power directly off of Panels?

240 w-h of energy per day.

50% depth of discharge for battery health.

240 x 2 = 480 watt-hours of battery capacity

15% battery charge discharge inefficiency

480 w-h x 1.15 = 552 watt hours of battery capacity

we will assume room temp for batteries for this case.

2 days of autonomy

552 watt hours x 2 = 1104 watt hours of battery capacity

1104 watt hours / 12 volts = 92amp-hours of battery capacity which works out to be a group 27 or group 31 12v battery.

---

pv system

for 2 days of actual autonomy to be possible, our array must have the ability to produce 1104 watt-hours in a day, so that after one cloudy day, the next sunny day the array is able to cover the loads as well as make up for the lost day.

without knowing your location, we will assume 4 sun hours per day.

1104 watt hours / 4 sun hours = 276 watts of pv at STC

276w of pv x 1.2 for difference between STC and Reality = 331w of pv

it is important to understand you can do less pv than this, but that the system design will not handle bad weather without another charging source.  just matching the loads alone in good weather would put you at 72w of pv power, so your 80 example could work, but when it is not a great sunny day, your system will not function at its best or for more than another day, maybe, without sunshine.

john's math is correct, but i wanted to spell out how i would do it if it were my situation, i am a little more cautious.

james
Alt-E staff

James,

I hope we haven't put you off by giving you the worst case scenario, but you should know it and you can decide for yourself what amount of risk you can live with.

Typical battery efficiency is usually given between 70 and 80 percent, but I was quite surprised to read a report on the Sandia Labs site somewhere that under certain conditions it can be as low as 50 percent. It's a very good site for research.
http://www.sandia.gov/ess/

Now on to a practical answer to your question. The first thing that a charge controller does is take whatever energy it has available and put the battery through a charging cycle. Once the battery is fully charged, the charge controller will actually turn "off" the pv panel so that no current is flowing. Now if you put a 40watt load on the battery then you interrupt the charge controller in its single-minded pursuit of charging the battery, and knocks it off its bulk/absorb/float cycle.

So if you have a pv panel capable of delivering 7 amps (84 watts for easy math) at 12v, and say your fully-charged battery requires 1 amp to maintain the float, then your 40 watt load would be drawing its 3.3 amps directly from the charge controller. The charge controller would also be delivering 1 amp to maintain the float voltage, so it will effectively be "de-rating" your 84w panel down to about 50w.

Now if you add a second 40w load, the charge controller would turn the panel power back up to 72w and service both loads while still maintaining the 1 amp battery float current. Add a third 40w load and suddenly you are drawing more current than the pv panel can deliver so you start taking current out of the battery and the battery voltage drops. The charge controller "sees" the drop in battery voltage and says "hey, I'm not in float mode any longer. It's time to re-evaluate the situation".

For practical purposes you can think of the charge controller as dumping everything that it gets from the pv panel into the battery, and the load as drawing all of its current from the battery, but in reality it is slightly more complicated than that.