Which SMA Inverter?

Lei_Chat

3000 vs 3600 vs 4000
Aside from a small difference in price, for a sub 4kW system is there any significant reason to opt for one over the other please? I believe they all are available in a dual string option allowing us to split the panel system in half. (We fortunately have very minimal shade but I've been given the impression it's worth doing anyway.)

http://www.sma-uk.com/products/solarinverters/string-inverters/sunny-boy-3000tl-3600tl-4000tl-5000tl-with-reactive-power-control.html#Downloads-129505

tunnel

There's only £30 difference between the 3600 & the 4000tl, I'd opt for the 4000 but just consider that the max output will be capped at 3680W to comply with DNO regs.

Lei_Chat

Thanks.
If your panels and inverter were 100% efficient am I correct in thinking there would be no point having a system over 3.68kW or is this yet another thing I'm getting confused about?
With panel degradation over time is it better to have an inverter that will match the reduced output?

EricMears

Lei_Chat wrote: »

. . . am I correct in thinking there would be no point having a system over 3.68kW or is this yet another thing I'm getting confused about?. . . . .

3.68kW is NOT the absolute limit that you're allowed to generate ! It's the limit that you're allowed to export without first getting approval from your local DNO. Providing you get their approval first (and there's unlikely to be any great problem in approval for an extra 320W) you'd be able to fit as big a system as you want/need/can afford/have space for.

tunnel

EricMears wrote: »

(and there's unlikely to be any great problem in approval for an extra 320W) you'd be able to fit as big a system as you want/need/can afford/have space for.

Care to run that past western power for my parents who were originally limited to 3.2kWp maximum system Eric. One size definitely doesn't fit all

Martyn1981

Lei_Chat wrote: »

Thanks.
If your panels and inverter were 100% efficient am I correct in thinking there would be no point having a system over 3.68kW or is this yet another thing I'm getting confused about?

Partly right, partly confused. This makes sense if the panels are always in direct sun, but of course, they won't be.

When the sun to panel angle is poor, or there is cloud, then generation will be lower. Imagine a 4kWp system in cloud with a 4kW inverter putting out 2kW (despite being 100% efficient), then in that example you can see that 8kWp of panels would still work.

Also, heat is a major issue, though you may have covered that in your '100% efficient' criteria. But if not, then as panels get hot, generation drops. They'll rarely hold more than 90% in sustained direct sun*, and in very hot weather with still air, say June and July, they could drop down to 70%.

* In April and October, when the sun may be strong, clear skies, good angle to panels, and cold temps with strong wind, then we get some interesting spikes, 100%+ and these can last for a few minutes, rather than seconds.

Lei_Chat wrote: »

With panel degradation over time is it better to have an inverter that will match the reduced output?

I wouldn't worry about that factor, as panel degradation may only be about 0.5% pa. The inverter may need replacing before the numbers match.

However, there is an argument that in the UK, where our weather is more intermittent, it's better to undersize an inverter by about 10%. This makes it a little more efficient at lower levels of generation, at the expense of some capping during peaks.

I think this works, but location and roof orientation have to be considered, to work out efficiency gains v's capping. Compare a south facing system in the south west getting good sun all day, to an off south system further north where efficiency may be more important.

Due to the mismatch in the UK of FiT levels encouraging 4kWp, and DNO rules encouraging 3.68kW, then the issue of slightly undersizing is already decided for most of those installing 4kWp systems.

Mart.

EricMears

tunnel wrote: »

Care to run that past western power for my parents who were originally limited to 3.2kWp maximum system Eric. One size definitely doesn't fit all

I suspect there were some very unusual issues with the power supply to their property ! And of course if they hadn't asked WPD first, they'd have been allowed 3.68 anyway

For the vast majority of people there would be no problem at all in fitting a 4kWp system. There was certainly no problem when my installer applied to WPD to do just that in 2011.

ely_ellis

Hi,
Not too up on solar PV, still looking into it myself, but I have a question and comment.
Are the 3000, 3600 & 4000 units the same unit with a restriction or are these the rated capacity?
If the latter, then surely a 4000 unit restricted to 3600 would be running at 90% of its design capacity where as a 3600 would be running at 100% capacity. Like all things, something running at 100% is more likely to fail over time than something running at 90%
Or am I completely off the mark?
Cheers
Martin

Martyn1981

ely_ellis wrote: »

Hi,

Or am I completely off the mark?

Cheers
Martin

Nope. You are spot on. [Edit: Though the smaller units won't be running at 100% all of the time, just more of the time. M.]

A 4kW inverter capped to 3.68kW would be operating slightly under its max, and that might mean it lasts longer. But it could also mean that it's a little less efficient at lower levels, where a smaller inverter, say 3.6kW might be slightly more efficient.

[Think about cars and engines, a large engined car might drive at 100mph at moderate revs and complete ease, but burn more fuel at tickover and 30mph city driving than a small engined car. Whilst a small engined car, that is very efficient at low speeds, might be struggling at 100mph, burning more fuel at higher revs. This is a very bad example, as leccy and combustion engines don't work the same, but hopefully it gives a sense of the comparison.]

BUT life's not that easy. You would really need to read the specs of different inverters as the names (numbers) of different makes aren't necessarily comparable.

Some model names, such as 4,000 may refer to max DC watts in, whilst others to max AC watts out. Others may have numbers that are a little lower, or a little higher than either their AC or DC ratings, eg a 3600 model with a max 3.5kW DC and a max 3.4kW AC. So before assuming it does what it says on the side of the tin, you may first have to read the side of the tin.

In the case of this thread, sticking to one make, and one basic model package, then you can compare the 3000, 3600, 4000 and 5000 models as they are all related. In the specs they share a package and all weigh the same.

Mart.

Martyn1981

EricMears wrote: »

For the vast majority of people there would be no problem at all in fitting a 4kWp system.

But 4kWp of panels wasn't the issue, it was 4kW (or to be more precise, exceeding 3.68kW) of inverter that was raised. T only mentioned a kWp issue to point out that applying for permission wasn't as simple as you'd implied. Your posts (and your system) relate to, or certainly appear to relate to a 4kWp system using a 4kW inverter instead of a 3.68kW capped model.

EricMears wrote: »

There was certainly no problem when my installer applied to WPD to do just that in 2011.

But a lot has changed since 2011. There have been some changes to the procedures (G83/1, G83/2 and G59/1), and when considering permission the DNO has to take into account the amount of local SSEGs (small scale electricity generators), which will of course increase as more people connect.

Also, and whilst this relates to the supply side, WPD infrastructure is already creaking:-

National Grid cannot take any more power from renewable schemes in the Westcountry

Western Power Distribution said the grid was being temporarily closed to new, large-scale renewable energy projects for the next three to six years while new infrastructure is installed.

While opponents of wind and solar farms may rejoice at the news, it is a significant setback for the renewable energy sector in the South West which supports 16,000 jobs and is worth £1.1 billion to the economy.

It will not initially affect small, domestic installations, but threatens to make many major solar, wave or wind energy projects – which will be unable to feed their “spare” electricity into the national grid – unviable.

Of course, if the local infrastructure can cope, then it might be argued that increased demand side generation would help. But the complexities of that question are well above my pay grade.

Mart.