New to the side and have very little knowledge on electrical systems. I'm trying to come up with a good off-grid system for my small house and have the idea of a PV array with a battery bank, charge controller, and inverter to charge during the day, and then have a bicycle running a car alternator to charge the batteries/help with load during the evening when I workout.
Now my question is, we intend to run some high-draw appliances such as a vitamix blender, food processor, etc. from time to time, and will these appliances need a capacitor to help with the large draw when we first turn them on? Both to protect other devices running off of the electric system and also for the batteries? I'm assuming so, but wanted to make sure so I don't spend money unnecessarily. Also, how would I calculate the size of capacitor I'd need if it is necessary?
Also, correct me if I'm wrong, but I should not be overly concerned with the battery bank being depleted from appliances like this since they are on for a minute or less at a time? I'm planning on a bank of about 400-800 amp-hours. Please correct me if I'm mistaken!
Thank you all very much in advance!
Best Answer
The quick answer is no. A capacitor won't help, AND you don't need it. If you have a 400 - 800 AHr battery, it will provide you with all the juice you need for a small appliance such as you've specified. Additionally, without going into details, practical capacitors simply don't have the energy and power density you want.
If you're worried about transient load currents (as you should be), you need to pay attention instead to your inverter specifications. Inverters don't generally have much excess margin on their load current specs, so you need to get one which seems oversized compared to your average load. The exact amount of this overage will depend on exactly what appliances you're using.
Recharging via exercise bike won't hurt, but it won't help a whole lot, either. The general rule of thumb for humans doing work (like driving a generator) is 100 watts. This is (roughly) the solar power incident at noon on a 1 square foot solar cell. Figure 20% efficiency for the cell, and a 1-hour workout on the bike is more or less the equivalent of an extra square foot of solar cell over the course of a bright day. Like I say, it won't hurt, but its major impact will be to make you feel better about exercising and not wasting energy.
By the same token, running appliances for short periods of time need to be taken into account, but in general they are not dominant in your energy budget. Let's say a blender draws 10 amps at 120 volts. For a really good inverter with a 12-volt input, that's an inverter draw of 100 amps. But, it only lasts for 1 minute, so the total battery drain is 100 amps x 1/60 hours, or about 1.5 amp-hours.