Hi, 

I'm considering a 3,5kw inverter with 24v 100ah battery.

 

Basically I just want to be able to run the lights and the microwave. A few extra appliances (electric blanket and  a fridge or two) would be nice. Considering adding solar to offset the eskom bill slightly. The two fridges use around 500W. I'll be downscaling on a microwave that will either use 1450W or 1250W (depending on which size I go with).

 

When running on battery power (with 80% DoD, gives 80AH of useable capacity):

1) If for example, the constant current draw is 30AH, and then I switch on the microwave which draws 70Amps for 10mins. The microwave draw is less than 70AH but while the microwave is running, I'm not sure if this will exceed the battery's capacity and will this damage the battery?

 

2) Can someone please explain the C rating of a battery and how to select a battery accordingly. An example here will help.

 

Regarding solar panels, our roof faces East and West. Was thinking of getting 4 x 550W solar panels. And maybe placing all East facing or two East and two West. 

3) What should I know regarding shading? It's likely that some of the panels will be in the shade  at certain times of the day. And which method parallel or series connection will be better suited in this situation?

 

4) What happens to excess solar power generated once the batteries are fully charged, and supply exceeds usage?

 

With a hybrid installation, I'm thinking that preference would be set to use solar first, and then grid and during load shedding, battery.

5) So is a changeover switch required? When does one typically use a changeover switch or what purpose does it serve? 

 

Any other advice is always welcome.

 

 

Edited June 10, 20233 yr by Munzee

1 hour ago, Munzee said:

1) If for example, the constant current draw is 30AH, and then I switch on the microwave which draws 70Amps for 10mins. The microwave draw is less than 70AH but while the microwave is running, I'm not sure if this will exceed the battery's capacity and will this damage the battery?

Both inverter and battery will have a peak current that they can sustain, so check both of them. The trip will be triggered by total draw on the backed up circuits at any given moment. And it will be triggered whether or not there is load shedding.

The lowest rated device will trip first. If you look at my system, the inverter will supply 20A on the backed up side, whilst the battery can supply a big 100A. So my inverter will always trip whilst the battery still has more to give. The system is just as down whether the inverter or the battery trips, but in my case the inverter automatically restarts if there is grid or PV.

1 hour ago, Munzee said:

 

2) Can someone please explain the C rating of a battery and how to select a battery accordingly. An example here will help.

The C rating is a ratio of maximum discharge allowed to capacity. The battery you are considering is 100aH. So if you can draw a steady 100A from it, it is C1. Note that you don't want to take full advantage of this, because you can empty your battery in 1 hour. (In reality the battery will likely shut down when SOC reaches 10% remaining).

Some brands claim to be C1 but if you read the small print they will have something like (this is not from the spec sheet of any given battery) "100A for 5 minutes, once every 30 minutes". 

The C rating is also an indicator of the maximum discharge current the battery allows. 100aH, C1 = 100A. 50aH, C1 = 50A. 100aH, C0.5 = 50A.

Conversely, if they don't quote a C number then divide maximum allowed discharge current by the capacity. EG 50A/100aH = 0.5C

1 hour ago, Munzee said:

 

Regarding solar panels, our roof faces East and West. Was thinking of getting 4 x 550W solar panels. And maybe placing all East facing or two East and two West. 

3) What should I know regarding shading? It's likely that some of the panels will be in the shade  at certain times of the day. And which method parallel or series connection will be better suited in this situation?

 

4) What happens to excess solar power generated once the batteries are fully charged, and supply exceeds usage?

If you are selling back to the grid, then it gets exported. If you are not then the system will just reduce what it draws from the panels.

1 hour ago, Munzee said:

 

With a hybrid installation, I'm thinking that preference would be set to use solar first, and then grid and during load shedding, battery.

5) So is a changeover switch required? When does one typically use a changeover switch or what purpose does it serve? 

The inverter should be configurable in this regard. Most hybrid inverters will use PV first (when available) ahead of batteries, but usually you can change the priorities.

Edited June 10, 20233 yr by Bobster.

Quote

4 hours ago, Bobster. said:

Both inverter and battery will have a peak current that they can sustain, so check both of them. The trip will be triggered by total draw on the backed up circuits at any given moment.

 

How do I check what the peak current the inverter can sustain? I've attached a photo of a 3,2kw inverter specification, is this value shown here? I don't know what to look for 

4 hours ago, Munzee said:

How do I check what the peak current the inverter can sustain? I've attached a photo of a 3,2kw inverter specification, is this value shown here? I don't know what to look for 

3200W/230V=13.91A is the maximum. A bad label to not show it in the top portion of the sticker. 

8 hours ago, Munzee said:

How do I check what the peak current the inverter can sustain? I've attached a photo of a 3,2kw inverter specification, is this value shown here? I don't know what to look for 

On the input side to the inverter (battery paramrter), it takes 148A at 24V to deliver 3.5kW.

The battery should be able to deliver at least the same. Looking at thd Pylontech UP2500 (0.5C), 24V 100Ah battery as a benchmark, it delivers 56A per batt, so you'd need 3 to get full power out of the inverter.

On a 1C battery you would need 2.

 

Experts can't agree here. 

Is there not a spec sheet with the packing or that you can download? 

As I see it, 3200W (rated power) with a 24V battery gives 133.33A drawn from the battery. So it could work a single battery quite hard. A C1 100Ah battery will not put up with this. 

You've said 3.5kWh, the inverter you show is 3.2. That may be enough for your needs, but please read the specs carefully so you know what you're getting. 

The max current on the AC output side will be less, as @Scorp007points out. That's because we take power to be constant, so if voltage increases, current goes down.

Now we use the simple calculations that we looked at earlier: 100ah, C1 battery, will trip if you hit the limit on this inverter. It will try to pull 133A, the BMS will say howzit at 100A and shut the battery down to protect it. 

This inverter is not a good match for a single 100Ah C1 battery. Two such batteries, connected in parallel, will do better, but will hurt your pocket more. 

If you can manage the loads you'll get away with it. You'll be marginal for a 4 hour shed (if you don't use the microwave). But we all have to do what's within our budget. 

 

On 2023/06/10 at 2:42 PM, Bobster. said:

And it will be triggered whether or not there is load shedding.

Question: will this still happen even if the priorities is to use grid power before batteries when not loadshedding? And the current draw is based on the battery voltage and not the AC 230v even when using grid power? This means it will be necessary to rewire plug points and have a dedicated inverter circuit.

We have the toaster, microwave, kettle, bed room plugs (hairdryer), though most of these won't be used during load shedding, will cause the inverter to trip when there is grid power because it will be on the same line.

Perhaps it will be worth considering going for a bigger inverter. The price of two 24v batteries is about the same as one 48v.

12 hours ago, Bobster. said:

You've said 3.5kWh, the inverter you show is 3.2

Yeah, both inverters are made by the same brand ECCO, and I'll expect the specs to be very much the same. They don't have a website unfortunately but this brand has been around in SA for sometime making dvd players, kettles, irons etc. 

AC DC have a very neat looking 5kw Luxpower inverter on the what they call an easy install kit. It comes prewired with AC and DC boards for R21300. The inverter alone is around R11500 elsewhere. I'm not sure if it's good value.

And I seriously need to rethink the system I would want have installed. I wasn't wanting  to spend too much and early estimates is around 20k difference between 3 - 3,5 KW system and a 5kW system.

10 hours ago, Munzee said:

Question: will this still happen even if the priorities is to use grid power before batteries when not loadshedding? And the current draw is based on the battery voltage and not the AC 230v even when using grid power? This means it will be necessary to rewire plug points and have a dedicated inverter circuit.

Splitting the DB is always recommended. That way things like the geyser or an electric stove cannot deplete the battery during outages. Unless you are going to run this is a plug in device and then control the loads by the simple means of deciding what to plug into it.

I'd think of the current draw on the AC side. For the example inverter that's 3.2kW divided by 230V (what your inverter should produce when it's backing up) and that's just under 14A. The 3.5kW unit will give you 15A (round numbers). But as we said, the batteries you are looking at won't allow 3.5kW, so the battery now imposes the limit. That limit is (rough numbers) 100A at 25V, which gives us about 10A on the AC side (round about there, there will be losses in the conversion from DC to AC).

So you need to manage loads to suit the limits of your system. We all have to do this. We just have different limits. Start with the bare bones and see how your system does. Then maybe permit another device and again see how the system responds.

There are some people here with good technical skills. I am not one of those people. I just have 4 years experience living with PV and I know Ohm's law 🙂 I particularly don't want to comment on the inverter that you are considering. It is clearly not like my own.

10 hours ago, Munzee said:

We have the toaster, microwave, kettle, bed room plugs (hairdryer), though most of these won't be used during load shedding, will cause the inverter to trip when there is grid power because it will be on the same line.

Perhaps it will be worth considering going for a bigger inverter. The price of two 24v batteries is about the same as one 48v.

I'd expect a single 48V to be cheaper, if we're talking the same brand and quality (check the cost of two Hubble AM4s V one AM5 as an EG.

10 hours ago, Munzee said:

Yeah, both inverters are made by the same brand ECCO, and I'll expect the specs to be very much the same. They don't have a website unfortunately but this brand has been around in SA for sometime making dvd players, kettles, irons etc. 

AC DC have a very neat looking 5kw Luxpower inverter on the what they call an easy install kit. It comes prewired with AC and DC boards for R21300. The inverter alone is around R11500 elsewhere. I'm not sure if it's good value.

And I seriously need to rethink the system I would want have installed. I wasn't wanting  to spend too much and early estimates is around 20k difference between 3 - 3,5 KW system and a 5kW system.

I hope I wasn't too pessimistic. Your system CAN work. You obviously understand that you can't back up the whole property. It is thus a question of managing loads and expectations. The other thing with going to 48V is that your cables on the DC side are lighter for the same current, and you have many more choices for the initial system, and if you want to upgrade.

A DB split is recommended. You can manage loads conscientiously, but if the geyser decides to turn on then it will, and that could easily put you over 10A by itself.

On 2023/06/12 at 8:21 AM, Bobster. said:

It is thus a question of managing loads and expectations

Is my understanding here correct, based on the following assumptions:
An ECCO 3,2kW inverter
1 x 100Ah 24v battery with C1 rating

With one 24v battery, the maximum continuous power the inverter can get is 2400W from DC side. The continuous current draw will be 100Amps. And maximum current draw will depend on the inverter (148Amps for the ECCO 3,2kW) and battery's maximum current.

On the AC side, the maximum continuous output power will be 2400W when running on battery only (ignoring inefficiencies). This equates to 10,4Amps (at 230v).

With PV or grid power available, the maximum output power is 3200W. This equates to 13,9Amps (at 230v).

So when deciding how to split the loads, the essentials on the backed up circuit should not exceed 10,4Amps or 2400W when on battery.

And when grid power is available, a few extra items can be added, but in total should not exceed 13,9 Amps or 3200W.

I think you're right. Bearing in mind that although you can pull 100A from the battery, if it's true 1C, you don't want to do that all the time because you will flatten the battery in an hour. 

Read the specs & warranty carefully. 

10A on the AC side is not nothing. 2.3kW. Looking at data from my system I see at one point we had multiple lights (led) on, TV, fibre, Wifi, electric fence, fridges running, kettle on and were under 10A.