Zero Sum vs. Sliding Window Charging
Zero Sum charging
is a charging strategy where by any charge used is fully replaced in the cycle time. Typical cycle times are one day and the bank is sized to be 1.5 to 2 times the cycle (days) use.
So if you were using 200 AH per day you would size solar
to fully replace your usage in 1 day or around 200 AH worth of solar
. With 1.5x to 2x daily usage we end up with a bank size of 400 AH.
This is a pretty normal setup. When we exceed the capacity of solar charging we run our engines or generator
to bring us back to fully charged (e.g. 80% SOC). With 2x bank capacity we are able to “slide” out of zero sum for a day or 2.
But in the Sliding Window mode This sliding is built into the design by choice.
In that case we size our house bank to be 5 or 10 times the daily usage. With 200 AH daily usage our bank becomes 1000AH to 2000AH in size (cost, cost, yes we know. Just ignore it for now. This is theory).
Solar (or other prime charge sources) can be sized at 100% of daily usage more or less.
The “window” is 200 AH and is 1/5th of a 1000 AH bank. With a Min and Max bank SOC of 20% and 80% we have 600 AH for the 200 AH daily use window to “slide” in.
When the daily charge equals the daily use the window stays put. When the daily charge is greater than the daily use the window slides up to 80 SOC and we stop charging for the day.
When the daily use is greater than the daily charge the window slides closer to 20%SOC.
When the window slides to 20% SOC we bring on additional charging capability e.g. we run the engine
So if we were to use some FRN (Farcical Random Numbers) to show the min-max window sliding we could get something like (starting with 80% SOC)
80% - 20% usage = 60% SOC + 20% charge = 80% (Day 1)
80% - 20% usage = 60% SOC + 15% charge = 75% (Day 2, a touch cloudy)
75% - 20% usage = 55% SOC + 15% charge = 70% (Day 3, still cloudy)
70% - 20% usage = 50% SOC + 10% charge = 60% (Day 4, really cloudy)
60% - 5% usage = 55% SOC +20% charge = 75% (Day 5, low use with good sun)
75% - 25% usage = 50% SOC + 5% charge = 55% (Day 6, high use, low charge)
55% - 25% usage = 30% SOC + 5% charge = 35% (Day 7, new beer
in fridge, towel on solar array)
35% -25% usage = 10% SOC + 40% charge = 50% (Day 8, run generator)
50% - 25% usage = 25% SOC + 40% charge = 65% (Day 9, ran generator again)
65% - 25% usage = 40% SOC + 40% charge = 80% (Day 10, ran gen, resynced SOC)
I would venture to say that most of us are in the zero sum charging mindset but in fact use some form of sliding window. This is more or less a default.
But to actually use sliding window to its advantage we need to change our mindset and slid the window by design with an increase the total house bank capacity to cover more than 2x daily usage. 5x to 10x being the ballpark size.