# using a capacitor for an off-grid system

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!

• I have a 488 Ah battery bank, and Dr. Watt is the brand of power factor correcter I use for a fridge and an air conditioner. Not only does it help with starting but on inductive loads it will save you a lot of power over the long run. The rule is ELI the ICE man. ELI means the voltage sine wave preceeds the current sine wave in an inductive load, so the power factor controller will switch in capacitance as necessary so the peak voltage and current are in phase. Mar 25, 2017 at 7:31

## 4 Answers

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.

• Upvote. I totally agree. I've got 600 Watts of panels, a 20 Amp MPPT Renogy Commander controller, and a 1000W inverter that claims 90% efficiency. So far I have not run inductive loads much, except for the air conditioner with the power factor controller. It seems to absorb the startup peak current just fine. But it won't run long unless the sun is out, which is actually perfect. I have 5 22000uF capacitors and was thinking about how they might help and have come to the same conclusion as you have in this answer. No. Apr 24, 2017 at 3:12

A 1500W appliance running for one minute consumes 25 W-h. Compared to your total proposed capacity of 4800 to 9600 W-h (I'm assuming 12V batteries), this is a drop in the bucket.

As long as your inverter can handle this peak power level, your batteries will be fine, and you don't need any extra capacitors, either.

on paper you guys are right, however deep cycle batteries often have trouble starting certain appliances such as a freezer, hi amp blender or juicer etc. I recently set up a system with 4 deep cycle 12v batts that together ran a 720ahr bank. Also i bought a pure sine wave inverter rated at 3000w peak 6000w. This system should have been way overkill to start a small chest freezer which ran at 150w but starting it was drawing 1800w and immediately draining the bank while never fully starting the freezer. The answer was a car audio capacitor. I bought a 4000 ferad capacitor for 35 bucks online and hooked it up in parallel with my batteries. It works like a charm. there are hazards involved with charging capacitors but this system works well. Deep cycle batteries and capacitors are on opposite ends of the spectrum. the batteries deliver a consistent voltage but have poor immediate surging power. A capacitor can surge immediately but voltage will drop until it is recharged. hope this helps. Get a capacitor that is rated somewhere around your inverter wattage output. hope this helps.

• If you know where to get a 4000 Farad capacitor for 35 dollars, that can actually handle 12 volts, you need to share a link, buddy.
– user39962
Mar 15, 2015 at 5:06
• Your capacitor is most likely compensating for you having wires too small or too long (or both) between the battery and the inverter. Mar 15, 2015 at 6:31
• It is most likely a 4 farad capacitor and not 4000 farad. Mar 15, 2015 at 8:19

I have exactly the same setup as 69992, and its true. even though the system should handle a load such as a freezer pump, juicer sometimes they just dont and voltage drops quickly. First thing people underestimate is wire size. from batteries to inverter wires should be 0 guage or bigger like 0/1 or 0\2. overkill these wires and make them as short as possible. Now some cheaper deep cycle batteries like 24 or 27 series simply dont preform like a trojan or rolls and dont have the dump capacity needed to surge start loads like these. My solution to this problem was a car audio capacitor. These can be bought on ebay for like 30 bucks. Now these need to be wired in parallel with your batteries with the same heavy guage wiring as before as they are part of the same 12v loop. this loop should be capable of handling loads up to 200 amps as after the inverter the most current you would probably use with a etup like this is 20amps at 120v. thats why people go to 24 or 48 volt systems as they can get away with smaller wire sizes at the 12v loops. The capacitor was the key, i have a 4ferad cap in parallel with my batts. I was told that you size the cap to thw general wattage used so a 4 ferad cap would couple to a 4000w inverter perfect. these caps are not well known about in the off grid world, they solve alot of problems. The one unknown aspect of this setup is the unknown behavoir of battery charging as you are also charging the capacitior as well. hope this helps

• It's funny that you post the same information as user69992, have the same trouble capitalising words properly, use the same sign-off and make the same spelling mistakes. Anyone else think so? Feb 25, 2016 at 19:11
• And the same email address! (The user icons are based on a hash of your email.)
– user39382
Feb 25, 2016 at 22:07
• We have almost the same setup and have the same problem. Oct 31, 2019 at 22:30