Very New to Solar, what formula's need to be applied for calculating Battery etc

prabakaran Registered Users Posts: 1
Very new to Solar, so i like to learn the formula and calculation with examples to design PV system, Charge Controller, Invertor, Battery selection for storage.

Example: if my home average consumption per day is 1kWh (i know how to calculate this). it will be good for me and other beginners who like to learn the complete Solar design.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
    Re: Very New to Solar, what formula's need to be applied for calculating Battery etc

    Welcome to the forum Prabakaran.

    Were you hit by the blackouts in India the last few days? Hope everything works out well in the end for you and your family.

    Anyway--You are starting out correctly... Asking questions.

    Solar power is not cheap, and, usually it is not "simple" to implement and maintain. Especially if you want off grid and/or emergency backup power. You have to add a good sized battery bank to store the electric energy from the panels during the day and provide stable electric power 24x7.

    So--First thing first. You need to measure/estimate your daily electrical loads. For example, there are now consumer kWH meters (kilo Watt*Hour meter) that allow us to (cheaply) measure the power used by each plug in appliance. There are also whole home metering systems that can measure your A/C, electric stove/water heater, etc. for energy usage too.

    It is almost always cheaper to conserve energy than to generate it. So, measuring your loads (Watts, peak Watts, Watt*Hour or kWatt*Hour per day, etc.) and then comparing that to your needs and available alternatives (are lights/computer/tv/sat receiver on when kids are out of the home, can you use a 30 watt laptop computer vs a 200 desktop computer, etc.).

    Also, certain uses of electrical power have new devices that can really save energy. For example, there are now "heat pump" type water heaters. Basically instead of using a resistance heater for the hot water, a refrigeration compressor takes heat from the warm air and pumps it into the hot water (2-3x more efficient than resistance heating). If you have A/C (air conditioning), there are "de-super heaters" that attach to your A/C system and heat your hot water for almost free.

    And, depending on your climate, Ceiling insulation, double pane (two layers of glass) windows, wall insulation, changing to florescent or LED lighting will all save lots of power. As will many modern refrigerators (at least in North America).

    Once you have done all that--Then you can look at solar and its various options.

    In north America, the average home uses around 1,000 kWH per month. I am fairly energy efficient with my home and use around 200-300 kWH per month (natural gas for all heating, no A/C system). An off grid home that is every energy efficient can be down in the 100 kWH per month (3.3 kWH per day or 3,300 WH per day) and still have a refrigerator/freezer, lights, tv/computer, electric cloths washer.

    And there are folks here that live on 1-2 kWH per day very nicely too... Energy usage is a highly personal choice. We help you understand those choices and your options.

    Just to give you an idea of US/Canada's power costs:
    • $0.10 to $0.30 per kWH typical utility power pricing in US/Canada
    • $0.10 to $0.30 per kWH typical cost of Grid Tied / Utility Interactive solar PV power (no battery, no emergency power)
    • $0.45 to $0.75 per kWH for GT solar power with emergency backup capabilities (hybrid battery based system)
    • $1 to $2+ per kWH for off grid solar power (panels, chargers, inverters, batteries, backup genset, etc.)

    So--For us, off grid power costs ~10x what utility power costs. So, first, imagine your utility power bill and multiply it by 10x. Now conservation looks much better.

    For most grid connected people, emergency power is best supplied by a generator (diesel, gasoline, propane, natural gas, etc.). They sit there 99% of the time doing nothing (and not much costs for maintenance)--and "high costs" of fuel and maintenance for a few hours/days per year.

    And many of the folks here with off grid homes are doing it because it can cost $100,000's to run a power line to their property (if not $1,000,000's).

    You also need to have "good sun" for solar power too... You will collect very little solar power with overcast skies (50% or less). And during stormy weather, about 5% or less). Also, no shading/power lines/other people's homes blocking sun to your solar array is imperative. If you get shade until noon every day, your system will lose 1/2 of its generating capacity.

    Anyway--that is a start. We can run through a calculation for you--But I would really like to better understand your loads (type, kWH/WH per day, emergency backup vs full time off grid, vs Grid Tied -- if legal with your utility, etc.).

    Small systems to run a few lights/radio is one thing... A system to run a full off grid home with A/C, computers, etc. is something el$e.

    I have a thread here with lots of solar/conservation ideas and links to many more sources of information:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset