Part of my off-grid energy community involves one-on-one question-and-answer interaction with me online. People ask questions about solar and wind energy, they enter those questions in the course interface, then I answer. Sometimes learners and I get on the phone together or we do a video conference, too. The question below is from a learner named Greg. He was in the last session of the course, wondering about connecting an off-grid energy system to the grid, to feed power back into the system and, ideally, getting paid for it. Here’s our interaction . . .
Feeding Power Back Into the Grid
Greg: How do I determine the equipment I need for a grid-tie system to compensate for my grid energy use? I don’t want to use batteries and my current electrical use is about 800 kilowatt-hours (kWh) per month.
Steve: That’s a great question and the answer depends largely on the sun and bureaucratic conditions where you live. And not just the sun conditions at one point in the year, but the conditions as they vary across the year. Here’s what I recommend . . . in a sense, you should think of the grid as a kind of big battery that you can draw from as needed.
Step#1: Identify the local situation as far as tying back into the grid goes. You want to find out for sure if this is allowed where you live, and if your power meter is the kind that can run backwards in your favor. Most places are okay with this, but you need to find out for sure.
Step#2: Identify the kind and size of inverter you need. Inverters convert the direct current (DC) power produced by solar panels and turns it into alternating current (AC) of the kind that comes out of a wall outlet. This inverter would be the kind that can connect to the grid (not all can), and it would need sufficient capacity to handle the flow of electricity from your panels into the grid. 800 kWh divided by 30 days per month equals about 27 kWh per day on average. Assuming 10 hours of some kind of sunshine per day, that works out to 2.7 kilowatts going into the grid each hour on average – at least in theory, anyway. In practice, depending on the sun and cloud conditions where you live, you will probably need to put energy into the grid at a faster rate than the 2.7 kWh sometimes, to make up for times when it was cloudy. So, in this sense, you need to take something of a guess. I’m thinking that a 5000 watt grid tie inverter should be sufficient, with solar panels to match.
Step#3: Determine the wattage of panels you need. This also requires something of a guess, but the guesswork is risk-free. Here’s how the process works . . . If you need to average 2.7 kWh of power going back into the grid 10 hours per day, then you’ll need something more than 3000 watts of panel capacity. How much the “something more” works out to depends on exactly how much power you use, and how much real-world sun you get. So you might want to start with 3000 watts of panel capacity, then see how it goes. You can monitor how much energy you produce versus how much you consume from the grid, then add more panels until you reach a balance that you like. You’ll certainly need more than 3000 watts of PV capacity, and watching the balance between usage and production will tell you how much more.
Does this make sense? If this were my situation, I’d make it a point to find people in your area doing this kind of installation and begin to learn from their experiences. This will give you confirmation about all the things you’ve determine in these three steps. When it comes time to choose specific pieces of equipment, you and I can talk about that then.
What are your thoughts, Greg? I hope this has been helpful. Please let me know. We can get into specifics of equipment type and wiring later, as we finalize your situation.
Watch the video below to learn about the unique approach I use for online teaching about off-grid energy. Under the video window you’ll find a place to join my Off-Grid Energy Community. You’ll get free information about building your own off-grid energy system, and you’ll get a coupon code for $30 off the next running of my off-grid course if you choose to be part of it. The next session starts in October.