Recycler Math

Recyclers stack with each other, but only to an extent. The percentage of the best recycler on the vessel determines the maximum percentage of the other recyclers on board. For example, if the best recycler is an RT-5000 (79%), then no matter how many RT-500s (60%) or MPLs (50%) you add, you will never exceed 79% recycling. However, if the RT-5000 only covers some of your crew, the others will stack additively with each other to meet it.

1 kerbal consumes 0.00050 supplies/second, or 0.50/1,000 seconds (the rate shown at 1,000x time warp). Let's represent this with the variable k = 0.50 supplies/1000s.

16 kerbals consume

16k = (16 * 0.50) = 8.00

If we add an RT-5000 in there, it will reduce supplies consumption by 79% for 3 of them, so we end up with

13k + (3k * 0.21) = 6.50 + 0.32 = 6.82

Adding an MPL in there will affect 4 more at 50%, so we get

9k + (3k * 0.21) + (4k * 0.5) = 4.50 + 0.32 + 1.00 = 5.82

If we add enough RT-500s to cover the remaining 9 at 60%, we have

(3k * 0.21) + (4k * 0.5) + (9k * 0.4) = 0.32 + 1.00 + 1.80 = 3.12

Notice that each RT-500 reduces supplies consumption by 0.30, since 0.50 (the normal amount for one kerbal) is reduced by 60%, which is 0.30. This means that every RT-500 we add will reduce supplies consumption by 0.30. So when we added 9 RT-500s, we reduced supplies consumption by 5.82 - 3.12 = 2.70 = (0.30 * 9).

Now that we have all our kerbals covered by recyclers, the math is a bit more difficult to follow, but what happens is that each recycler will continue to subtract its amount until the minimum consumption is reached. The minimum consumption is the amount that would be consumed if the largest recycler applied to the entire crew. In our above example, that would be the RT-5000 for the entire 16 kerbals, for (16k * 0.21) = 1.68.

Since the floor is 1.68, and each recycler will continue subtracting its amount, we have a couple options. A MPL will subtract 1.00, since 4 kerbals consume 2.00 without it and 1.00 with its 50% recycling. Therefore, two MPLs will reduce it by 2.00. Normally this would give us 3.12 - 2.00 = 1.12, but since our minimum consumption is 1.68, it will simply peg it at 1.68.

We could instead add 5 RT-500s. This would reduce it by 5 * 0.30 = 1.50, which is enough to bring it down to the minimum. Since 2 MPLs weigh 7.0t and 5 RT-500s only weigh 0.5t, this is definitely the better option in this case.

Recycler	Mass	Percentage	Capacity	Rate reduction	Reduction/ton	EC/s	Mod
RT-500	0.1t	60%	1	0.30	3	0.50	USI-LS
RT-5000	3.75t	79%	3	1.185	0.316	18.75	USI-LS
MPL	2.5t	50%	4	1.00	0.4	1.00	Stock
'Duna' Pioneer Module	2.31t	70%	4	1.40	1.2987	3.00	MKS
'Duna' Kerbitat	4.45t	77%	4	1.54	0.3461	21.00	MKS
'Duna' Kerbitat Water Purifier	4.45t	88.5%	4	1.77	0.3978	21.00	MKS
'Tundra' Pioneer-Logistics	3.11t	70%	4	1.40	0.4502	3.00	MKS
'Tundra' Kerbitat 2.5m	7.65t	81%	4	1.62	0.2118	37.00	MKS
'Tundra' Kerbitat 2.5m Water Purifier	7.65t	90.5%	4	1.81	0.2366	37.00	MKS
'Tundra' Kerbitat 3.75m	9.48t	86.5%	4	1.73	0.1825	138.00	MKS
'Tundra' Kerbitat 3.75m Water Purifier	9.48t	93.25%	4	1.865	0.1967	138.00	MKS
'Salamander' Command Pod	2.0t	25%	2	0.25	0.125	1.00	MKS
'Orca' Command Module	17.5t	80%	6	2.4	0.1371	40.50	FTT
Malemute Geology Lab	1.2t	25%	2	0.25	0.2083	1.00	Malemute

From this chart, we see that the best way to reduce supplies usage is to get the best recycler you can afford (usually an RT-5000) and then slather RT-500s on the outside of the craft for any kerbals not covered by it. They have a much better reduction/mass ratio than any other part by a large margin. Here's a helpful cheat sheet for life support modules if you have one RT-5000 already:

Number of kerbals	Number of RT-500s	Total recycler mass	Consumption Rate	500 supplies will last for
1	0 (covered by RT-5000)	3.75t	0.105	220d
2	0 (covered by RT-5000)	3.75t	0.21	110d
3	0 (covered by RT-5000)	3.75t	0.315	73d
4	2	3.95t	0.42	55d
5	3	4.05t	0.525	44d
6	4	4.15t	0.63	36d
7	6	4.35t	0.735	31d
8	7	4.45t	0.84	27d
9	8	4.55t	0.945	24d
10	10	4.75t	1.05	22d
11	11	4.85t	1.155	20d
12	12	4.95t	1.26	18d
13	14	5.15t	1.365	16d
14	15	5.25t	1.47	15d
15	16	5.35t	1.575	14d
16	18	5.55t	1.68	13d

Note: Multiply these rates by 3.6 to get the rate in supplies per hour.