I just purchased a Xantrex XPower Powerpack 1,500 watt battery backup. I'm trying to have enough backup power to operate the fan for my gas heat in case of power outage this winter. What would be a reasonable arrangement of solar panels to recharge the Xantrex in one day? Of course, cost is a concern. But I'd like to have at least a small amount of backup power without using a gasoline generator. There is a Sunforce kit on Amazon for a little over $300, but I haven't seen anybody here mention Sunforce. So that makes me think it might be too lightweight.

Thanks

The wattage you call out, is likely the capacity of the inverter. We need to know the size of the battery, and it's unlikely it's large enough to run the fan for more than an hour. I suspect the inverter is also a mod-sine wave inverter, and it will fry many things, fan included, beware, or correct me if I'm wrong.

Yes, you're right. The 1500 is the capacity of the inverter. And it is modified sine. The reviews on Amazon, and there were quite a few, said they had run some fairly large appliances including refrigerators with the thing. But, as you can tell, I'm very new at this. Here is a document from Xantrex that describes the spec in more detail.

http://www.xantrex.com/web/id/38/docserve.asp

I certainly don't want to fry the fan in my central heat unit. But I'm not sure what this thing is good for if I can't trust it for that purpose. I'm trying to avoid a gasoline generator. But the prices I've seen for a true sine inverter are out of my range at the moment.

I've realize I've wandered off-topic here. My original question was what sort of solar arrangement might be capable of recharging the Xantrex in a reasonable amount of time. Of course, that question may be moot if the battery pack is not going to be useful for my purpose.

First, you need to know the minimum amount of power in Watt*Hours (or kWatt*Hours) that the emergency loads require...

For example, lets guess that the your central heat requires 600 watts and runs 4 hours per day to keep your house minimally warm (your need to measure this somehow--if you don't know how, you should get a friend or electrician who can). Also, you may wish backup power for a fridge/freezer and a few lights/radio.

For general appliances (in the US), you can always use Kill-A-Watt meter ($25-$40 each)...

In my example, the result would be 600 watts * 4 hours or 2,400 Watt*Hours (2.4 kWH if you are reading your electric meter/bill).

Next, you need to figure out how much sun you get in your area. One place to look is here:

http://rredc.nrel.gov/solar/pubs/redbook/

As you look through the data (I would suggest using the PDF files--they are easier to read and understand)--you will find that, usually, there is a difference in that number of hours of sun per day based on month/season... So, lets assume you need heat in the winter and your area gets 3 hours per day of sun...

Based on average efficiencies of charger, batteries, and inverter, you get about 60% overall system efficiency:

Solar Panels=2,400WH/D * / (0.60 * 3Hsun/D)= 1,333 watts (PTC) of solar panels.

Add about 13% for PTC (real solar energy) to STC ("data sheet" solar ratings):

1,333*1.13%=1,506 watts (STC) of solar panels... That is about $7,500 of solar panels (at $5/watt--you can probably find them for $4/watt or a bit less)... (excluding all of the other costs for Charge Controller, batteries, replacement batteries every 5-10 years, inverter, installation)...

If your furnace runs on natural gas, you might be better off just installing a small home backup generator that runs off of natural gas for less money and higher availability (if you have natural gas, you have heat and fuel to run your generator--if you don't have natural gas, you don't need to run the generator--and with solar, your might lose your power when you have little sun, so solar panels become less useful).

A better "investment" for you money might be to go with a Grid Tied system (or Grid Tied with battery backup). You would really like to use your solar system's panels 365 days per year to generate power for daily use--rather than use it may be one or two weeks per year when you have ice storms (or whatever knocks out your power).

What you are asking for can be done--but it would probably be better if you approached it from another angle (full solar power system for home, and/or backup generator). Money wise, just creating a solar system for
emergency backup power (used only a few days or weeks out of the year) does not give a good return on investment.

Or just install several vented gas wall heaters (that don't need AC power) in a couple of rooms to keep the home livable during cold weather.

-Bill

elaborating a bit on bb's excellent post i'll say that most of this depends on the circumstances you wish for it. now if you have a backups type of thing in mind that like the ups on a computer it kicks in line automatically for the furnace to stay active in your absence will that xpower powerpack do that? (i didn't read the spec sheet) also some motors have a high start surge that may or may not be handled by the inverter you have chosen. i do agree with bb that the modsine can be bad on motors and if you wish to try that do so under non emergency conditions to see if the inverter, motor, etc all handle the task you wish for it. if it works then great, but if it doesn't you could blow the fan and or the inverter. if it all works then don't bother with the pvs as odds are you will need more battery power than you have to run the furnace any appreciable amount of time. even if a modsine inverter handles your task keep in mind that it can shorten the lifespan of your fan. generator backup is best in your case if no sinewave inverter with a larger battery bank can't be had at this time.
the idea of the lng generator is a good one as many of those can be set to start automatically during an outage. you won't need to store gasoline or worry about it gelling with lng. the lag time is higher with a generator set to start than with an inverter so keep that in mind too. save your money for that and get into solar as a seperate venture be it battery backed or gridtie.

OK, you link claims the battery is a 60AH battery. you will only use it in emgerencys, so 70% discharge is fine, the inverter shutdown will likely kick in before then.

So 70% of 60AH is 42ah. at 12.5V is 525 Watt Hours, which would run a 100 watt bulb for 5.25 hours, then the battery is dead. (actually less with system losses, if the inverter didn't shut off due to low voltage)

To recharge, you will need about 600 watt hours (there are losses), I don't know your location, so I will guess worse case, that you only have 4 good sun hours to charge from. If you had a single 150W panel, in the sun for 4 prime hours, you could recharge the battery in 1 day.
If you run the fan, while recharging, you will need more time, and/or more panels.
You will also need a charge controller, to avoid overcharging the battery.

But the bigger problem is : the battery is not likely to run your furnace fan very long, before it is drained.