Calculating Electrical Production and Use

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 A PEAK SUN HOUR (PSH) is defined as an hour in the day when the intensity of the sunlight reaches an average of 1000 watts/ square meter. For example, a location that gets 5 PSH (kWh/square meter), means that area gets 5 hours of solar power when the average intensity of sunlight is 1000 watts/square meter.

Now, let’s do an example of energy calculation of a solar PV system using the peak sun hours. Say we have a 5kW solar PV system in a location which gets 5 peak sun hours a day as an annual average. Knowing the system wattage and peak sun hours we can find out how much electrical energy the system will produce annually:

• Daily Energy: 5 kW x 5 hours (PSH) = 25 kWh
• Annual Energy : 25 kWh x 365 days = 9125 kWh

The average US household uses around 30 kWh of electricity per day. This can vary greatly depending on the house and the occupants. Generally, it would require a 6kW to 10kW solar system to offset the average, US, full-size, American home energy use.

A 3-5 kw system can run a small, efficient home and may handle needs of up to 20 kWh per day.

An energy efficient Tiny home, Bungalow, Water well pump, farm outbuilding, and other special applications may only require a 1-3 kw system with 800–2400-amp hours of battery storage capacity for autonomy during nighttime and cloudy days for 3-4 day periods. The exact sizing depends on each applications specific circumstance and on user preferences.

Watts= (Amps) X (Volts) 

Amps = Watts divided by Volts 

Volts = Watts divided by Amps. 

Amp Hour:  If you use one Amp of electricity for one hour, you have used one Amp-hour (1 Ah). 

Kilo-watt hour:  If you use 1000 watts of electricity for 1 hour you used 1 kilo-watt hour (1kWh) of electricity. So, if you use a 100 watt light bulb for 10 hours, you used 1000 watts, or 1kWh, of electricity.

An average home uses about 30 kWh of electricity per day, but this varies greatly from home to home. An energy efficient household or tiny home can get by comfortably on half that much electricity. ZEB’s single solar electric unit is 1100 watts of panels and four 200Ah batteries with a 4000 watt inverter. Each battery weighs 120 pounds. One of these batteries can supply 20 continuous amps for 10 hours or 10 amps for over 20 hours (200Ah). Each single unit makes approximately 5kWh of electricity each day. Several single units can be strung together to make larger solar arrays. 

If you connect two 12v 200Ah batteries in parallel, it will still be a 12 volt system, but the amps will double to 400Ah, so the batteries will last longer. On the other hand, when batteries are connected in series, voltage is increased while capacity (Ah) stays the same. The ZEB solar electric batteries are connected in a 24 volt system and the charge controller cuts off the charging at 25.5 volts. The inverter changes this 24 volts DC to 120 volts AC for household use and cuts off the household load at 50% battery discharge.  

Examples of typical electric use: 

Fridges, standard size, use between 3 and 6 amps but can go up to 15 amps. Plug your fridge into a 20-amp dedicated circuit. Look at the manufacturer's details for amperage info or calculate it from the wattage. Use a power meter to accurately measure the amps used; 

Heater, 1,000 watt - uses 1000 watts (1 kWh) per hour; 

TV, modern, 50 to 100 watts; 

Computer, 30 to 70 watts; 

Fan, 15 to 90 watts; 

Air Conditioner Capacity (BTU)   Amps 

6,000 BTU                                       5.22 Amp 

12,000 BTU                                   10.43 Amp

How long will a 5000 BTU air conditioner run on a 200 amp hour battery? 

Four (4) 200 amp hour 12 volt batteries will keep this AC unit for running about eight hours maximum. This is assuming these batteries are fully charged to begin with and that you use all the available charge. 

To determine the number of watts needed to charge a 200 amp-hour, 24 volt, dead battery in 5 hours, we need to use the formula: Watts = (Amp-hours) x (Volts) / (Hours). Plugging in the numbers, we get: Watts = (200) x (24) / (5) = 960 watts. So in order to charge your 200 amp-hour, 24 volt, 4 battery system in 5 hours when it is 50% discharged, you need 1920 watts of solar panels. Missouri averages 4.5 peak sun hours a day. So, ZEB’s basic solar electric unit has 2200 watts of solar panels and four, 200 Ah batteries wired in a 24 volt configuration with a 4000 watt inverter and 100 amp charge controller. 

To convert amp hours to kilowatt hours, multiply amp hours times volts, then divide by 1000.

- kilowatt hours = amp hours x volts / 1000

-watt hours = kilowatt hours x 1000

To convert kilowatt hours to amp hours, divide kilowatt hours by volts, then multiply by 1000.