Run A/C on a generator or on an inverter with generator powering a charger?

jcheiljcheil Solar Expert Posts: 722 ✭✭✭
I was wondering if anyone had ever done any research or an experiment regarding the following:

I an off-grid setup, occasionally I would like to run a small 5000BTU A/C unit at night. The unit draws around 550w when the compressor is on and around 100w when it is off. I would think it would be safe to assume that it is pretty much 50% runtime with the compressor on. I will run it with the KAW meter next time I need it just to find out for sure. Also I realize the runtime will be affected by the time of year but that is not a factor in my question.

The question being, would it better (fuel efficiency wise) to run the A/C directly off of a small 1400/1800w generator (non-inverter generator) or simply have the generator power a battery charger and allow the A/C to run off the inverter? I know most (non-inverter) generators like mine have a break-even point at which they are going to use the same amount of fuel if you have no load vs. a certain load somewhere in the middle of their rated maximum. I have no idea where that point is on my generator (wish there was a way to find out) but I know that while running that A/C by itself, it will run about 10ish hours on the 1.4gallon fuel tank.

The charger in question (proposed) would be the IOTA DLS-27-15 which seems to pull about the same amount of power as the A/C when the compressor is running when the charger is at full output with low batteries; and obviously much less when the batteries are closer to being charged.

I guess I am thinking down the lines that if the generator has to be running, I might as well put it to some “other” use, if I can keep it in that “window”; but not so much that it increases fuel consumption. I rarely, if ever, need to run the main (DLS-27-40) battery charger anyways (have plenty of panels and sun) and if I ever do, it has to run on my larger 3500/4000w generator because of the load draw from it. And neither the A/C nor the chargers are power factor corrected which I know just adds to the problem.

So again, I was wondering if anyone had any experience with this, or even some theories on it.
Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Run A/C on a generator or on an inverter with generator powering a charger?

    Oh, yes. A few of us have plug the window rattlers and other types of A/C into the K-A-W and got the horrible results. Your running Watts and fan-only Watts are pretty much spot-on.

    Now here's the thing: if the generator is going to have to run anyway, you may as well power the A/C from it directly. It's more efficient that way. The other way you get losses when the power is dropped down to DC and then back up to AC.

    Think about this consumption: 500 Watts half the night or 4 hours = 2 kW hours. That is quite a lot of power for off-grid. Do you really want to run a gen all night? Even an inverter-generator will produce some noise (more noticeable at night) and consume fuel. Maybe you could run it beforehand and "pre-cool" your sleeping area?
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Run A/C on a generator or on an inverter with generator powering a charger?

    A lot depends on your over all system, You can put your A/C on a 24V thermostat and Relay and put the charger on the relay N/C side and the A/C on the N/O side and let it flip flop back and forth ( you might have to add a time cube delay on make ). That way you get the most out of the generator, the charger charges on the A/C off cycle.

    I have mine on a Inverter with Generator Support that will start the load and it cut's the charger back during the A/C cycle, but it will also carry the refrigerator on pass through at the same time. On a Honda EU 2000 it stays in the ECO throttle idling.
  • BB.BB. Super Moderators, Administrators Posts: 31,171 admin
    Re: Run A/C on a generator or on an inverter with generator powering a charger?

    I believe that running with a small Honda eu2000i/similar Yamaha inverter-genset directly will be, overall, more fuel efficient vs using a battery charger+genset.

    0.80 charger efficiency * 0.80 battery efficiency * 0.85 AC inverter eff = 0.544 = ~54% worst case efficiency

    The system in your signature line looks pretty good sized... Do have you have an estimate of how much power you could harvest in a day vs your current daily loads?

    Also, you have a choice where to put your money... Insulation + Mini-Split A/C system may cost you about the same (conservation) vs having to increase the size of your system for the extra loads.

    In general, if these are short term random usage of A/C--Then a small inverter-genset is probably the way to go (low capital costs and a gallon of gasoline per night when needed.

    If your usage is ~9+ months of the year (humidity control in Florida)--Building the system out to support the A/C loads is probably worth it.

    If you need to better integrate your generator with your off grid system--Generator Support mode (in some newer AC inverter-chargers) may be worth it. Basically, you program the inverter-charger for the maximum AC amps from the generator. The Inverter-charger will split current from the generator to AC loads and battery charging. If the AC loads exceed the AC Generator current limit, the AC inverter-charger will draw battery power and support the AC loads.

    But generator support assumes (at a first guess) that you are willing to operate the generator for many hours per day during 6 weeks of summer to support short term/higher power loads (A/C system)... If you do not want to run a genset that much--Then I would not rework your system for this capability.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: Run A/C on a generator or on an inverter with generator powering a charger?

    Well, I always have an excess of power. I average using about 6-7kwh per day.
    Usually, I am floating by no later than 3pm each day, after a 4hr absorb time.

    I can actually run that 5000BTU A/C all day long and still not consume all that I am producing, however at night, I am not sure that I would want to risk running it even though I have 860ah of batteries and technically I believe I could and still maintain half way decent DOD. I just always error on the side of caution and never try to allow my batteries to get below 70% SOC.

    And as I said, I can count on one hand the number of times in the past YEAR I have had to run the generator to actually charge the batteries (because they were at *my* level of concern at 70% SOC) so I can't see the investment in a larger inverter-generator with auto-start.

    And like someone mentioned, Florida, Humidity, that is the killer. So even though sometimes it is cool like in the 70's at night, the humidity makes it really sticky trying to sleep. So pre-cooling helps, but midway thru the night the humidity comes back; thus the need to be able to run it at night.

    And I dare to take this thread off topic, but from what I have seen, the inverter-generators pretty much use the same amount of fuel as a non-inverter generator when you have a "larger constant load" on them (smaller loads I agree they will be better). For example, I have a simple non-inverter generator 1400/1800 $199 and runs that a/c for 10ish hours on 1.4 gallons and is pretty quiet. My neighbor has the small Honda inverter generator (1000 I think?) $900 and he runs the same unit for 8 hours on 1.1 gallons. So doing the math they are pretty much neck and neck in fuel consumption. Yes his is a "little" bit quieter, but $700 worth of quiet? Nah :)
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • BB.BB. Super Moderators, Administrators Posts: 31,171 admin
    Re: Run A/C on a generator or on an inverter with generator powering a charger?

    With good deep cycle batteries, you might want to let them get down towards 50% State of Charge before looking to cut loads/turn on genset.

    In general, the batteries will output the same number of kWH over time (i.e., draw 20% more power from them, they will "wear out" 20% sooner). So, the price of battery per kWH vs the price of generator+fuel+maintenance.

    Roughly, the overall costs are probably very similar. You either pay a big chunk of cash every X years, or pay a smaller amount of generator fuel every few days/weeks.

    I agree a standard genset running >50% rated load probably is more fuel efficient vs an inverter generator. Running the genset on a variable load with "eco throttle on" with average loads down in the 25% average rated load range, the inverter-generator will be more fuel efficient.

    The huge costs of fuel are, generally, when somebody is running a 8+ kWatt generator and has typical ~2 kWatt or less loads.

    Noise is always a personal issue--If your property is large enough and you can isolate the genset--You will do fine. If you are in a city/urban area and have smaller lots--with emergency backup power--The low noise gensets are generally worth it (at least too me).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Run A/C on a generator or on an inverter with generator powering a charger?

    You could put the A/C on a Voltage controlled switch that would prevent it from running if the batteries got too low based on Voltage (not the most accurate SOC indicator, but the easiest to deal with). That way if the batteries did start to go low at night the A/C would be shut off first, and everything else could be kept running 'til morning.
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