Garage Wiring, Coffee Roasting, Voltage Drop and Solar

It's probably simplest to think of the grid-tied inverter as being a "negative" load.


So if the PV was producing 1A and the roaster was drawing 5A, then 4A would be drawn into the garage from the house. (Load = 5A + (-1 A))


If the inverter was producing 6A then the overall load would be -1A - i.e. 1A would flow from the garage back to the house.


Voltage drop would be in proportion to the load (in both magnitude and direction) . If you're losing 8V with a 5A load you, reducing the load to 4A would reduce the voltage drop to 6.4V so you'd have something like 222.6A instead of 221V (ignoring any voltage drop within the garage for simplicity). With a -1A load you'd have a -1.6V voltage drop in the garage supply - i.e. an increase of 1.6V from house to garage giving you something like 230.6V.


That said, the practice might not be as simple as the theory suggests. Ordinary PV inverters don't produce a nice constant current just when you want it, but vary considerably with the intensity of the sunshine. You'd probably need rather more than a garage roof full of PV panels to get anything like a reliable 5A even during clear days. Adding batteries might seem attractive but makes things much more complicated - how would you regulate the output from a battery driven grid-tied inverter - when the connected load includes the national grid?  If battery driven, an islanded system that only supplies your loads is probably more practical - but then you probably want a mains supply to re-change the batteries when the PV isn't sufficient. There comes a point where replacing the 30m of cable with something more sensible starts to look attractive....


Actually 8V v.d. for a 5A load over 30m feels like rather a lot - that would imply a cable with a loss of more that 50mV/A/m - which would suggest it's rather smaller than 1mm² - which feels rather unlikely. Perhaps you've a loose connection or corroded joint somewhere that's the real cause of your problem.


   - Andy.

Convert to gas.


A resistive heater will kill batteries.


Andy Betteridge

As has been pointed out to me in the past, AC and DC electrical theory should not be confused.


Raising the local voltage using batteries and an inverter may be a nonstarter if the inverter is pushing current into the installation and grid by skewing the frequency rather than raising the voltage.


Andy Betteridge

I'd be looking very hard at the cabling if adding a 5 A load really loses you as much as 8 or 9V - that suggests the best part of 2 ohms in series in the loop.

It's not  ' a bit underspecced ' it is 'woeful' .Consider a length of 2.5mm or ideally 4mm SWA as the most sensible minimum for even a couple of sockets and a light at that sort of distance.

Is it running on a length of old lawnmower flex or something ? Even 1mm2 cable (the thin lighting stuff) would be about half that. (1mm2 is about 16-18 milliohms per core per metre, depending how hot or cold it is and how near the minimum of the production tolerance. )

Any attempt to generate and push power back towards the meter with that sort of resistance in the way is pretty much doomed to fail, as the solar inverters cut off if their generation voltage rises too high. If you have 230V at the meter (and it sounds like you do more or less) then if the solar panels tried to generate say 10A, then the voltage at the panels would need to exceed 250 - and around then the over-voltage trip kicks in to protect the electronics  and any grid connected kit.

Great feedback guys, thanks.


An update since last post - I have recently added in 2 new sockets on a spur over where the roaster sits. Last night, at 11pm ish I needed to do a roast (ran out, forgot to do it earlier, don't want to go shopping for 1 bag of beans) and it was sitting at 227V / 1250ish Watts. Not quite the full 1300, but certainly closer than the 1200 I was getting before. I should have checked the voltage in the house for reference, but I think there maybe was a bad connection in a terminal box - there was a screw terminal that was stripped and not actually clamping very hard.


The wire from the house looks to be 1.5mm2 or so (it is armoured), certainly not 2.5 or 4.


Still need for solar, but I'll do some more research. What I really want is to use fill the batteries during the day when our loads are low, and use them in the evening (induction hob, washing machine etc.) when the loads are higher - peak shaving. Seems like the setup is a bit more complicated, and potentially not very economic in the short term - but if I can keep picking up cheap/free parts I might be able to cobble something together. In a way it's more about learning how it all works than the actual outputs.

Connecting something you have cobbled together to the mains could result in a death or injury as well as your mains supply being disconnected and possibly a prosecution.


Apart from that the cable is not big enough to connect a inverter to.


Just move the roaster into the house, change to gas or replace the cable.


Andy Betteridge

You could always revert to the easier option of just buying "easy coffee" like the rest of us. Is all this hassle so advantagious?

What I really want is to use fill the batteries during the day when our loads are low, and use them in the evening (induction hob, washing machine etc.) when the loads are higher - peak shaving.

There are systems about that'll do that sort of thing - one of the IET publications talks about them at some length. It's not a trivial problem to solve - generally you'd want to regulate inverter output (and battery charge) according to overall household demand, available local generation, and possibly time of day and/or market electricity price (if you're on one of the smart meter tariffs that offers lower prices at times of low national demand). At the very least you'd need some current sensors a long way from your garage!


Then there's the issue of grid-connected inverters - as soon as you start exporting back into a grid connected system there's a whole stack of legislation and technical standards to comply with - not least so DNO workers can't be endangered by a cable that's disconnected from their supply, but possibly back-fed from yours. Commercially produced inverters will be type-tested to show compliance - that wouldn't be economic for an experimental setup.


   - Andy.

If you're going grid-tied, then it all needs to be off-the-shelf components are fully type tested.  There's no way you can lash something together from some solar panels, a pile of leisure batteries and the inverter you use when going camping.

Small-scale solar power is barely economically viable in the UK these days.  The Feed In Tarriff has been scrapped for new installs.  If you shop around, there are electricity companies that will pay for exported power, but they pay you at wholesale rates for export and charge you at retail rates for anything you import.

Connecting a battery to the inverter is possible if you buy the right one.  I have one, and you can feel smug running off free electricity at 10pm.  But the economics of it are extremeny doubtful, given that the battery is intended to last for 10 years, and will gradually lose some capacity over that time.