I have been getting lots of questions from people on designing & selecting supplies for their solar power system. This page is to help you figure understand the basic supplies and get a good start on your system. I will be happy to answer questions and help you design or give you tips on assembling your solar system. Post your question below in the comments below and send Craig an email at Johanna@HoneymilkHomestead.com. (This is our farm account)
Note: This page & my replies may contain affiliate links.
Solar Introduction:
There are 4 main parts to an off grid solar system.
Solar Panels (PV modules): The solar panels are what convert the sunlight to electricity. Typically 12V to 36V. They have MC-4 or equivalent connectors on them and they can easily be connected in series or parallel. You can get them from 10 watts about the size of a laptop to approximately 40”x79” for a 400W.
Batteries: The sun only shines during the day. If you need power any other time you will need batteries to store the suns energy. I recommend LiFePo4 batteries. If the price intimidates you get the smaller ones. You may be able to get by with a Lead Acid deep cycle for really basic projects but you will get better results with the LiFePo4 batteries even if they have less than half the amp hour rating. Your DC battery system voltage is typically 12-48V. RV is typically 12V. Small cabins 24V. Large cabins and houses are typically 36-48V.
Charge Controller: This takes the electricity from the solar panels and converts it to the voltage needed to efficiently charge the batteries. There are two main types of charge controllers. PWM and MPPT. PWM is cheaper and MPPT type is more efficient. You can put multiple charge controllers to charge the same battery but make sure the type (PWM vs MPPT) matches. Depending on the type they can take anywhere from 12V to 200V solar voltage in and depending on the model they typically output 12V, 24V, 36V, or 48V depending on how powerful the system is. Note that the solar panel voltage typically doesn’t match the voltage of the batteries. The actual voltage from the panels will change all day and the charge controller converts the voltage to match what the batteries need.
Inverter: (Note some modules have the charge controller built in) this converts the power from the battery to your typical wall outlet type electricity that you run all your appliances off.
FAQ:
I want a 24V system but the solar panels are 36V. Is this a problem?
This isn’t a problem. You need to install a charge controller between the solar panels and your 24V batteries even if your panels are 24V output just because the voltage changes with the sun. First check what the OCV (Open circuit voltage) spec is on your panels (Typically about 45.5V on a 36V panel) and make sure the charge controller input is larger than 45.5V. Then verify that the charge controller works with your battery type and voltage.
How big of a system do I need to make a completely off grid system for my home?
Take your electric bill and subtract $30. Then divide the remainder by 15. This should give you an estimated number of kW of solar you need for an off grid system. Note: This assumes you have the solar panels with an adjustable mount angle with an average 6 hour solar day. You will need more if it isn’t at the optimal angle or if your location has shorter solar days.
How big of a system do I need to make an off grid system with grid backup?
Take your electric bill and subtract $30. Then divide the remainder by 19. This should give you an estimated number of kW of solar you need for an off grid system. Note: This assumes you have the solar panels at a 45 deg angle in MN.
What if my completely off grid system is at a fixed angle?
Add an extra 20% of solar panel capacity if your system is fixed at the optimal angle for your location. (45 deg in mn) to account for the times of the year it is less efficient. Add an additional 2% of capacity for each degree they are off the optimal angle. For example a shallow roof would require more capacity to account for the times of the year it is less efficient.
What is a typical price for a professionally installed system?
Scale matters a lot. Small systems cost more per watt. Note that the prices below don’t include the battery. Add an additional 20-30% for a full battery system.
1kw - 8kw or smaller system will probably cost around $3/watt
8kw - 20kw around $2.25 to $2.75/watt
20kw - 40kw around $2 to $2.25/watt
What is a typical cost for a DIY system?
Scale matters a lot. Small systems cost more per watt. Note that the prices below don’t include the battery. Add an additional 30-40% to the cost for the batteries
1kw - 8kw or smaller system will probably cost around $2.25/watt
8kw - 20kw around $1.5 to $1.75/watt
20kw - 40kw around $1.1 to $1.5/watt
How much can I make selling electricity back to the electric company.
The price for electricity in MN right now is $0.13/kwh. For each 1kw of solar installed at 45 degrees will provide an average of $18/month or $216/year.
Design Assistance
Want help designing your system? Answer the following questions in a comment and email them to me as well. If I am not overloaded with work I can write some notes for your setup.
What is your monthly bill?
Do you want to be completely off grid, just have an off grid backup, or have an off grid system with grid backup for days you need extra power?
What size is your building?
Does the roof face south?
What angle is the roof? (Can send a pic if not sure)
Is there any shade on the solar panel site?