You need to charge a battery (or a set of batteries - lots of batteries) to where you have enough power to power that 1500 watt heater. It is not a 'no brainer' it is actually a bit of math mixed in with time and solar gain and how efficient the panel and the operating system (battery, DC/AC converter and their losses). You need to think in terms of watt hours (like kilowatt hours) in order to better understand you problem. Power off the box outside is measure in kilowatt hours - The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h) of time. I think you need to read through this: http://www.solarbuzz.com/Consumer/FastFacts.htm It starts off telling you that a 100 watt light bulb uses 1 kilowatt of power in ten hours. and gives you some other maybe interesting information. Since you are planning on using a 1500 watt device for 4 hours then that means you will use 6 kWh per day - that means you need to be able to produce kilowatts of power. If you have 8 hours of day light then you will need to generate 6000 watts over 8 hours or 750 watts per hour. To give you an idea of what we are talking about. A 10 watt solar power at optimum solar exposure generates 10 watts of power per hour (of solar exposure) For your little heater you need at least a 750 watt solar panel system if you live in a bright sunny location getting full sun 8 hours a day. The reality is you do not want to try to use solar power to heat a space with an electric space heater. Although possible, it is actually a huge waste of energy.