Days of Autonomy

Novaz

Step 1
Please Select Your System Design Parameters
System Loads and Battery Capacity Requirements

Values below will change as you enter system parameters and load estimates.
Battery watt-hours per day for AC loads
(including 15% AC inverter loss) 25218.49 Wh/day
Battery watt-hours per day for DC loads 0.00 Wh/day
Total battery watt-hours per day
(assuming 97% wiring and distribution efficiency) 25998.44 Wh/day

Having found the very helpfull Trojan battery calculator
I wanted to make sure i understood this field correctly
is it the days when you are either not drawing power from the system or is it days when not receiving enough charge
Thanks




Avg daily battery amp-hours needed
(with 24V battery system )
1083.27 Amp-hours (@24V)
Required system capacity*
(based on desired 50% DOD) to achieve 1 days of Autonomy. 2166.54 Amp-Hours (@24V)
Choose system design battery voltage (12V, 24V, or 48V)

Choose desired battery depth-of-discharge (DOD)

Type of Battery

Days of Autonomy

Section Divider

Cariboocoot

Re: Days of Autonomy

Days of autonomy is the number of days the batteries must supply power on their own while receiving little to no charging.

It is also known as "the autonomy trap" because people think they need several days' worth and end up with an over-sized system.

My own preference is for 25% average daily DOD leaving one day "in reserve" before reaching 50% DOD, and a generator for day three. Most of the time you won't have to run the gen, and when you do it will probably be for several days in a row.

BB.

Re: Days of Autonomy

All I can say is understand and parallel calculate the battery capacity/etc... Many times, these calculators will make other assumptions that you are not aware of (sometimes it is just a definition, other times, they derate to "save you" from your mistakes).

So, what did you start out with? Something like 10kWatt*Hours of AC power per day (that is a good sized off grid power system--~300 kWH per month)?

Anyway--1 day of autonomy and 50% maximum discharge is a bit small for a full off grid home used 9+ months of the year... For many reasons, 2 days and 50% works out better (optimum) for system design (too small of bank, more stressed when discharging, and if you want to "charge it fast", cannot support as large of array, smaller bank has lesser surge capacity for starting well pumps, etc.).

If you are only looking for emergency power/weekend use/RV use, 1 day + 50% discharge of storage may be more cost effective.

Anyway... How I would run the numbers with 2 days (1-3 days is "practical") and 50% maximum discharge:

10,000 Watt*Hours per day * 1/0.85 inverter eff * 1/24 volt bank * 2 days of storage * 1/0.50 max discharge = 1,961 AH @ 24 volts

Now--More or less, once your battery bank exceeds ~600-800 AH--It becomes a bit more of a nightmare. Heavier copper cables, more Solar charge controllers (to manage more than 60-80 amps of charging current for 10% rate of charge)--So, I would be suggesting a 48 volt battery bank (its own issues with fuses/breaker ratings, more of a shock hazard, more cells to monitor/water):

10,000 Watt*Hours per day * 1/0.85 inverter eff * 1/48 volt bank * 2 days of storage * 1/0.50 max discharge = 980 AH @ 48 volts

And then there is the whole question of what size of AC inverter (or inverter/charger) you want to support... More or less, a good starting rule of thumb is ~100 AH @ 48 volts per 1 kWatt of AC inverter maximum draw... So a 980 AH battery bank would support around a maximum of 9.8 kWatt of continuous draw and ~2x that or 19.6 kWatt of starting current (few seconds to start a well pump, etc.).

So--Based on my guesses about your input--It does appear that the Trojan calculator produces similar results to my hand calculations. But--Each "factor" in the calculation really requires lots of thought and discussion about requirements and tradeoffs. When you start using online calculators--Much of that decision process can get "lost" when the numbers pop out magically.

-Bill

Cariboocoot

Re: Days of Autonomy

BTW if you're running into 1000+ Amp hours you should be increasing the system Voltage. For one thing you'll need fewer charge controllers and for another you will improve over-all system efficiency.

It looks like you're planning on around 20kW hours AC per day. That is very large for off-grid and should be on a 48 Volt system. It would use about 463 Amp hours @ 48 Volts as opposed to 926 @ 24 Volts. A difference of 1852 Amp hours (needing 3 charge controllers) vs. 3704 Amp hours (needing 5 charge controllers). In either case about 12kW array.

Faced with those numbers I'd be revisiting conservation efforts intensely.