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This question is similar to two others, here and here, but with a small difference.

To allow a bit of context: Suppose I have a winters day with lots of sunshine. I have an off-grid PV system, and the PV modules are making more power than can be used at the moment. My inverter is however limited to 1600VA, which limits what appliances I can use to dump this excess.

I also have a 3kw hot water cylinder, which presently runs from grid power. It has a standing loss of at least 2kwh a day, and there is a good 5kwh space between 55 and 85 centigrade (assuming 150 liters of water). It would make a good candidate for dumping excess, especially in winter when more hot water is used.

I can obviously not do this at full power due to the small inverter. In theory, however, the heating element has a power factor of 1 (or very very close), and if I drive it at half the voltage/current it would just take 4 times longer to get the same heat in there, but it should still work.

Traditionally you would do this with something that resembles a dimmer switch, some kind of PWM control, or you'd just rely on the thermal mass of the water and turn it full-on/full-off, but that can't be done here due to small inverter.

My question here: How does it affect overall required VA (because inverters are rated in VA, not watts) if you use something akin to a dimmer switch or PWM control? The overall average consumption may be below the inverter rating, but if it does so by creating extremely high current spikes at low duty cycles, I cannot imagine the inverter would like that very much.

I suppose you can shorten this question to: Can you use a dimmer switch (of sufficient capacity) to drive a large heating element from a small inverter. I have a feeling the answer is no. What are the alternatives?

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izak
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    Do you have to power the heating element from the inverter? Maybe you can connect your heating element directly from the source. – Christian Mar 29 '16 at 11:21
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    Agreed. The inverter will have losses and probably won't like the dimmer switch. DC heating will be most efficient. Your problem will be finding a suitable means of attaching a heater to your hot water cylinder. – Transistor Mar 29 '16 at 11:44
  • Good point. I know there are people selling DC elements. Something to investigate. Thanks. – izak Mar 29 '16 at 12:53
  • Many water heaters actually have three heating elements. In single phase operation they are connected in parallel, while in three phase operation they are connected in a [star AKA Y connection](http://m.imgur.com/D6dW76v). If your water heater fits this category, you could just switch the three elements individually (each element = 1 kW) to vary the power. – jms Mar 29 '16 at 13:47
  • In your country that may be common :-) In South Africa, they are mostly single phase. – izak Mar 29 '16 at 13:54

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You could use a transformer or a Variac to reduce the voltage without affecting the power factor much. It should be easy to find a 2:1 transformer. Be sure to prevent the transformer from back feeding if full power is applied, or you will have a bad day.

Spehro Pefhany
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    Indeed. This is a rather obvious answer, come to think of it, and I'm ashamed to admit I didn't think of it. The simplest solutions and all. Finding a 2:1 transformer will be easy. We're a 230V country, so existing adaptations for American equipment will do the trick. – izak Mar 29 '16 at 12:55