Residential Solar Calculator - Affordable Solar

3000 kwh per month solar system

System / December 28, 2016

Step #2: What do you want to do with the power?

If you are already served by a power company, it is considerably cheaper to purchase a “grid-tie” system. If you want to be totally independent of your energy company and/or you need electricity in a location that is not yet served by your energy company and it would be very expensive to connect, you will want to consider a “Battery Stand-Alone” system. A “Battery Back-up” system is a combination of the two systems, where batteries are used as a back up for a power outage in an emergency situation.

  • Grid-tie: You generate power during the day (while the sun is out) so if you’re producing more than you’re using, your meter will run backwards. Your energy company supplies the power you use at night, so your meter will run forwards. This push-pull will “net” at the end of the month with a balance either in the customer’s favor or in the electric company’s favor. In this dynamic, the power company acts as a huge energy storage device/battery.
  • Battery Stand-Alone: Along with the solar panels and support system, you will need to purchase batteries. Your solar panels charge the batteries and you then draw energy needed for your house from the batteries. You would need to determine all energy needs and make sure your system is large enough to acquire and store enough energy.
  • Battery Back-Up: This is a hybrid system that is useful when power goes out. You can pre-determine which appliances should never be without energy, and then store that needed power. For example, you might have emergency back up for your refrigerator, computer, and 4 lights.

Step #3: Consider how much power you actually use.

Realistically, you will probably look to supplement your power needs via solar (“grid-tie” as described above) rather than use a Battery Stand-Alone system. If you want more information about battery back up and true system sizing, please let us know – we have an overview you can use to determine your full power needs. However, if you simply want an understanding of how much electricity you use and how much a solar system will produce, follow the steps below.

One way to look at the math:

  • Solar systems are generally sized in 1 kW – 6 kW (and larger) systems. A typical size is 3 kW.
  • 3 kW, or 3 kilowatts = 3, 000 watts
  • A 3 kW system will generate around 3, 000 DC watts per hour
  • Multiply the per hour generation by 5.4 which is an average number of sun hours in a day (3, 000 x 5.4 = 16, 200)
  • Multiply the new total by the average number of days in a month (16, 200 x 30.5 = 487, 620)
  • Multiply the new total by .77. This is the “derating” factor, or the amount of energy lost when DC current is turned into AC current. (487, 620 x .77 = 375, 467)
  • So, a 3 kW system will generate about 375, 467 watt-hours per month, or about 375 kWh.
  • Now compare this number with the kWh usage noted in your electric bill. How many kWh do you use in a typical month? Twice this amount? Then you would save roughly ½ your electric bill if you installed a 3 kW system.
  • Consider how much money you save per month to figure out how long it will take to pay off your system.

Another way to look at the math – in reverse:

If you want to get all of your energy needs met through solar power (and get a “0” bill from your electric company) calculate how large a system you will need by following the steps below. Before you start, choose an average electric bill. Look for how many “kilowatt hours” you consumed. This is generally expressed as “kWh”.



YOUR info

Note the average number of kWh you use per month


kWh X 1000 = total AC Watts used per month

550, 000

Total AC Watts / 30.5 (days in a month) = AC Watts used per day

18, 033

AC Watts used per day / Sun Hours per day (Central Texas = 5.4)


AC Watts needed per hour per day X 1.29 (AC to DC conversion factor)


Solar array in DC Watts to reach a Zero electric bill

Solar array in kilowatts, or kW


Step #4: Information to keep in mind when considering a solar system.

  • Up to 30% of the total cost of installation can be taken as a federal tax credit. Previous versions of this credit had a cap of $2, 000; there is no longer a cap.
  • The most productive system is located on a South- or West-facing roof (or area) that is shade-free from 9 am – 3 pm every day of the year.
  • Generally speaking, you will need ~ 1 square foot of space for every 15 watts. So a 3 kW system would take ~ 200 square feet; a 4 kW system would take ~ 266 square feet.
  • You do not need to size a system to meet all your energy needs. You will remain a customer of your electric company, so you can use more (or less) power as needed.
  • When considering “payback” time, think about the price of electricity per kWh. Has it increased in the past? Will it increase in the future? By how much? We do not know the answer either, but the question should be factored in to your decision-making.
  • All energy providers are obligated to allow “net-metering” which means that the electric meter must be able to run backwards (i.e. when you are generating electricity). However, they are not obligated to pay you a set $ amount, or anything at all, if you generate more energy than you use in a month. Check with your energy provider to find out their particular “Tariff Agreement”.