SodaStream DIY Recoup and Calculations

sodastream_diy.soulver

@tl;dr
5.3 months // how often 3 sparkling water drinkers will go through a 20 lbs CO2 tank
$90 // SodaStream
$29 // 20 lbs tank refill every 6 months, at Robert's Oxygen in Dulles, VA
$201 // initial investment in tank, refill attachments/adapter, first CO2 fill
5.4 months // time to recoup initial equipment investment (sans SodaStream)

@initial equipment costs (excluding SodaStream, which we got as a gift)
diy_tank_20lbs = $115 + $21.34 shipping // 20 lb CO2 tank
adapter_kit = $30 // kit to attach tank to sodastream bottle to refill (tubes, connectors, valves)
co2_refill = $29 // at Robert's Oxygen in Dulles
initial_diy_cost = diy_tank_20lbs + adapter_kit + co2_refill

@refill interval
refills_per_month = 2.5 // the rate 3 sparkling water drinkers go through normal sodastream bottle in our house
diy_months_until_refill = 20 lb / (1.5 lb × refills_per_month) // when we'll need to refill the DIY CO2 tank in months

@refill cost breakdown
non_diy_refill_cost_at_target = $15 // 1.5 lb CO2 bottle
non_diy_cost_per_month  = non_diy_refill_cost_at_target × refills_per_month
diy_cost_per_month = co2_refill / diy_months_until_refill
diy_cost_per_sodastream_fill: diy_cost_per_month / refills_per_month

@initial equipment cost recoup 
recoup_months: initial_diy_cost / non_diy_cost_per_month // months_to_recoup_initial_diy_investment