# Battery size calculations

Options
Solar Expert Posts: 218 ✭✭
I want to a 3920 per day using a 800 watts array. the average sunshine hours are 6 hours per day. What should be the battery bank size ?

consumption is not continuous, it is like
• 1 hour before sunrise
• 1 hour from 1pm to 2 pm
• 1 hour from 4pm to 5pm
• 1 hour from sunet
• 1 hour from 8pm to 9pm

Is the size of array good enough ? Can I use Morning star MPPT 45A or do I have to use a 60A MPPT ? Or should I use a Midnite or outback charge controller ?

Options
Re: Battery size calculations

The basic recommendation is for the battery bank to support 1-3 days of energy usage (bad weather) with 2 days and 50% seeming to the "optimum" design point for most people (use genset for beyond 2 days of poor weather).
• 3,920 Watt*Hours per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days * 1/0.50 max discharge = 769 Amp*Hour @ 24 volt battery bank

This is about the maximum size 24 volt battery bank I would suggest. You may want to look at 48 volt battery bank--If it makes sense for your power needs (24 VDC vs 48 VDC appliances, etc.).

Getting 6 hours of sun per day minimum:
• 3,920 Watt*Hours per day * 1/0.52 system efficiency * 1/6 hours of sun = 1,256 Watt array minimum "break even"

Then there is the amount of charging current the battery bank requires. We suggest that 5% to 13% rate of charge (based on banks 20 Hour AH rating) would be:
• 769 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,448 Watt array minimum
• 769 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,896 Watt array nominal
• 769 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,765 Watt array "cost effective" maximum

So, here we are--This design assumes that you generate power during the day and use power at night. And you want 2 days of energy storage.

And, as you see, in your case, the "needs" of the battery bank actually exceeds the solar array size needed to power your loads.

You can look at a smaller battery bank... Say you only need 1 day of storage and 50% maximum discharge. That would reduce your charging current needs by 1/2. Note--This also reduces the ability of the battery bank to support surge currents and high charging current--So you need to look at your load needs/backup power for bad weather requirements.

You can also reduce battery bank size if you are willing to reduce loads during bad weather/have more usage during the day vs night--But that makes the system a bit less "friendly" (your spouse, others using the system may not want to turn off "optional" loads during bad weather, etc.).

Anyway--Given the above numbers--What is your "pain point" for system size/costs?

If you are off grid full time and cycling daily--You should really be looking at 10% minimum rate of solar charging--Unless you are willing to really monitor the system and run the generator more (the closer to "run to the edge", the more attention the system requires).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭✭
Options
Re: Battery size calculations
drraptor wrote: »
I want to a 3920 per day using a 800 watts array. the average sunshine hours are 6 hours per day. What should be the battery bank size ?

consumption is not continuous, it is like
• 1 hour before sunrise
• 1 hour from 1pm to 2 pm
• 1 hour from 4pm to 5pm
• 1 hour from sunet
• 1 hour from 8pm to 9pm

Is the size of array good enough ? Can I use Morning star MPPT 45A or do I have to use a 60A MPPT ? Or should I use a Midnite or outback charge controller ?

Okay 800 Watts isn't going to do 3.9 kW hours per day. It just isn't.
Average sunshine hours are irrelevant; it is minimum hours of equivalent good (direct) sunshine that matters. Even if your day is 16 hours long like it is up here in Summer that does not mean 16 hours of direct sun on the panels. You rarely will see more than 5.5 no matter where you are or how long the day is (unless you use a tracker).

What you can expect from 800 Watts of panel is about 1.6 kW hours AC if everything has to go in/out of the batteries. You can raise that up to about 2.6 kW hours if you can use loads during the day (which improves efficiency by making use of power that would otherwise go unrealized). I know because I have 700 Watts of panel.

800 Watts would support around 256 Amp hours of 24 Volt battery. That would be 1.3 kW hours AC at 25% DOD (see the importance of making use of daylight power?)

If you need 3.9 kW hours AC per day you need a larger system. You can't squeeze blood from a stone. Realistically you are looking at 1800 Watt array and 578 Amp hours of battery @ 24 Volts. Minimum. You probably would indeed be better off at 48 Volts for that power level.