What can you tell me about inverters Jeff?
www.caravantechnology.com/Error.aspx?aspxerrorpath=/caravaninverters.aspx
Bottom of the page - can they run a vac?
Could a vehicles engine run one? Even if it could i suppose it might be an offence to leave an engine running?
If its to run your vac from it then your really need one with some meat on it, I'm assuming your vac is 3,600W if it is, Then you really need to get the 5000w one, the 4000w will be running close to capacity and draw a bigger current on start up.
You'll also need to keep your battery charging, I couldn't see the current consumption in the specs but I know its going to be high, forget to keep the battery continuously topped up for even a short time will result in a flat battery.
I would personally only use it incase of emergency, use a Genny or custy's electric.
Sorry to be thick, but could an invertor-as described by jefftemperly- run a mains electric pressure washer?
Check the wattage of your washer, (example) if its say its 1000w then you need to buy an inverter that is higher than 1000w, its no good buying one that runs 1000w as it will be running at full capacity continuously and would soon break down.
If you look at what jeff has 1500w, then to run say a 800w drill or a 1000w router he covered himself very well