An AC is usually on when it’s hot – and at least here in Hawaii that means the sun is shining, and the heat-pump runs directly on energy generated by 3 solar panels. The solar-powered air conditioner is in it’s 3rd generation and has most of the problems worked out. The installation without modifications is really easy. It takes as much power as is available from the solar panels, and makes up for the difference from the grid. The SEER rating is 35 for DC power, that makes it one of the most efficient units on the market – units more efficient than 14.5 SEER can get energy star certified. Since it is connected to the grid, it can be used at night as well – 900W is the maximum usage. As most heat pumps it is reversible, so it also works nicely as a heater.
Often the sun is shining, but there is no need for air conditioning, even during overcast conditions the panels keep producing, so we added a charge controller and batteries into the system. The batteries store the energy to power LED lights and USB chargers. Almost all rooms now have dual light – the incandescent lights stayed, and we installed flicker free pure DC lights. Theoretically the batteries will power the house for 3 nights without a recharge, but so far the weather was too good to ever reach the limit.
We selected Nickel Iron (NiFe) batteries, because good quality Lithium batteries were still prohibitively expensive. Lead-acid batteries are toxic, notoriously hard to maintain, and have a much shorter lifespan – at 25% Depth of Discharge (DoD) 1500 cycles are expected, but at a 50% DoD that’s already down to 1000 cycles – 3 years. For a Nickel-Iron battery the life expectancy is 10,000 cycles – 27 years, at 80% DoD. The DoD difference means, if 1.8 kWh are needed out of a system the size of the lead acid battery has to be 3.6 kWh (50% DoD) to 7.2 kWh (25% DoD). The NiFe battery has to be only 2.4 kWh (75% DoD)*.
The 12V direct current LEDs that are installed now give very even and beautiful light at a fraction of the price of AC LEDs, unlike LEDs that run on rectified DC they are completely free of any detectible flicker, hum or buzz. They are the most energy efficient lights available at the moment.
* In Ah instead of kWh: The DoD difference means, if 150 Ah are needed out of a 12 V battery bank the size of the lead acid battery has to be 300 Ah (50% DoD) to 600 Ah (25% DoD). The NiFe battery has to be only 200 Ah (75% DoD).