What battery will give me the best return of investment?
Batteries have a ROI? lol
100% of the financial ROI comes from the PV. The batteries are a pure cost. You can, however, try to determine which battery has the lowest cost over a time frame like 10 years. Lithium is winning that battle.
Intangible returns include: ability to consume more, safety (LiFePO4 is the winner here), UPS when grid is down (which is 24/7 if totally off grid), how long you can maintain power when grid is down or it is cloudy if off-grid. Investing in these intangibles often means purchasing batteries, and this can dent your payback period really fast.
The best payback period (best direct financial ROI) is currently grid-tie w/o batteries.
If your income depends on power, then UPS can potentially be translated to a financial ROI. E.g., data centers. I'm in this category. I view batteries as short-term UPS, and solar as long-term UPS. Business continuity has financial value, and thus increases the ROI calculation. Though business continuity is only a portion of my consumption.
The variations in how people value the intangibles is why there is no one size fits all for how much battery capacity a person should have.
If you can get a 48-volt lead acid (LA) forklift battery cheap then it can be the best place to start cash flow-wise. If buying new for long-haul, you'll probably do better with LiFePO4 due to the higher cycle life. If you are only doing 50% discharge on the forklift battery to extend its life, then you'd want to compare 510ah of forklifting to 255ah of LiFePO4. When you then include the number of cycles until a certain point in capacity is reached, such as 80% of original, you'll likely find that LiFePO4 costs less over 10 years, though it's upfront costs are certainly higher.
