Quote:
Originally Posted by Zeke
I like where this is going. The only thing I see as a precaution is what happens if there is more power produced than is being drawn (clear summer day and no one home) and the cut off protection. I assume there is an auto switch for that like when using a generator (there is).
The reason for 'selling' excess power back to the grid is so excess power has a place to go when not being used at the home. There needs to be a battery and some way to buffer the production of power when 'off grid'. There are hybrid solar power systems but what you propose ain't one of them. It's not going to be as simple as you think. Read this for some en lightenment.
Not my area of expertise but I'm real leery of the solar installation companies. |
The standard "Grid-Tied" systems are setup to flow any excess out to the grid. They have no storage and most cannot modulate the power. And they also need a voltage/frequency signal from the utility to work. They are designed to shutdown if the power goes out.
Grid-Interactive systems have batteries. They also use charge controllers to collect the solar power and keep it DC. Depending on the manufacturer and the programming options, you can set up to sell excess, store excess, never sell. But these systems are more sophisticated and they include batteries.
Batteries have a finite life. After so many charge-discharge cycles, they stop working. And the number of cycles will depend on how deeply you discharge the batteries.
Li-Ion (NCA is most common) are terrible for storage. They are good for about 1000 cycles. I have special lead-acid types that allow 60-70% usable capacity and have 2600-3000 cycles. LiFePo4 are "the best", but they are expensive. They will last 7000-10000 cycles, which on a daily charge/discharge is 20 years.
I calculate the storage cost by taking the cost and dividing it by the (#expected discharge cycles * usable storage * the round trip efficiency).
If a 1.2kWh battery has a 1.0kW usable range and lasts 10000 cycles and has a 93% efficiency and costs $1K, then the battery charge/discharge cost is 10.8 cents/kWh.
If a 2.4kWh battery has a 1.44kW usable range and lasts 3000 cycles and has a 90% efficiency and costs $550K, then the battery charge/discharge cost is 14.1 cents/kWh.
And this is JUST for the battery storage. It does not price in the cost of the solar equipment. For most of us, that is more than we pay for electricity. So self storage, unless you are paying over 20 cents per kWh (California/Hawaii/Puerto Rico) it doesn't make sense.
What we sell is Hybrid Solar: Solar PV plus battery backup, backed up by a generator. It operates like a grid-tied system, except when the power is out. The batteries are kept full are are for short term power outages. During the day, the solar helps to supply power. At night, it is all battery. If you are short of power while the utility is out, the generator can supplement. It power the house and recharges batteries when the batteries reach a defined point. Once the batteries are recharged, the generator shuts off.