Solar-electric Coffee Roasting and Brewing

The Freshroast is rated at 1100W/120V (9A). An older model roasts a batch of 60-75 g (0.14-0.16 lb) of green coffee in less than four minutes, about 72 W·hr/batch. Although I haven't measured it precisely, the newer model appears to operate proportionally.

{(1100 W) (4 min)}/{1 batch} ({1 batch}/{60 g}) ({1 hr}/{60 min}) ({1000 g}/{1 kg }) = 1200 W·hr/kg

Two 60-g batches are enough coffee for three, three-mug press-pots of strong coffee. If you drink an average of three mugs each day, you will need to roast about two batches every three days. This will require 144 W·hr every three days. If you get 5 hr sunlight each day and have an 80% system efficiency, you will need only a 12W PV panel.1)

However, you will also need a 1.2 A·hr, 100 A car battery and an 1100W inverter (or voltage booster) . A twelve-watt panel is small enough to charge a car or motorcycle battery without a charge controller.

In addition, to brew the coffee from a solar-electric source, you will need about 100 W·hr to heat three mugs of water each day to near boiling.2) This will require about 25W more PV and 10 A·hr additional battery capacity. Although the following figures have not yet been updated to reflect these additional costs, the analysis should not be materially effected.

Equipment

$150 1Inverter (>=1100W, 10 yr)3)
100 1PV Panel (>=12 W, 30 yr)4)
60 1Motorcycle Battery (>=1.2 A·hr, 100 A, 3 yr)5)
180 26)Freshroast Coffee Roasters (>2500 batches, ≈10 yr) 7)
70 1Hand-crank Coffee Mill8) (>3500 batches, >10 yr)
40 1Stainless Steel French Press (>1000 batches, >3 yr) 9)
1Immersion heater or electric kettle
600Initial Investment
7¢/mug10)<25¢/batch11) + 4¢/pot

Supplies

$0.4260 gGreen Coffee Beans ($3/lb)12)
9¢/mug13)

Labor

20¢/mug5 min/batch Roasting14)
56¢/mug10 min/pot Brewing15)
76¢/mug≈ 4.4 min/mug, 10 usd/hr16)

Note you will actually lose money buying your coffee to go if it normally takes you about five minutes or longer to purchase it retail, since this is more time than it would take to roast and brew it yourself and the difference is enough to completely offset the fixed costs.

Total

JpGraph Function Plot

Supply costs: $100/yr (14.6 kg (32 lb) Green Coffee, Labor costs: $800/yr 80 hr Labor (20 roast + 60 brew)) Money Flow

JpGraph Table Data Plot 17)

Consider this. Suppose you have been spending $3/day on coffee and somehow, it takes you no time to get it. You decide to switch to roasting and brewing your own by purchasing your initial equipment and supplies on credit so it costs $800 to pay it back over the first year. You are still monetarily ahead by $300, but it cost you 80 hours of labor. But now look at the next three years. Instead of spending $3300 on coffee, you only spend $400, a savings of $2900 for these additional 240 hours. You have now accumulated a savings of $3200 for 320 hours of work and have broken even at $10/hr. The next three years are the same, and will have accumulated a savings $6100 for 560 hours labor and are now ahead both monetarily and in equivalent labor time.

2) ({1000 ml}/{3 mug})(100-15 C)({1 Whr}/{860 cal}) ≈ 100 W·hr/3 mug
6) Using two roasters halves the 10-minute roast/cool cycle time to 5 min/batch by roasting with one unit while the other cools.
10) ({0.25 USD}/batch) ({2 batch}/{3 pot}) ({1 pot}/{3 mug})+({0.04 USD}/pot) ({1 pot}/{3 mug})≈ 7¢/mug
11) 450+100+600+540=$1690/30 yr (7300 batches/30 yr) → 23¢/batch
13) ({0.42 USD}/batch) ({2 batch}/{3 pot}) ({1 pot}/{3 mug})= 9¢/mug
14) ({5 mn roasting}/batch) ({1 hr}/{60 mn}) ({2 batch}/{3 pot}) ({1 pot}/{3 mug}) ({10 USD}/{1 hr}) ≈ 20¢/mug
15) ({10 mn brewing}/pot) ({1 hr}/{60 mn}) ({1 pot}/{3 mug}) ({10 USD}/{1 hr}) ≈ 56¢/mug
16) ({5 min}/{1 batch})({2 batch}/{9 mug}) + ({10 min}/{1 pot})({1 pot}/{3 mug}) ≈ 4.4 min/mug ≈ .074 hr/mug
17)
Annual Cost Flow
YearFixedVariableComments
0 600 Initial Investment
1 100 800Supplies, Labor
2 100 800Supplies, Labor
3 200 800Supplies, Labor, Battery, Press Pot
4 100 800Supplies, Labor
5 100 800Supplies, Labor
6 200 800Supplies, Labor, Battery, Press Pot
7 100 800Supplies, Labor
8 100 800Supplies, Labor
9 200 800Supplies, Labor, Battery, Press Pot
10 500 800Supplies, Labor, Inverter, Roasters, Mill
11 100 800Supplies, Labor
12 200 800Supplies, Labor, Battery, Press Pot
13 100 800Supplies, Labor
14 100 800Supplies, Labor
15 200 800Supplies, Labor, Battery, Press Pot
16 100 800Supplies, Labor
17 100 800Supplies, Labor
18 200 800Supplies, Labor, Battery, Press Pot
19 100 800Supplies, Labor
20 500 800Supplies, Labor, Inverter, Roasters, Mill
21 200 800Supplies, Labor, Battery, Press Pot
22 100 800Supplies, Labor
23 100 800Supplies, Labor
24 200 800Supplies, Labor, Battery, Press Pot
25 100 800Supplies, Labor
26 100 800Supplies, Labor
27 200 800Supplies, Labor, Battery, Press Pot
28 100 800Supplies, Labor
29 100 800Supplies, Labor
30 700 800Supplies, Labor, Battery, Press Pot, Inverter, Roasters, Mill, Panel

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