Winter is near and I want to be able to run my Trane HVAC 1 hp motor and my Dell abd Mac desktop computers off an inverter or generator. Should I get something that puts out a pure sine wave or can I safely use less expensive, modified sine wave equipment?

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    You may want to start here – Kris Dec 16 '15 at 2:48

the laptop wont care if its modified sine wave power but if you plan on running an ac get a generator the trane is using a 1 hp capacitor start capacitor run induction motor, it requires a true sinewave to run using modified will ruin it and void the waranty. also truesine inverters are very costly. you should be fine using a 25kw generator.

  • The computers are desk top models. – Yehuda_NYC Dec 16 '15 at 2:56
  • 25kw is gross overkill. I run my whole house on 70 amp 220v service. That equates to 140 amps at 110v, which is much less than 25kw. – Yehuda_NYC Dec 16 '15 at 2:58
  • you are forgetting that the starting inrush of a 1hp induction motor is quite significant. so while you can get by on 10kw under running conditions. it will overload the geny during startup of the central hearing/air unit if you have a heatpump/ac the outdoor unit requires a 30A 240v circuit. and the indoor unit will be either a 15 or 20A 120v circuit the combined running load for both will be 9kw and easily twice that during starting so 25kw is not overkill if you expect to avoid brownouts for a few seconds when the heat/air-conditioning kicks in. – Arion Mitutoyo Dec 16 '15 at 3:23
  • @Yehuda_NYC Arion Mitutoyo isn't that far off, assuming single phase service, 70a x 220v = 15400w x 125% = 19KW. And that's not including any special considerations for motor starting requirements. – Brian Duke Dec 16 '15 at 15:40
  • @user3623501 I just want to run one appliance, and he is suggesting a power supply larger than the service to my whole house. That does not compute. – Yehuda_NYC Dec 16 '15 at 23:47

even though this is a single phase system there will be some harmonics. all systems are susceptible to them. they are however a more serious issue on 3 phase systems. most insidious are the 3rd order harmonics. inverters that are not true sine are themselves a source of harmonics and that being said so are generators. and a contributing factor to the harmonics problem on any system are non liner loads such as computers and other equipment that has a switching power supply. now if you wish to have a fully scale able inverter system that is able to be tailored to your needs. im using a system made of these one of these and one of these additional racks can be added for more capacity as needed just like the inverters may be paralleled as demand grows.


Pure sine is better; no harmonics. Modified sine (really a stepped approximation) is usually close enough and cheaper; it has much weaker harmonics than the old square-wave sources and the remaining ones don't generally cause problems.

  • Hmmm. What do you mean by "don't generally cause problems"? By HVAC is an expensive unit, multiple thousands of dollars. I would not want to have to replace or repair it. My desktop computers as well are costly to replace. So, what are the risks and how serious are they? – Yehuda_NYC Dec 16 '15 at 15:40
  • Motors should ignore the difference, power supplies should filter out thd difference without strain... Can't vouch for specific equipment but I'd probably go for it, or if time permits ask the manufacturers just to be sure. – keshlam Dec 16 '15 at 18:28

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