Well that is one of the most frequently asked questions I see asked on many Renewable Energy forums. Unfortunately there is not a hard dollar figure you can put on it because the answer depends on your location. Location means everything because of what is called your Solar Insolation Sun Hour Number for the month of December in the northern hemisphere. June for those Down Under.

To demonstrate this I will use the USA best case (Tuscon AZ) and worst case (Seattle WS) locations as a design example. The objectives are:

Generate 1000 watt hours of usable energy per day or in a 24 hour period.
Maximum connected load = 250 watts

OK from the objectives there are three fixed cost items we can lump together; battery, inverter, and misc wiring fuses, and electrical hardware.

We know we want 1 Kwh of usable energy per day. In a off-grid battery system the minimum battery capacity is 7.5 times the daily capacity. So 7.5 x 1000 wh = 7500 wh of battery capacity or 7.5Kwh. A good quality 7 year warranty battery is going to cost $140 per Kwh of storage capacity. You can expect it to last 5 years on average with excellent TLC. So the battery is a fixed cost of $140 x 7.5 Kwh = $1050.

For the inverter the right size is twice the maximum connected load. We know the max load is 250 watts. So 2 x 250 watts = 500 watts. A good quality True Sine Wave Inverter will cost you $1 per watt. 500 watts x $1 = $500.

Misc wiring, fuses, and electrical hardware for distribution $500.

Total Fixed cost per Kwh = $2050. OK so the fixed cost total is $2050/Kwh for each two examples. Let’s move on to the variable cost of solar panels and charge controllers.

In Seattle, December Sun Hour rating is 1.4 hours best case. Battery off-grid systems at best using MPPT charge controllers are 66% efficient. That means to have 1 Kwh of usable power your solar panel must generate a minimum of 1500 watt hours or 1.5Kwh per day. So to find the solar panel wattage needed you need to know the minimum watt hours the panel must generate each day, then factor out the time element in hours. The formula is Watts = Watt Hours / Hours. So in Seattle 1500 wh / 1.4 h = 1071 watts of solar panel wattage minimum to get the job done. UL qualified panels cost around $2.50 to $3 per watt. So using the low number 1071 watts x $2.50 = $2677

In Phoenix the same applies above. What changes is the Sun Hours from 1.4 hours to 5.6 hours. So 1500 wh / 5.6 hours = 270 watt solar panel $2.50 x 270 watts = $675. Interesting fact huh? Seattle solar panels cost roughly $2700, and Phoenix cost $675. Could the location be a critical factor? Let’s move on to Charge Controllers.

In Seattle since we have more than a 1000 watt solar panel array so we have to use a 24 volt minimum battery system because the largest Charge Controller available is 80 amps. So at 12 volts would exceed the 80 amp limit (1080 watts / 12 volts = 90 amps) So at 24 volts the minimum charge controller rating is 1080 watts / 24 volts = 45 amps. Closest match we can find is 60 amps MPPT charge controllers that cost $200.

For Phoenix we can use a 12 volt battery system with a 35 amp MPPT charge controller that cost $120.

So let’s add up the damage.

Phoenix 1 Kwh system cost:
Fixed items = $2050
Solar Panels = $675
MPPT Charge Controller = $120
Total Material Cost = $2845

Seattle:
Fixed items = $2050
Solar Panels = $2677
MPPT Charge Controller = $200
Total = $4927

The above does not include any permit/inspection fees. Labor, or engineering certification and plans. All that is extra. The above cost can give you a rough idea of how much any size system will cost based on your Kwh requirement. If you need 4 Kwh multiply the above by a factor of 4.

Motto of this story is location means everything. If you live In Seattle WS and want to go off grid, sell your home and move south.

__________________
Dereck, PE, MSEE
Moderator