Small House System for Blackouts

Parts list

 

Description

When you experience a blackout because of a hurricane, or load shedding in South Africa, you can have a backup power system. It works like a UPS.

I chose the Victron Multiplus 2 48V 3000VA because it has a fast transfer switch. You can add a generator to this system as well to recharge the batteries.

You don’t need the solar panels, you can just used the inverter/charger in combination with a generator.

This system uses a 48V server rack battery. This battery is stackable next to each other or with a server rack so it’s easily expandable later. Because of the 48V, the current will be lower than a 12 or 24V system. This makes it possible to save money on wiring costs and charge controllers.

Since we have to recharge the battery in one day and the average sunhours is 3 per day, we need the following amount of solar panels:

Total battery power: 100Ah ×51.2V=5,120Wh
5,120Wh/3 sunhours=1,700W of solar

We are going to use the same solar panels as the previous example. We need:
1,700W/385W=4.4 solar panels

We can round down to 4 panels because it’s a backup system that is not used daily.

The solar panel specs are:

  • Pmax: 385W
  • Operating Voltage (Vmp): 39.37V
  • Operating Current (Imp): 9.78A
  • Open-Circuit Voltage (Voc): 48.4V
  • Short Circuit Current (Isc): 10.38A

We need to wire the panels the same way (2S2P) we did in the previous system. Refer to those for calculations. This is the calculation for the charge controller. Max input voltage:
48.4Voc ×2 panels in series ×1.25=121V DC

Current to the batteries:
(385W ×4 panels)/51.2V=30A

We can use Victron’s 150V/35A charge controller. Notice that if we use a higher-voltage battery, the current is reduced? In the previous example, we used a 24V battery, which required a 60A charge controller for the same amount of panels. Checkout my video about using 12V, 24V or 48V systems.

If the distance is shorter than 70 feet or 20 meters, we need an 8AWG or 10mm² solar PV cable. I calculated this in the previous system, so take a look at how I calculated it there.

Calculate runtime

Lets say you have a fridge and freezer and you want to keep it running because a hurricane left you without power for a few days.

  • Fridge: 250W
  • Freezer: 250W

It’s safe to say they have a duty cycle of 30%, meaning that the compressor will run for 8 hours per day in total (24h/3). This is normal as i have demonstrated in my video about my fridge power consumption.

We must also take into account the consumption of the inverter. This is to keep the electronics running and is on 24/7.

  • Fridge: 250W*8h=2000Wh
  • Freezer: 250W*8h=2000Wh
  • Inverter idle consumption: 15W*24h=360Wh

If we add these up, we get a total of 4,360Wh per day.

Our battery total power is: 51.2V*100Ah=5,120Wh.

This is how long you can run your fridge and freezer for:

5,120Wh/4,360Wh=1.17 days, so you can only run them for one day. Not very long.

You can either:

  • Expand your battery
  • Add solar
  • Add a generator

Lets explore these options.

Expand battery

We can add more batteries to the system. If one battery (48V 100Ah) is good for one day of running, then three batteries are enough for 3 days of running. You will have to recharge the batteries eventually. If the power comes back online after three days you are good. But what if it takes longer? Batteries are not cheap either ($1,200 a piece), so adding solar or a generator might be a cost-effective solution.

Solar

In this example we have added 4 panels of 385W each in a 2S2P configuration for a total of: 385W*4panels= 1,540W of potential solar power.

Depending on your location and time of year, the power they produce will be different. Checkout my video here to find out yours.

Let’s say you have an average of 3 sunhours per day.

1,540W*3sunhours=4,620Wh per day.

Remember that our fridge and freezer consumption was 4,360Wh per day?

So the solar power would be able to recharge the battery in one day (on an average day).

This looks like it’s correctly sized, but this is without three days of autonomy.

You see, your loads should be supported for three full days, and the battery should be charged in one day of solar energy. That is a correctly sized system. But because this is a backup system we don’t really have to.

What if it was correctly sized?

3 days of autonomy= 4,360Wh*3days = 13,080Wh battery or 3 48V 100Ah server racks.

Recharge the battery in one day= 13,080Wh/(385W solar panels * 3 sunhours)=12 solar panels to recharge the battery in one day.

Generator

We can add a generator to recharge the batteries in winter, or in case you don’t use solar panels.

The victron multiplus has a 30A battery charger. The battery voltage is 51.2V, so 30A*51.2V=1,536W of charging potential.

If you use the honda generator or a champion generator (cheaper), this is the time it will take to recharge a single 48V 100Ah battery:

5,120Wh/1536W=3.3 hours.

So it will take 3 and a half hours of running the generator per day to keep your fridge and freezer running.

 

Do you have questions about this system? Send me an email here.