Solar panels with storage battery - some questions

Not very critical, there may be a slight difference in output versus south facing but you could compensate by having a few more panels on e/w.

Loss of grid should not be an issue so long as there is charge left in the battery.

A Smart meter with import export facility will be necessary.

It should all happen automatically with the right installation.

Of course you will need a cable from the panels to the meter, as to the specific difficulties you have they would need to be overcome but in the scheme of things should not be unsurmountable.

Unless you specify a system that can supply the house on it’s own, a “normal” solar/battery system will not allow this. It adds a fair bit of complication and cost I believe.

As I’m sure you understand, it would be potentially very dangerous for any linesman repairing power lines if your system “back-fed” into the mains supply. It might not be *that* expensive so do get a quote, just don’t expect a normal system to do this.

Tim

You lose ~20% by going E-W, but OTOH many people consume more power in the mornings and evenings than at midday so the power might be more useful. Panels are cheap (~£170 per 400Wp + fixing hardware), so if you have space putting more on would compensate.

See the calculators linked from here, and for some examples:

You'd need an automatic transfer switch to provide that. Tesla's batteries do it, others typically don't. However it's the same setup as used with a generator and transfer switches start at £30 (of the Chinese ebay variety, more for better quality versions).

Often the inverter will have an 'emergency' output that goes live when the grid goes down, and this can be wired to trigger the transfer switch and feed circuit(s). For example you may not want to power the oven or electric shower, but maybe sockets and lighting only.

They will shut down the output that feeds the grid: they must for linesman safety. As above you can make your own switching arrangements, but you need to make sure there's no connection back to the grid if the grid is down.

Some old mechanical meters spin backwards, but the majority of meters (including all electronic meters) don't. Either you have a separate export meter or it's integrated into a smart meter. The export tariff depends on your electricity supplier. You have separate import and export readings so the tariffs are different and not in lockstep.

Typically yes. That's a setting on the battery inverter, which is configurable via apps, webpages, serial ports... depending on the model.

The inverters typically have current transformers around the cabling from the panels, the grid feed and your house loads, so they measure which power is flowing were.

You can decide how you want to build the system: panels and batteries in the same building (combined PV inverter and battery charger/inverter), spread out via a DC interconnection (limited distance), or separate PV inverter and battery inverter with regular mains cabling between. Or microinverters, where a small inverter is on the back of every (pair of) panels, meaning they just hook into regular mains cabling without a PV inverter box on the wall.

With mains interconnection you can locate units on your usual mains circuits, as long as there's isolator switches (although it's better to run separate circuits to your consumer unit). Current transformers (CTs) can signal wirelessly, so don't need to be cabled back to the units. However any transfer switch would need to be wired to your consumer unit so it can interpose on the supply, and for that you really need your battery inverter 'emergency' output wired to it.

In your case, it would be OK to have everything in the garage and use the garage supply to backfeed, as long as your system used wireless CTs. But I think the transfer switch would need some physical wiring, unless you could site the battery in the house. Many batteries come in server rack form factor these days, so maybe there is somewhere to wall mount it (can be sideways on)?

Theo

Fairly so. You'll have to look up the angles involved. I'm sure there are tables you can use. South facing is always going to be best.

Most systems are designed to shut down if you lose your mains supply.

I don't think you get any (much) credit these days. The usual technique is to dump any excess power into your water cylinder via an immersion heater.

There will be a battery management system (BMS) to ensure the batteries are not fully discharged.

Sounds inevitable. Perhaps use a duct so you can have a feed to the garage, if there isn't already one?

One clarification: there's no 'switching' from grid to battery mode or vice versa. It's simply a balance between inflows and outflows. Your battery inverter outputs max X watts. The load is Y watts. If X<Y we draw all the power from the battery, if Y>X we draw Z=Y-X from the grid. This is necessary because many loads are variable: eg you have a 3kW inverter but the washing machine might take 5kW for a few seconds when the motor starts up. That would trip out the battery if the battery was the sole supply, but in this scenario the grid just makes up the difference. We can also configure things so that we export by the inverters outputting more than the load. We can separately bring in additional loads (like solar hot water diverter) if the battery is full and that is preferable to exporting.

So to rephrase what you're asking, you can configure the inverters to control the power flow between the three sources (battery, PV, grid) and two sinks (house, battery) as appropriate. Power can change direction by the second depending on how things vary, and the controls allow you to configure the decisions it takes.

Theo

It's also not practical to power everyone else's house too! As a rule the householders will be asked if they are running generators, sources etc that can back power.

I'm sure there is a system available that will isolate the incoming in the event of a fault. Not sure if that needs to be reset manually or automatically.

As Tim mentions, I dont think having mains failure "protection" is a standard thing. Our installation came with the option of having a double socket located under the inverter which becomes live when the inverter detects mains failure. I told the installer not to bother as the inverter is in the loft and climbing into the loft to plug in a (for example) phone charger didnt appeal to me. There are more complicated installation options available that will allow the whole house to be powered from the inverter but these add more cost. I cant remember the last time we had a power failure so I didnt bother investigating any further.

We had a smart meter installed prior to our installation. The installers said it wasnt completely necessary but was a good idea. After we had the solar panels and batteries installed, we notified our supplier and they "created" an export meter on our account that tracked the electricity that we exported. Again, I dont know if it was necessary to have a smart meter for this to be done.

Our batteries have the ability to set the minimum capacity at which to stop discharging. Its at the supplied setting of 10%. On ours, we can change the order/priority of things. Its setup so that electricity from the panels is used to power the house. Any excess is then sent to the batteries. If there is still excess (batteries are being charged at their max charge rate or they are full) then it is exported to the grid.

You will need to get power from the installation to your home. You will need to think about where you want to place the inverter and the batteries. Both the inverter and the batteries have operating temperatures so its worth thinking about that. In particular, Lithium-Ion batteries dont have a very high tolerance for cold (well, ours dont!). I found this out last year (the first winter) when they suddenly stopped charging when the outside temperature dropped below 0deg. When I looked at the fine print for the batteries, it stated that their performance degrades below 10deg. I build a frame out of celotex which solved the problem. I think some batteries have internal heaters to get around this.

Its also worth thinking about how much power you need at a given time when sizing things. Our batteries can only charge and discharge at a max rate of 3kWh. I have found that a higher rate (3.5-4kWh) would probably have been better.

You will still have a daily usage from the grid too. Ours is around 0.5kW per day. This is where the power usage for the house fluctuates and the inverter is trying its best to match it all the time. This results in several watts still being imported from the grid and the same is also exported to the grid from the batteries.

Alan

Octopus have a few tariffs that may mean you are better off exporting than heating hot water. Its dependent on what setup you have.

Their Flux tariff has a day rate, Flux rate (02:00-05:00) and Peak rate (16:00-19:00). The day export rate is just over 21p/kWh. Their gas charge is just over 10p/kWh.

If you heat water using a gas boiler it may be cheaper to carry on doing that. You would also save on the cost of the additional equipment needed to divert excess solar energy to the immersion element (and installation if you arent DIY'ing it).

Alan

Why not spend it on lottery tickets. At least you stand a chance of getting it back

My solar system was installed a couple of years before the smart meter was installed. There is a solar meter installed that reports the output by wireless.

The first thing to consider is your consumption and the pattern.

For example, we seem to hover around 20 kw per day on average.

That, of course isn’t used at constant rate, I know for example about 1/3 is used on econ 7, some of which is used to charge our hybrid. ( I don’t charge every day.)

We cook using electricity. The demand of the induction hob - not to mention the oven- would outweigh anything a normal domestic system with a battery could supply rapidly. You could supply one ‘ring’, perhaps two.

Even if somehow we ‘saved’ the solar system battery to charge the car, most systems seem to have about 3kwh batteries, a fraction of what even my hybrid needs.

During the day, consumption tends to be things like fridges etc- the solar would run those. At night the lights are mostly, if not all LED, so the battery would probably be ok for those.

So, all the high power devices would still be powered by the grid - cooking, charging the car, probably the washing machine, dishwasher, … some of which we run on econ 7.

Our heavy use items are washing machine, dishwasher and oven which all seem to operate at around the 3kW mark. We dont have an electric car and the hobs are gas. The house has a "background" usage of around 3-400w. When we use the heavy use items we only go over the 3kW mark for short periods so only a small amount of electricity is drawn from the grid.

We try to avoid using the heavy use items at the same time to limit the draw on the grid and run mostly off the battery.

During the winter months we charge the battery from the grid over night. We also use the timer function on the washing machine and dishwasher so they also run over night (econ 7).

Now that we are getting more solar, we charge the battery from the solar panels and run the dish washer and washing machine during the day. I usually check on a couple of solar forecast sites for the next days estimated solar and decide if I should give the battery a little boost on the econ 7 or leave it to completely charge from solar.

Our daily usage is between 15-20kW.

We have 2 6.5kW batteries (13kW total). Over winter they are charged between 02:00 and 07:30 (which is when our econ 7 ends). They will normally last from 07:30 until 02:00 the following day depending on what we are up to allowing us to run on the econ 7 rate electricity all day. This year will be the first full summer with the system (they were installed end of July last year).

As long as you aren't buying into a proprietary solution like Tesla's, a lot of the batteries come in modular server rack form, and you can just add more capacity as needed.

If you're prepared to order from China you can get them cheaper in larger sizes. (typically 280Ah LiFePO4 cells in a 48v configuration, which is 13.4kWh)

One of the main limitations is the output of your battery inverter - many are max 3kW (which is also around the G98 single phase 16A limit).

If you either get G99 (>16A per phase) or somehow limit your export capability to 16A, you can either buy a bigger inverter or stack smaller inverters, each with their own battery, to allow a larger output power which would cover multiple hungry appliances running together.

Theo

I h> Our daily usage is between 15-20kW.

Again, I assume this is kWh.

Yes, finger trouble 😀

It's usually dependent on the output of your battery charger/inverter - there is typically a limit to the speed the batteries will charge and another limit for discharge. Add more batteries and/or inverters to increase the power.

See my other post - you need an automatic changeover switch. It's not a standard part of an install, but it's possible to fit them. The £30 jobs from China seem to work ok - they do it by mechanically motoring a lever from one position to another, so the time gap while it's moving means the two circuits being changed over are never connected. That time gap might be a fraction of a second, so it depends how well your power supplies hold up during that gap.

Example:

Theo

Ah, so the changeover is done by (automated) mechanical switch rather than by electronic means. I wonder how well the PSUs of computers, router, mesh network nodes, Alexas will stand up to a short break. I'm surprised the switch is a motor (which may be fairly slow) rather than a relay that defaults to one position (grid isolated) but is normally held in the other position (grid connected) while grid voltage is available: that would be a faster changeover. Maybe these motorised switches really are as fast as "a fraction of a second". However the one you posted a link to quotes "Conversion time < 2 seconds" which is *way* too long: I've had power cuts that last less than that (at a guess, about a second) and all computers throw their toys out of the pram.

Presumably in normal operation (when there is grid mains) there is circuitry which is determining which source (local or grid) is supplying the house load, based on power required and local power available. And it must be able to switch seemlessly between the two if you suddenly need more power than the panels/battery can supply, and then later you stop needing so much power and the local supply can cope again.

I can see the need for an isolation switch (whether mechanical or electronic) to prevent the possibility of local power find its way onto a "dead" grid circuit.

No, as I said upthread there is no switching. It's simply a balance of power in and power out. If there's more power being pushed out of your PV+battery than your local demand it's exporting to the grid, if there's less it'll import the remainder from the grid. Power can be pushed in either direction, and which way it flows just depends on the balance.

The control system just tells the inverter(s) how much to generate. For example, imagine if the grid voltage was exactly 230V. If your inverter generates 231V then power is flowing from you into the grid, because current flows from places of higher to lower voltage.

Instead let's say you turn on the oven and that normally causes the grid voltage to sag to 228V (since the grid has a certain impedance which causes a voltage drop at a given current). If instead the inverter generates 229V then you're sharing the oven's load between your inverter and the grid (half of your current will come from the inverter, and half from the grid). You don't need switching for this, you're merely adjusting the inverter voltages to change the quantity and direction power flows.

If your grid connection was interrupted upstream, this would just try to try to power the neighbourhood from your inverter. That's not going to end well, on top of the linesman safety aspect. Which is why loss of grid frequency means the inverter refuses to output on its regular connection.

That's why a transfer switch is not integral to every install, because it's doing something different from when everything is grid-tied, and that needs extra hardware.

Theo