# Isaias is coming my way

Registered Users Posts: 378 ✭✭
edited August 1 #1
Lately Heat index here in nc has been 106.
my SO has breathing issues so ac is a necessity
Last hurricane sept 19 power was out for 2 weeks.
I was calculating what i could run with my 1700 2000 surge genny.
My 115 volt 10000 btu window unit list 8 amps and 825 watts?
I'm confused shouldn't 8 amps x 115 v be 920 watts?
My chest freezer 200 watts running.
50 watts 40" tv for weather updates
100 watt box fan on high speed
24 watts for 2 12 watt led lamps
11 watts for 2 5volt 2.1 amp phone chargers
385 watts plus ac 850 or 920
Say 1300 watts worst case 77%
Of 1700 watt generator rating..
Does this sound doable thought anyone?

Blue ridge mts. Renogy pwm 4 100 watt and 2 190 evergreen on Epever mppt 30. 4 Gc 208 ah @12 volts 300 watt psw inverter. 2 kw genny. Iota 45.

## Comments

• Solar Expert Posts: 792 ✭✭✭✭
edited August 1 #2
Test it so that you have time to decide if need to change anything!
I always have more questions than answers. That's the nature of life.
• Registered Users Posts: 378 ✭✭
.@Marc Kurth very smart thinking Thanks.
What's the deal with the ac amps vs wattage?
Blue ridge mts. Renogy pwm 4 100 watt and 2 190 evergreen on Epever mppt 30. 4 Gc 208 ah @12 volts 300 watt psw inverter. 2 kw genny. Iota 45.
• Super Moderators, Administrators Posts: 30,721 admin
AC "Math" is actually quite complex... An analogy:

Use a rope between two cars. If you have the two cars in line, and pull 500 lbs on the rope with the first car, the second car will also see 500 lbs pulling it forward too...

Second example is to have the first car 60 degrees off to the side of the first car (pulling rope at 60 degree angle). Cosine of 60 degrees is 0.5 or 1/2 of the force of the first car is applied to the second car pulling forward. If you still want to pull the first car forward with 500 lbs, you would need to put put 1,000 lbs of force on the rope to pull forward (the rope needs to be 2x stronger to do the same amount of "work").

Similar with AC power... You have a 60 Hz voltage sine wave. If the load is a resistor--The current will follow the sine wave exactly in phase. Cosine of zero degrees is
If you are, for example, driving an induction motor, the current will typically "lag" the voltage by some amount. That is the "cosine" of the phase angle between the voltage and the current. That Cosine of XX degrees we also call the "power factor".

A typical induction motor may have a PF somewhere between 0.65 to 0.80 or so... The math looks like:
• Power = Voltage * Current * Power Factor
That is the power/energy that is needed to "do the work" (i.e., power from battery, generator fuel, etc.). However, the wiring, transformer, generator/alternator/AC inverter needs to be "heavy enough" to move the "extra current" because of the Power Factor issue. We can use Voltage*Current or VA to represent the sizing of wiring/fuses/etc. to carry that "extra current".
•  VA = Voltage * Current
The name plate ratings of many appliances are difficult to use in accurately predicting performance/energy requirements. Sometimes they are "worst case" numbers--Other times, they seem to be "nominal" or average numbers. And the name plate rating almost never takes into account "surge current" (induction motors can take upwards of 5x running current for starting current).

Take the 8 amp @ 115 VAC A/C unit and 825 Watts:
• VA = 8 amps * 115 VAC = 920 VA
• Power = VA * Power Factor: PF = Power / VA = 825 Watts / 920 VA = 0.90 Power Factor
So, in this case, the wiring has to be 1/0.90 heavier because of the PF (1.11 x heavier wiring/AC inverter/Genset/etc.).

0.90 PF is not bad... Typically anything >=0.95 PF is considered pretty much "perfect PF" (1.0 is perfect).

There are things the manufacture can do to make for better PF... For example, they can put a properly sized capacitor in parallel with the motor and "correct" the power factor to ~0.95 (for other reasons, correcting to 1.0 PF can cause major issues with induction motors and capacitors).

Or the Mfg. can put an "inverter" between the input main power, and the motor/load. The input of the inverter is near 1.0 PF, and the inverter does what is required to run the motor (i.e., motor PF is not "seen" on the inverter's input connection).

https://en.wikipedia.org/wiki/Power_factor

This is not simple stuff... And separated Tesla from Edison (Tesla the math genius, and Edison the practical physical researcher):

https://en.wikipedia.org/wiki/War_of_the_currents

Using a Kill-a-Watt type meter which can measure Volts, Amps, Power Factor, Watts and kWatt*Hours can give you a pretty good estimate of what is happening on your power cord (estimate, still an inexpensive instrument doing the measurements, not a lab grade measurement system).

https://www.amazon.com/s?k=kill-a-watt+meter

So, for our needs... Use Watts to estimate battery power (rate of energy use: Watts) and energy usage (amount of energy used: Watt*Hours). And use VA to size the AC wiring/AC inverter/genset/etc.

Note: There are different types of meters out there... There are the "cheap" meters which make assumptions (like Sine wave AC voltage and amperage wave forms), and others that have more powerful chip set which can measure "arbitrary" wave forms (sine, square, pulses, etc.)... They are called "True RMS" reading meters.

https://www.fluke.com/en-us/learn/blog/electrical/what-is-true-rms

A standard meter only measures the "average" voltage or current waveform and applies a conversion factor for Sine Waves (i.e., Peak Voltage * 1/1.414 (or sqrt of 2)) to convert peak sinewave voltage to the "true RMS" value of the voltage:
• 120 VAC * 1.414 conversion = 170 Volts sine wave peak
Or, work wise (Watts, Watt*Hours) a 170 Volts peak sine wave does the "same amount of work" as a 120 Volt DC power source when driving a resistance heater.

A True RMS meter will "sample' the wave form something like 50,000 times a second (for a meter used on 60 Hz power lines) and "do the math" to estimate the true RMS value of the signal (RMS=Root Mean Square==Basically the "average power" under the wave form==I.e., Power=V^2/R -- That is why "square root" average, and not regular averaging).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,278 ✭✭✭✭✭
Maybe it will miss you?  For the next one you really need to deal with this in a realistic manner. A larger genset or an inverter based variable speed cooler. There will be no surge and all of your loads can run with some sanity on your part. The small LG unit is still out of stock but if this one will "fit" I would take a look at it. Once you have lived with an inverter based cooling, there is no going back.

https://www.amazon.com/dp/B08677DCKN/ref=sspa_dk_detail_0?psc=1&pd_rd_i=B08677DCKN&pd_rd_w=xu9Dq&pf_rd_p=48d372c1-f7e1-4b8b-9d02-4bd86f5158c5&pd_rd_wg=ybdMB&pf_rd_r=4F63KJTDBTGHMWW7XDV7&pd_rd_r=dfddcf2f-8eea-407f-a00e-9da0d03e0294&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFQT1RFMU9RSjRaT1kmZW5jcnlwdGVkSWQ9QTA0OTMzOTEyWFBSTEk0U01EU1omZW5jcnlwdGVkQWRJZD1BMTA0MzkwNjFLUTMyUzhaWTlTRE0md2lkZ2V0TmFtZT1zcF9kZXRhaWwmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Solar Expert Posts: 8,864 ✭✭✭✭✭
It's likely too late now, but harbor freight has some pretty decent gensets.   To account for Power Factor (AC watts, VoltAmps) take your calculated V * A  = W  and multiply by 1.3, which will give a closer VA for sizing a generator.  if one chiller is running, you have to have enough generator overhead to be able to start the 2nd cooler when it starts up (the compressors cycle on and off pretty frequently).
For general house loads, I'd stick with old fashion alternator gensets, and skip the inverter generators, you don't need super clean power for an air conditiooner.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 1,292 ✭✭✭
edited August 4 #7
You have to get past the air cond starting surge first.  Unless it is an inverter mini-split unit. For 120vac 10k btu likely about 40-45 amps.  There are booster starter cap with softstarter that can knock it down by 60-70%.  See Micro-Air EasyStart.

I took my 240v 48k btu central air from 200 amp startup surge to 73 amps.  Run current is 14 amps.  I run it on two old SW5048+ units that have the needed surge capabiliity.
• Registered Users Posts: 378 ✭✭
Cat 1 hurricane. Almost a direct hit.
And the power is out.
1700 watt old style genny running 10 btu ac a box fan and a couple 12 watt led lamps. It sounds like its loaded.
Come day light I'll see how much damage there is.
Blue ridge mts. Renogy pwm 4 100 watt and 2 190 evergreen on Epever mppt 30. 4 Gc 208 ah @12 volts 300 watt psw inverter. 2 kw genny. Iota 45.
• Solar Expert Posts: 792 ✭✭✭✭
Cat 1 hurricane. Almost a direct hit.
And the power is out.
1700 watt old style genny running 10 btu ac a box fan and a couple 12 watt led lamps. It sounds like its loaded.
Come day light I'll see how much damage there is.

Good to hear that you are up and running! Hope the damages are not too bad.
Marc
I always have more questions than answers. That's the nature of life.
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