Battery solar charging system for a 2KW home as backup

hello members, (this request is in the wake of hurricane Sandy aftermath)

I am planning to build a DIY solar charging system at home in NJ to power some battery that lets me run 2KW worth of home appliances including fridges, sump-pump, etc run for some 2 or 3 days. Could you break it down by the items (brands are welcome for easy lookup/understanding only) that need to be purchased/configured.

My research so far tell me to install: Solar PV panel, Charge Controller, Inverter, Batteries and wires of all sorts.

Please tell me the spec for each of the components I need to procure and configure.

I am no electrician. A schematic diagram would help a lot.



  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Battery solar charging system for a 2KW home as backup

    Welcome to the Forum.

    For this kind of use, a generator will be far cheaper and more reliable. Think about that before spending too much time looking at solar PV options.
    To limit the generator running time, an inverter system with PV panels and batteries may be a useful supplement, but please do look at what you can do with a generator first.
    SMA SB 3000, old BP panels.
  • ydnrao
    ydnrao Registered Users Posts: 2
    Re: Battery solar charging system for a 2KW home as backup

    inetdog - that's great idea. thanks. could you elaborate on an inverter system with PV panels and batteries supplement part.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery solar charging system for a 2KW home as backup


    First thing to do: buy a Kill-A-Watt meter. About $30. Use it to measure the average daily consumption of everything you want to keep running in an emergency. This information is vital to proper planning.

    You are going to find that refrigerators have high consumption. They also have high start-up demand which won't be noticed by the meter.

    Once you have that number you can plan a system. As inetdog implied, taking it 'totally solar' will be far more money than it's worth. But you could indeed have a battery+inverter system for "quiet power" at night and then run the generator during the day to recharge. Power from solar comes with a hefty price tag, and the less you use it the more expensive it gets. Batteries will deteriorate over time whether or not they are used and will have to be replaced. Eventually so will other components.

    P.S.: the K-A-W meter will also allow you to identify power hogs in your home and do something about them, which will save you money all the time. :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,522 admin
    Re: Battery solar charging system for a 2KW home as backup

    More or less--Identify loads (typically smaller ones) that you critically need at any time or 24x7 (always on power) per day... That would be some LED/CFL lighting, laptop computer, cell phone charger, battery charger (flashlight/radio batteries), perhaps a small water pump (if you have a well or storage tank) and such...

    Get 2-4 6 volt golf cart batteries, a smallish MorningStar TSW 300 watt 12 VDC Inverter (300 watts/600 watts for 10 minutes, remote on/off, "search mode" for low power standby). Get a 20-40 amp or so AC battery charger (grid or generator), and a Honda eu2000i (1,600 watt) or Honda eu1000i (900 watt) inverter/generator (these use very little gasoline to run your loads--400 watts will run ~9 hours per gallon; 1,600 watts will run about 4 hours per 1.1 gallons of gas--Also, they can be connected to a 5 gallon fuel container so you have several days of fuel without refueling the small internal gas tank--of course, you need to watch oil levels).

    You can get a larger genset--But storing fuel and trying to find fuel for a generator that may consume 1/2 to 1 gallon of fuel per hour to power your whole home can become a big issue (12-24 gallons per day vs 1-3 gallons per day for a smaller Honda genset).

    So, say you want 4x 6 volt 220 AH "golf cart" type batteries (cheap, plentiful--there are better, but for a start and learning without dumping a heck of a lot of money--a good way to gain experience without emptying your wallet).

    Lets say you choose 4x 6 volt 220 AH batteries (series/parallel connected for a 12 volt @ 440 AH battery bank). The recommended solar array would be around 5% to 13% of battery capacity (you can go to ~25% charge rate--But not usually recommended unless you have special needs):
    • 14.5 volts charging * 440 AH battery bank * 1/0.77 panel+controller derating * 0.05 rate of charge = 414 Watt array minimum
    • 14.5 volts charging * 440 AH battery bank * 1/0.77 panel+controller derating * 0.10 rate of charge = 829 Watt array nominal
    • 14.5 volts charging * 440 AH battery bank * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,077 Watt array "cost effective maximum"

    The daily power usage from the battery bank would be 1-3 days of storage and 50% maximum discharge. A good sized system, lets say 2 days of storage and 50% maximum discharge, or 25% discharge per day:
    • 12 volts * 440 AH * 0.85 eff inverter * 0.25 discharge = 1,122 Watt*Hours per day of 120 VAC power (estimate)

    To make things easy, say you are in Atlantic City NJ, using PV Watts with a 1kW array (simple round numbers), fixed array, and 0.52 end to end system efficiency would be:
    "Station Identification"
    "Lat (deg N):", 39.45
    "Long (deg W):", 74.57
    "Elev (m): ", 20
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 39.5"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:","11.2 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.61, 59, 6.61
    2, 4.20, 62, 6.94
    3, 4.78, 74, 8.29
    4, 5.23, 76, 8.51
    5, 5.44, 80, 8.96
    6, 5.48, 74, 8.29
    7, 5.55, 77, 8.62
    8, 5.41, 76, 8.51
    9, 5.23, 73, 8.18
    10, 4.60, 68, 7.62
    11, 3.59, 54, 6.05
    12, 3.17, 50, 5.60
    "Year", 4.69, 824, 92.29

    You would average around 50 to 80 kWH per month. At this time of year, November averages around 54 kWH per month or:
    • 54,000 WH * 1/30 days per month = 1,800 WH per day (average) of 120 VAC...

    Just to give you an idea, a standard energy star refrigerator uses around 1-2 kWH per day (1,000 - 2,000 WH per day).

    For an AC battery charger, you can pick a "simple" AC charger for ~20 amps maximum for the Honda eu1000i generator, and the eu2000i would support 40 amp or larger... For a 440 AH battery bank with 5% to 13%/25% rate of charge, the suggested chargers would range:
    • 440 AH * 0.05 rate of charge = 22 amps minimum
    • 440 AH * 0.10 rate of charge = 44 amps nominal
    • 440 AH * 0.13 rate of charge = 57 amps peak "cost effective"
    • 440 AH * 0.25 rate of charge = 110 amps not to exceed (can overheat battery bank--remote battery temp sensor, etc. suggested)

    If you are in the technical side of the "most efficient" battery charger/small genset combination, here is a good thread:

    Question about battery charger selection with EU2000 generator.

    There are some nice options for charge controllers in this size array... Simple and less expensive PWM type charge controllers--But you probably will end up with MPPT charge controllers which are ~3x as expensive as the PWM types. MPPT controllers are more flexible and support arrays with longer wire runs from array to battery shed. Looking at ~$150 to $500+ for charge controllers.

    Note, the above system is really too small for powering a refrigerator--I would use a genset to power that (running the genset ~12 hours per day to keep cool/frozen). At the same time, the generator can also help recharge the battery bank (assuming you have bad weather and poor sun at the same time).

    For a ~1,000 watt solar panel system + batteries + equipment--You are probably looking at $5k to $10k+ or so (you install, vs somebody else, bells and whistles, etc.)... (Honda eu2000i ~$1,100 or so over the Internet).

    Some vendors (including our host--above links) can even configure/build/wire/test a system if you need it now, delivered.

    The links I have provided are suggested starting points for good quality equipment. You are welcome to purchase anywhere--We support all.

    I am a big believer in conservation and "balanced" system design. Too big of battery bank need a larger solar array+backup genset. A larger genset uses more fuel, etc...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,522 admin
    Re: Battery solar charging system for a 2KW home as backup

    A way to start would be:

    If you have power, use a Kill-a-Watt meter on your AC loads (refrigerator for 1-7 days, laptop for 1-7 days, washing machine per cycle, etc..). That gives you a list of power used (i.e., 120-700 watts running/1,000 watts starting/1.2 kWH per day for a fridge, do you have an oil/central furnace that needs AC power? etc.).

    You can do the same thing with a genset--And I would suggest starting with a Honda eu2000i (1,600 watt max) or eu3000i (~2,800 watts, pull start or electric start models). And again, take some readings with a kill-a-watt meter.

    1. Size/install a battery bank+inverter+AC Charger (grid/generator charging)
    2. Size/Install a smaller solar array (as time and funds permit)
    3. Add a Battery Monitor / log your usage and see if meets your needs

    Note: it is very difficult to "cost effectively" increase the capacity of an off grid power system--Keep your first one small/reasonable for your emergency power needs.

    Getting backup units (such as a propane tankless water heater for camping to take a shower/wash dishes, camp stove that runs off of gasoline/propane/etc.). Water storage/filtering/disinfecting systems... All help to keep your electrical demands low (keeps cost and fuel storage requirements low too).

    You can revisit the whole question 1-5 years down the road and see if it makes sense to install a larger system with more features if your needs require it.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery solar charging system for a 2KW home as backup

    Just to put another point on it.

    A Honda eu2000 will supply 1800 watts of power 24/7 using less than 2 gallons of fuel per day, for under $1000.

    10 gallons of fuel, stored properly, and cycled into your vehicle will last nearly a week. Cutting run time to 1/2 time, or reductions the loads will increase the run time. A $4/gal you might spend $10/day, or $70 for a week.

    Much cheaper, much more reliable than a small battery based PV system that will cost way more, and will have very definite limitations.

    Just a thought,


    PS the time to buy a genny is 6 months AFTER such an event, since all those that buy them for an emergency, tend to sell them since they don't think they need themo any more.

  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Battery solar charging system for a 2KW home as backup

    I second a 2 or 3 kw Honda inverter-genset. I started out buying a 12 kw genset. After swallowing the cost, my wallet then ran away screaming when it saw how much it cost to then run it. I lost power for 55 hours after Sandy and if I ran my big genset the whole time it would have cost me nearly $300.

    I wanted a way to increase the overall system efficiency and a way to run silently at night so then I bought a large UPS and a 22 kw battery bank. Standard gensets operate much more efficiently as you load them up, so running big loads and recharging an empty battery bank gave me more kw per gallon. And silence.

    If I had to do it all over again I would buy a Honda inverter genset for my base loads. ~300 w-h overnight and mid-day while at work, and up to 1500 in the evening and mornings. I've basically done this in an unconventional way (see the link in my signature). If you only can buy one generator, get the inverter genset to save your gas and give you silence at night.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is