This is my first post on this forum! Something I would like to get some input on, is whether there is an economic benefit of owning a large battery bank. That obviously depends on the cost of batteries at any given point in time, the cost of grid electricity and the specifics of the battery to buy. The reason for my post is because I currently have a Victron Multiplus 5kVA with 2xUS3000 batteries. I am contemplating whether to get a third, which seems to be more “in spec” than only two.

Some data I managed to gather:

- The Pylontech US3000 warranty can be summarised as follows:

1.1 4500 Cycles @ 90% DoD (My understanding is that 6000 @ 80% is also possible, and because 4500 @ 0.9 < 6000 @ 0.8, I’ll use 6000 @ 80% in my calculations).

1.2 60% nominal capacity remaining after 10 years (or 70% after 7 years).

1.3 I’ll assume that capacity degradation is linear over 10 years. - I’ll assume a 90% DC to AC conversion loss, assuming that you use the battery with an inverter.
- Current cost of electricity in Bellville, Cape Town is R2.12 for the first 600kWh and R2.92 thereafter.
- It is impossible to estimate future electricity increases and future investment return on other investments (if you didn’t buy a battery) so I will assume that the electricity increases and investment return are the same (negating the need for an unreliable time value of money adjustment). This would hopefully also lead to a somewhat prudent calculation.

Over a US3000’s life cycle - ignoring the 10 year lifetime, you can therefore expect to cycle the following amount of electricity from it:

6000 (cycles) * 3.5 (nominal capacity) * 80% (DoD) * 80% (average nominal capacity) * 90% (to get the power on the AC side) = 12,096 kWh

However, more likely that you get 365.25 * 10 (days in 10 years) * 0.8 (1x80%DoD cycle per day) = 2,922 cycles over the 10 years, assuming you have enough PV every day to cycle it. I’ll make some guess work and say that roughly 20% of the days in a year you don’t do a cycle: 2,922 * 0.8 = approx. 2,340 cycles.

As such, over 10 years only about 4,700 kWh.

With the price of electricity at 2.92 (the most I can pay), this adds up to R13,724 of savings. Currently the cost of a battery seems to be roughly R21,000 (just quickly checked on an online store, sure the price might fluctuate and this might not be the cheapest).

So my questions:

- How likely is it that you get more than 10 years of service from your battery if you treat it well? (I’ll make another post at some point asking advice on what it means to treat your battery well, let’s just assume for now that it is being treated well)
- Are there any assumptions in my calculations that are wildly wrong?
- Am I missing something because, unless you have a very large backup need, it would seem to me that it only makes economic sense to purchase enough batteries to match your inverter’s maximum demand, but no more than that?

(This edit was for science)