I am working on the design for the sump pump solar powered backup. There are several design criteria that need to be considered. These are:

1. The size of the electrical load.
2. The amount of usage.
3. Length of potential independent operation e.g. power outage
4. Cost effectiveness of the design

First consideration is the actual electrical load that will need to be satisfied. The pumps are rated at 0.3 HP each. The name plate rating gives the voltage as 115 volts AC, 9.7 Amps. Ohms law states that:

P=IxE

Where P is the power in Watts, I is the current in Amps and E is the Voltage. So we have P_watts= 115 Volts X 9.7 Amps = 1115.5 Watts. That seems like a lot for a 1/3 HP motor.

I find this curious. A unit of horsepower is equivalent to 745.6 watts of electrical power. A mathematically inclined person might take 745.6 watts and multiply it by 0.3 and get 223.68 watts. However, motors are not 100% efficient, so some efficiency factor needs to be figured into the equation. A conservative guess would be a motor is 60 percent efficient. If you take 100 percent and subtract 60 percent you have 40 percent, which is energy that is lost, usually due to heat. Therefore if you take 223.68 watts and multiply by 1.4 the result is 313.15 watts. Using Ohm’s law, P=I x E, The result for 313 watts/115 volts = 2.72 Amps.

I decided that I needed to confirm the power being used by the sump pumps. I did not bring my Kill A Watt meter home with me, so I used my clamp on ammeter to measure the current in the power cord going to each pump. Sure enough, as the picture shows, 10 amps. Bad power factor? It must be, I can imagine a motor that would be that inefficient. Power factor is something that can be fixed, some Saturday when I have a little tinkering time.

Regardless, the first result of 1115 watts holds, so that is the load we will have to calculate for. Their are two pumps in the system, so the total possible load will be 2230 watts when both pumps are running.

I am looking for a somewhat unique operation. What I want to do is power the pumps with the solar panels if possible and have the electrical grid available as a backup. Included in this would be some amount of reserve power in case the pumps need to run when the batteries are discharged and the grid is not available. This will give me the best of both worlds, using the PV panels to off set our electrical use for the pumps and providing a back up power source for the pumps when the grid fails.

Xantrex makes the DR 2424 inverter that has a battery charger built into it. This will work well in this application with a few external control devices to connect the grid power to the battery when the battery voltage indicates a 50 percent discharge state. It has a continuous rating of 2400 watts with a 6500 watt 5 second surge capacity, which is needed when starting motors.

The battery bank size needs to be calculated. This is going to be quite large because of the inefficient motors. I will use the basic 1 hour time unit for calculations. To run the pumps for one hour will require that the inverter provide 2230 watt/hour. The inverter is 96 percent efficient, therefore, 2230 watts x 1.04 = 2320 watts from the battery bank.

To convert this to Amps, divide by the battery bank voltage, which will be 24 volts, so 2320/24= 97 amps. Therefore, to run both pumps for one hour will require 97 Amp/Hours (Ah) of storage. However, the pumps do not run continuously, rather, they cycle on and off. The old main sump pump was never on for more than 50 percent of the time, even during heavy flooding. Often times it was much less, more like 10 to 20 percent of the time. Pump number one is in the same position as the old main sump pump, so I will assign a 50 percent duty cycle to that unit. Pump number two is on the other side of the basement, where it seldomly runs. I will assign a 15 percent duty cycle to that unit. To properly calculate the duty cycle we need to divide the total load by the number of pumps, then calculate the duty cycle load for each pump. Both pumps running will draw 97 amps. Divide 97 by 2 = 48.5 amps. 48.5 amps x 0.50 percent = 24.25 amps and 48.5 amps x 0.15 percent = 7.28 amps. Therefore, add the two pumps together and the amp hours (Ah) under average conditions is 31.5 Ah.

I would like the system to run autonomously (without input from the grid or solar panels) for 48 hours. 48 hours x 31.5 Ah = 1512 Ah. I would like to operate the battery bank so that in never exceeds 70 percent discharge voltage, so 1512 Ah x 1.3 = 1965 Ah battery bank storage capacity.

I would like the batteries to be charged by two 150 watt solar panels. This will require the solar panels, mount, and charge controller. I want the PV panels to operate at maximum efficiency, so I will be using a passive tracking mount and an MMPT (Maximum Power PoinT) charge controller. Both of those items will increase the amount of power from the PV array by about 25-30% each.

Finally, I would like the batteries to be disconnected from the inverter and recharged when they reach the 50 percent discharge voltage, which will require something with a low voltage disconnect relay. Here are all the parts:

• 1 each, Xantrex DR 2424 inverter, cost $755.00
• 4 each, Surrette S-530 6V deep cycle batteries, cost $1137.00
• 2 each, Sunwize SW150 solar panels, cost $1184.00
• 1 each, Blue Sky Energy Solar Boost 3024i charge controller w/MMPT, cost $248.00
• 1 each, Specialty Concepts ASC24/8 Low Voltage disconnect, cost $42.00
• 1 each, Zomeworks UTR-20 passive tracking mount, cost $382.00
• Misc wire, conduit, connectors, relays, shipping, etc cost $350.00

Total cost $4,118.00

This system qualifies for both State and Federal tax rebates. The Federal tax rebate is 30 percent of the cost, capped at $2,000.00 and the New York State rebate is 25 percent of the cost capped at $5,000.00. Apply those rebates to the total cost:

0.30 x $4118 = $1,235.00

0.25 x 4118 = $1029.50

Therefore, $4118 – 1235.00 – 1029.50 = $1853.50 final cost for the whole backup system.

Hmmm, looks like I found a good candidate for my Federal tax refund coming in May. The contractor offered a back up system that consisted of a separate battery powered pump that was rated at 1/2 the pumping capacity of the main pumps and ran for 12 hours. The cost of that system was $1,000.00. I believe my system is much better than what they proposed.

Diagram:

Click on picture for higher resolution version.

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