Measuring Wattage at different voltages

[Menticol]

Hello guys

My electronics knowledge is very corroded, I'm having troubles doing some simple calculations.

Let's suppose I have a 120V to 12V stepdown transformer, connected to a 12V 50W bulb.

The load draws W / V = A => 4,16 A at 12V

But how many amps does the transformer draw at 120V?

Another example: I have a solar cell capable of delivering 235W at 29VDC. How many of these cells do I need to power up, lets say, a 120VAC 50W load (using an inverter)?

I imagine that the efficiency of the transformer and the inverter must be taken in account, but I don't know how

Thank you in advance

 

[RCinFLA]

If the transformer is of reasonable quality and sized properly it should be greater then 98% transfer efficient. 50W/0.98 = 51 watts /120vac = 425 mA rms.

Your panel is a group of series connected cells. Silicon cells produce about 0.52 v per cell at maximum power point at 25 deg C. The cells produce about 35 mA per square centimeter at full sun (1000 W/m^2) for good quality cells. If your panel is max power point spec'd at 29 vdc there is would be about 56 series cells per panel. A pv cell is an illumination based current source so for a 235 watt, 29 vdc panel it would produce 8.1 amps at maximum sun illumination. You should not be powering the lamp directly from PV panel. Use an MPPT charge controller to charge a 12v lead acid battery and run the light from the battery.

 

[Menticol]

Thank you so much RCinFLA!!

I have forgotten the battery bank, every post of solar power talks about it.

Case 1:

According to your information, the formula is

(Load in W) / (efficiency in % * 0.1) / input voltage = Load consumption in mA at input voltage ?

Case 2:

Lets say I connected my panel to the battery bank, and the bank to the power inverter. In conclusion, the inverter capabilities will be determined by the battery bank, not the solar panel, right?

 

[KeepItSimpleStupid]

I think you mean "capacity" for "capabilities".

Capacity is part amount of sun, solar panel, battery, inverter, load and efficiencies.

Everything above creates issues.

The "charge controller" manages charging the batteries from the solar array. It has a lot of work to do such as managing max power transfer, battery temperature to eliminate overcharging, Low voltage disconnect, Overvoltage protection etc.

The load and how long it's on is the starting point. Batteries need to be sized primarily by how long the load can be supplied with no sun. The array can then be sized to be able to charge the batteries. A backup system may or may not be created to charge the batteries when there is no sun.

 

[bountyhunter]

Hello guys

My electronics knowledge is very corroded, I'm having troubles doing some simple calculations.

Let's suppose I have a 120V to 12V stepdown transformer, connected to a 12V 50W bulb.

The load draws W / V = A => 4,16 A at 12V

But how many amps does the transformer draw at 120V?

Another example: I have a solar cell capable of delivering 235W at 29VDC. How many of these cells do I need to power up, lets say, a 120VAC 50W load (using an inverter)?

I imagine that the efficiency of the transformer and the inverter must be taken in account, but I don't know how

Thank you in advance

What kind of AC are you running across the transformer? You can't simply do a power calculation, you also have to know power factor to calculate the AC RMS line current.

 

[Menticol]

I think you mean "capacity" for "capabilities". Capacity is part amount of sun, solar panel, battery, inverter, load and efficiencies. The load and how long it's on is the starting point.

You are right KIS, I'm sorry English is not my first language.

The load and how long it's on is the starting point.

I'm part of an Industrial Design team and we are proposing hypothetical scenarios for a mobile dental care unit.

The following are some of the loads (there are much more, but is just a sample to illustrate you the problem)

5 x Halogen lamp - 12V - 50W
5 x Electric Compressor - 120V - 920W
1 x Autoclave - 120V - 1000W (just a resistive load)
+ 5 x Small Computer - 120V - 100W
__________

= 2070 Watt running for 7 hours


Notes:
1) I noticed that the lamp cosumes 50W @ 12V, not 120V, so I would like to learn how to do the correct calculation
2) I didn't took in account the starting current of each motor, 225% during a couple of seconds according to a source.
3) I followed bountyhunter's advice and learn about power factor. For complexity and time reasons, let's assume that PF is 1.

The problem:

1) I suggested the team to use 5500W diesel generators. But another member is proposing solar technology. I find her idea non practical for a mobile unit working on a country with very variable weather and topography. But in order to prove my point I should do all the calculations for the total power consumption, then compare the numbers with the (solar cell+battery+inverter) capacity under ideal conditions (as RCinFLA noted)

If anyone can explain me how to do the above calculations correctly, I would be immensely grateful

PS: I understand all the concepts of capacity, Amperes / hour, C unit for charge and discharge, etc.

 

[Nigel Goodwin]

1) I suggested the team to use 5500W diesel generators.

You are obviously the sensible one in the team, that would be an excellent option, and probably the cheapest, most reliable and most viable.

As for calculating the loads, just add the wattages up - it doesn't matter what voltage they are, watts are watts (the halogen lights use 12V bulbs, but will have a 120V transformer feeding them).