Sizing an Off-Grid Array

We're seeing lots of increased demand for remote-sited off-grid PV installations. I know that in sizing the battery bank, you need to take into consideration your required days of autonomy.

What I'm curious about is how you size your PV array to replenish that reservoir. What is the rule of thumb for over-sizing your PV array, to allow for satisfying a typical day's demand, while also allowing for replenishing of the battery bank during normal production?

Comments

You can get a good introduction to what goes into sizing an offgrid system in Dunlap's Photovoltaic Systems book.

You size the PV array based on the load and the local conditions. The size of the battery bank and the PV array are not directly related, i.e. the size of one is not used to size the other. If they are both sized to the load then the PV array will be sized larger than needed to keep the battery bank charged under almost all conditions.
The problem with offgrid systems is that the PV array is oversized during average conditions of use since it has to be sized for the load under the worst case conditions. Offgrid PV arrays are often twice as large as you would need to zero out a grid tied home with the same annual energy usage. If we had a long term storage solution the off grid PV array could be much smaller and less expensive.

Reply

Hello Marvin,

I a solar installer and i have done a couple of off-grid system, but as always i am trying to get a better understanding every i do a new one. I try to do a lot of readings and comparisons to insure that i am going the right direction. Anyway, as i read your above comment I still see a misunderstanding of getting the correct PV array to charger the batttery bank. I read an articile from Adam Diehl, which is attach at the bottom. It is very good and i use the same method. Sizing the battery bank is simple, but sizing the PV array is a little confusing. As per the articile in sizing the PV array, it appears that it is sizing to only charge one battery bank. That is good, but if you have 3 cloudy day(autonomy) then the size of the battery bank goes 3x or 3 battery banks. And therefore, shouldn't the amount of the PV array increase to insure this larger size bank? Any comments please let me know.

Sorry, could not attach article. It is from Adam Diehl from Greentech Renewables written in September and October