I am working on plans for a new off-grid home in New Brunswick, Canada. I am trying to sort out the size of my system and I am getting quotes across a pretty big range.
My expected load is 5,500 kWhr/year (about 15kWhr/day) based on the design for the house. Solar potential for my area is shown in this chart:
Quotes are:
2.4kW panels, 26kWhr batteries
8.4kW panels, 65kWhr batteries
8.4kW panels, 139kWhr batteries
5.4kW panels, batteries TBD
7.1kW panels, 118kWhr batteries
From my understanding of the solar potential for my area:
I will produce 60kWhr per kW of panels installed in worst months
I need 15kWhr/day * 31 days = 465kWhr for worst months
Panels should be 465kWhr / 60kWhr/kW = 7.75kW
If I assume lead acid batteries and two days backup (as was done in my quotes) batteries should be 15kWhr/day * 2days / 30% depth of discharge = 100kWhr
Can anyone sort me out? Do I have an error in my understanding of the system size basics? Why would the quotes I am getting be so different?
It’s not unusual to get a wide range of quotes as some solar installers are not very familiar with off-grid solar system design and performance estimations. First question, is your electricity consumption (15kWh per day) in the winter months expected to be roughly the same as summer? Typically winter will be much higher.
The first quote with a 2.4kW solar array is clearly undersized and would not generate anything near 15kWh per day in winter. I would forget about this company.
The other quotes all seem to be much more appropriately sized in regards to solar. I am not familiar with your location but based on the estimated monthly generation you would need at least 8kW of solar. (based on 2 peak sun hours - PSH per day).
The big difference is in the battery sizing. The large variation could be due to lead-acid Vs Lithium batteries. In my experience lithium is far more efficient (approx 97%) and much more energy is available (up to 90% DOD) compared to lead-acid which only allows 30% DOD (depth of discharge) on an average day.
Also lead-acid batteries loose another 10-20% of capacity when the cell temperature drops close to freezing. That being said lithium batteries can completely shutdown at temperatures below 5degC, so the battery enclosure may need to be heated or very well insulated. (we don’t have this problem in Australia though
Thanks for taking the time to provide your thoughts!
Consumption will be a touch higher in the winter for us but not by much. Our off-grid plan keeps the main heating loads (space and domestic hot water) on alternate fuels.
The quotes we got are all lead-acid with the notable exception of the TBD one. They suggested further discussion and possible use of lithium. The size of our battery system and how we choose to run the backup generator will also be intimately related.
The battery storage will be inside the building envelop in well conditioned space so temperature issues should not be an issue.
No problem, just a quick question. What brand/type inverters where you quoted? Outback Power and Schneider Electric seem to be popular in North America.
