AC vs DC lighting efficiency

Re: AC vs DC lighting efficiency

newl wrote: »

Curiosity exercise here.

Situation:
12VDC LED strip lighting drawing 2.83A = 33.96W (measured from PWM controller on pass through)
240VAC lamp drawing 0.08A = 19.2W (calculated), 13W measured with inline watt meter (akin to kill-o-watt).

Knowing the inaccurate nature of the inline meter and losses due to the inverter and the possible calculated value of the PWM controller, how close are the two of these in terms of consumption? I'm wagering on the lamp.

Take the AC lamp--Actually both numbers can be correct. With AC, we have "power factor" to take into account. PF can be calculated by measuring the phase angle between the AC voltage and AC current wave forms. Plug a induction motor (inductive load) on an AC main, the current sine wave will be delayed from the voltage wave form by some number of degrees. Power Factor = Cosine (degrees delayed).

When the current is delayed from the voltage, the farther the delay, the less power the current will provide.

Sort of like pulling on a rope tied to a car--If you stand in front of the car, 100% of your pulling force is used to move the car forward. If you stand 60 degrees to the side, then Cos (60) = 0.5 PF -- Or only 1/2 of your pulling force is moving the car forward.

And for other circuits (like electronic power supplies), many just take current at the peak of the sine wave voltage. The "current efficiency" can be represented as power factor too (different/more complex math).

So, for the 240 VAC lamp:

Power = V*A*PF
PF= Power / (V*A) = 13 watts measured / (240 VAC * 0.08 A) = 13 watts / 19 VA = 0.68 PF

And that 0.68 PF is not unusual for CFL or LED lamps... Note I used the term "VA" -- the wiring and AC inverter need to be designed to carry the current (VA), and the battery bank (DC Side) will be discharged at 13 Watts (=19VA*0.68PF).

AC power is actually quite complex math--And if you have more questions, please ask. Some of this is actually quite important when designing AC Off Grid systems.

Similar issues with DC power systems. Many DC "electronic" devices do not take DC as pure current... There are switching power supplies that can also cause "pulse train" DC wave forms too--And there are VA (PF) issues too.

For example, remember one of the power equations is P=I²*R -- If you double the current (due to "poor" power factor), then the "self heating" due to resistance goes up 4x (and other effects too).

Sorry for going all over the map here--I am not sure if I am answering your question, and how much experience you have with power and the math.

-Bill

Re: AC vs DC lighting efficiency

The sad thing is that efficiency (and power factor)--And even specifications--All the "variations" add up...

For example, the standard derating for charging from solar during the day and running AC loads at night:

0.81 solar panel derating from STC * 0.95 typical MPPT charger eff * 0.80 typical flooded cell charging eff * 0.85 typical AC inverter eff = 0.52 end to end system efficiency

So--we are talking about almost 50% losses between that shiny solar panel marketing spec., and those electrons being consumed by your AC loads.

Why? Hot panels lose ~20% of their output voltage (all panels get hot under the sun--especially on nice summer days). A typical MPPT controller will average around 95% efficiency (less if you oversize the controller).

For PWM controllers, they are "more efficient" but because the way a solar panel (current source) interacts with a battery (voltage source/sink), there is also ~0.77 losses because of "miss-matches" in Vmp-array vs Vbatt-charging (just the way the physics/electrical engineering works out).

Lead Acid Batteries discharge at ~12 volts and recharge at ~13.5-14.5 volts--That is an energy loss. There are ways to operate a flooded cell battery bank at >90% efficiency--But most of us don't do it, or cannot really do it most of the time. You can get AGM batteries which are >90% efficient and can be almost 98% efficient. However AGM batteries cost 2x as much and (usually) do not last as long as comparable flooded cell batteries.

AC inverters--You run them at 40-70% load or so, and they can be >90% efficient. You run them at 100% load and they can be 80-85% efficient. Run them at 10% load, and you may get 65% or less efficiency... It all depends.

Add PF--Which is not great for induction motors (~0.67 PF is common) and poor PF for our "fancy lighting" (many CFL's have around 0.50 PF--"waste" 1/2 the AC current when making light)... Much of what we end up doing with off grid solar is conservation and efficiency improvements (i.e., buying smaller/energy start rated equipment) and/or using Kill-a-Watt type meters to figure out what is really happening with our AC loads (Name Plate ratings are almost useless for system sizing).

Energy usage is a highly personal set of choices... For many people who have never tried conservation before--Reducing power usage by 1/2 is not impossible.

In general, the rules of thumbs allow us to estimate/design a system that will, most likely, meet your needs very nicely--We try to be a bit on the conservative side and not bother with the detailed analysis--It gets confusing, there are all sorts of shifting values/efficiencies--Which, in the big picture--Can usually be just tossed into the overall rule of thumb design/efficiency numbers for "close enough".

Add that the energy received from the sun can easily vary by 10% or more from one year to the next, and nobody can predict when those 1 week of storms knock your solar panels out of the picture (with no sun)--You still have to have that backup genset or simply stop using most of your electricity when solar is not available.

We get very used to turning on a switch with reliable utility power... And we byte the bullet when the bill comes. Off Grid solar power will cost (out of pocket) something like 5-10x the cost of utility power (long term 20 year system life). All of a sudden, those conservation plans and electrical power usage/efficiency numbers really attract your attention.

-Bill