First Project

I've been trying to research different solar options for emergencies. My knowledge is very limited so please forgive me if I make any mistakes. Hopefully, you all can help me figure this out.

I'm looking to build something at my house to power a few items in the event of a power outage/emergency. The list of items I'm looking to power are: battery chargers, two way radio chargers, limited lighting, space heater, fans in the summer, maybe power tools once in a while. The only other thing I can think of to add to my list is maybe a Crock-pot.

I plan on attaching a number of marine batteries or 12v chargers to the panels. I'm not sure how many I would need for what I'm looking to do. Perhaps someone could give me a ballpark number. I also need an inverter and I'm sure other components I'm leaving out.

As for the panels themselves, I was thinking one or two 200 watt units. Will that be enough? If not how many would I need?

My current plan (if the two 200 watt panels are enough) is to purchase them online. I'm not sure if it will be that cost effective to construct them myself. If it will be within a few dollars a watt, I'd rather leave it to a professional.

This place has one for about 2.55 a watt. They have a few other models, but I'm not sure the differences between them.

So, am I on the right path? Thanks for any help you all can give me!


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: First Project

    Welcome to the forum!

    Shall we go straight to the bad news? :roll:

    Solar as emergency back-up power is not an economical way to go. Off-grid type systems will produce power at a cost of about $1 per kW hour - and that's for continuous use. For emergency situations you're better off buying a generator. Some of the inverter generators (type of gen - nothing to do with solar power systems per se) are extremely economical and quiet.

    Take a look at some of the things you want to run:
    Space heater. Now if that's a full electric space heater (500+ Watts) it's one of the worst things you can try to run off electric.
    Crock pot. Same problem. Anything that uses electric for heat is going to consume huge amounts of power.
    Battery chargers. There you can get into trouble if the battery charger isn't compatible with the inverter. Specifically some small tool chargers will actually burn up if used on MSW-type inverters.

    In any case, the place to start is with a Kill-A-Watt meter. Get some real world numbers of Watt hours used in a typical day so that you have some idea of how much power you would need to supply should the utility go down. Then size your generator accordingly. :p

    The only time solar makes sense for back-up is if the utility is really bad at maintaining reliable power (as is the case in some places).

    Your proposed two 200 Watt panels, btw, would supply approximately 800 Watt hours of AC per day under typical conditions.
  • System2System2 Posts: 6,290 admin
    Re: First Project

    Thank you for the reply and the good ideas! Yes, not long after I posted I realized space heaters and the crock-pot wouldn't be feasible. I'm not actually looking to go off grid per say, just more of a means of charging those marine batteries or 12v packs. Hopefully, they make inverts and chargers that would be compatible.

    I did invest in a generator, but gas is consumable and I wanted something extra.
  • BB.BB. Super Moderators, Administrators Posts: 33,313 admin
    Re: First Project

    Marine Batteries are typically not the best thing for use for deep cycle storage. They are a compromise between light/high current (starting battery) and some deeper cycles.

    However, from a charging point of view almost any solar charge controller will be capable recharging your 12/24 volt batteries. Many of the higher end models have adjustable Absorb and Float settings--helpful for keeping an emergency bank charged.

    Sizing your system... We can either look at your peak and average loads (peak Wattage needed and average Watts*Hours of use per day). For 120 VAC loads, the Kill-a-Watt meter is great for that. For DC loads, a DC Amp*Hour / Watt*Hour meter can be helpful (like one of these).

    Normally, for a small system (100-200 watts of solar panels or so)--You should be looking at a small TSW (True Sine Wave) Inverter. They can run any of your small electronic loads, charge cell phones, etc. fairly efficiently.

    All About Inverters
    Choosing an inverter for water pumping

    The Morning Star 300 Watt TSW 12 volt inverter is a great pick for a smallish system.

    You could purchase a big and cheap MSW inverter (1,200-1,500 watts or so)--But beware that running one of those at full power will use up your 1 day of solar energy from those 200 watt solar panels in about 20-30 minutes.

    It all depends on how you want to budget your power. 800 Watt*Hours will run a 50 Watt laptop computer, a pair of 2x13 watt CFLs, and a 10 watt battery charger for about (200 watts of solar panels, 4 hours of full sun per day--i.e., not the 3-4 months of winter):
    • 800 Watt*Hours / (50 watts + 2x13w + 10 w) = 9.3 hours per day
    That can be very handy.... But it is not going to run a full sized fridge which needs around 1,000 to 2,000 Watt*Hours per day (365 days a year).

    In the end, even if you build a nice little backup solar system (also a great educational tool)--You probably should have a small genset and siphon hose (at least) for power during bad weather (a little Honda eu1000i or eu2000i is very easy to carry and very fuel efficient--you can get 10+ hours on 1 gallon of fuel with light loads)... Larger non-inverter type 3-5kW gensets (which can cut back on engine RPM with light loads)can use a 1/2 gallon of fuel per hour with even light loads...

    Anyway, here are a couple of threads that may give you some ideas... One is an emergency stand alone system. And the other is a small travel trailer (basically similar size to what you asked about):

    Emergency Power Basically a very long thread that starts from the beginning with a few vague requirements through design and assembly for a "portable" solar RE off-grid power box.

    And here is another example by Mike90045 called the Solar Monolith:


    Here is a nice thread with video from Keven in Calgary Canada that shows designing and installing solar PV in a small RV trailer.

    In general, most people over estimate the amount of power they can get from a solar PV + Battery system, and underestimate their loads.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: First Project

    Thank you so much for taking the time to reply! That really helped!
  • System2System2 Posts: 6,290 admin
    Re: First Project

    One last question came to mind that perhaps you could help me with. A few years ago a company came to give a solar estimate to my employer. The salesmen mentioned needing to replace the inverter every five years. Would that also be true for what I'm building? Thank you again for the help!
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: First Project

    that's a loaded question for you can buy a brand new inverter and have it fail. for the good ones i'd say they'd average between 5 and 10yrs, but observe the warranty and don't count on beyond it even though you may get well beyond it. if it lasts 10 or more years you could have a situation of it being unfixable due to the lack of old parts.
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