things from freezing while the boat was still in the water, or when it was out of the water and we didn't want to winterize the engine for one reason or another.

You'd be surprised at how warm a small and well insulated <100sq ft shed/room gets with just a 60 or 100w bulb lit. And if you put that bulb in the center of some cinder (concrete!) blocks or similar, the heat will radiate for hours even after the light is switched off. If you can get the sun to do some of the heating of said cinder blocks during the day, it will be much easier to keep it hot at night with the bulb.

So if this is to keep batteries warm...think outside the box.

 

[Santa]

Anytime you are looking to heat something with electric, it's always more efficient to use a very low wattage device to heat a heavy block of metal or concrete slowly and have it radiate the heat. For many years we'd use a simple 60w incandescent bulb in our boat's engine room to keep things from freezing while the boat was still in the water, or when it was out of the water and we didn't want to winterize the engine for one reason or another.

You'd be surprised at how warm a small and well insulated <100sq ft shed/room gets with just a 60 or 100w bulb lit. And if you put that bulb in the center of some cinder (concrete!) blocks or similar, the heat will radiate for hours even after the light is switched off. If you can get the sun to do some of the heating of said cinder blocks during the day, it will be much easier to keep it hot at night with the bulb.

So if this is to keep batteries warm...think outside the box.

Reminds me of when I used to make (ferment) yogurt. I put the starter yogurt with milk in the oven at night with just the oven light on for 12 hours and it warmed it enough to have fermented yogurt in the morning. Interesting idea. ^ How long would the light have to be on to heat the cinder blocks and make a difference?

 

[SolarRat]

... How long would the light have to be on to heat the cinder blocks and make a difference?

The light itself heats the space immediately. The light also heats the block which is just a free bonus (heating just air is a waste). How long it takes depends on many factors. It doesn't have to be a light and it doesn't have to be a cinder block...it could be a heating pad that's heating anything with mass. Heating something with some mass is way more efficient that just heating air, for long-term heat anyway.

Using sunlight (or fire, hehe) in creative ways to also heat the "block" goes a long long way towards reducing the amount of battery necessary to keep that block warm.

Think of how a wood burning stove works...the heat from the fire is heating the mass of iron, which in turn keeps the house warm long after the fire is out.

 

[Supervstech]

How large a space are you trying to heat with the electric heater?

If 1000 watts in 1 hour will raise the temp 1 degree, you need more heat.
If 1000 watts in 1 hour raises the temp 40 degrees, you could do the same with 100 watts over 4 hours... but it needs to be able to maintain the temp desired.

With utility power... 1000w over 1 hour is exactly the same as 100 watts over 10 hours... no difference at all.
Resistive heat is 100% efficient.

Battery usage is different but not a huge amount.

I think you should size the heater based on the absolute minimum needed to achieve the desired temp.

 

[Santa]

How large a space are you trying to heat with the electric heater?

If 1000 watts in 1 hour will raise the temp 1 degree, you need more heat.
If 1000 watts in 1 hour raises the temp 40 degrees, you could do the same with 100 watts over 4 hours... but it needs to be able to maintain the temp desired.

With utility power... 1000w over 1 hour is exactly the same as 100 watts over 10 hours... no difference at all.
Resistive heat is 100% efficient.

Battery usage is different but not a huge amount.

I think you should size the heater based on the absolute minimum needed to achieve the desired temp.

Well, I am currently in the sticks and bricks and (only?) have several electrical sources for heat. (In my vehicle I will use a diesel heater.) I was just trying to see how I could save on electricity since they charge an arm and a leg. I only use the heaters less than a foot away from me in whatever room I am in so I can get warm and then turn it off.

 

[JeepHammer]

How large a space are you trying to heat with the electric heater?

If 1000 watts in 1 hour will raise the temp 1 degree, you need more heat.
If 1000 watts in 1 hour raises the temp 40 degrees, you could do the same with 100 watts over 4 hours... but it needs to be able to maintain the temp desired.

With utility power... 1000w over 1 hour is exactly the same as 100 watts over 10 hours... no difference at all.
Resistive heat is 100% efficient.

Battery usage is different but not a huge amount.

I think you should size the heater based on the absolute minimum needed to achieve the desired temp.

How much 'Heat' you need overall will depend on what the temp DIFFERENTIAL is you want,
AND,
How efficiency the space is insulated against heat loss.

*IF* the space is loosing heat as fast as you make it, zero sum gain, the space doesn't heat.
*IF* you put 100 Watts into an electric blanket under other covers, that's a very small, well insulated space and you get more use of the 100 Watts.
*IF* you make 100 Watts of resistance heat in a larger space, little felt effect since it heated the atmosphere (air).
*IF* you make 100 Watts in the infrared spectrum (light) you will feel it on your skin, the infrared light has to hit your skin, cloths, furniture before it changes frequancy and sheds it's energy as heat.


A 'Watt' is a unit of energy,
Doesn't matter if it's electrical, mechanical or thermal.
Petroleum engines are rated in kW, along with Horse Power & Torque,
While we are use BTU (British Thermal Unit) for heat, it's also rated in kW...

 

[JeepHammer]

While it's true one 'Watt' will convert from electrical energy into thermal energy,
It's the crap fight you go through to get and store that 'Watt' in the first place that makes electrical heating cost ineffective using solar panels (PV).

From sun capture costs, through charge controller losses, through battery cost and losses, through inverter cost & losses (as heat, but the cost is still there),

Fossil Fuels are still more cost effective at making heat directly though combustion.
Petroleum, Propane (LP/LPG) and natural gas will make a CRAP LOAD of of moisture when the burners aren't vented. Every gasoline & diesel heater will be vented...

For your van, YOU need to decide, there is a reason the motor homes are so expensive with furnaces, air conditioners, batteries/inverters, generators, etc.

 

[Deleted member 783]

Water is the most efficient thing to heat if you want the heat to dissipate over the evening. Sealed barrels line the walls of my greenhouse. It makes an amazing difference.

 

[JeepHammer]

Water is the most efficient thing to heat if you want the heat to dissipate over the evening. Sealed barrels line the walls of my greenhouse. It makes an amazing difference.

Thermal Mass.
In that case, you would be better off with Solar Thermal panels and a 'Mass' Tank/Pump in the RV...
Evacuated Tubes (Double Walled Glass With Vacuum In Between) will produce water that will SCALD you!