Minimum useful PV systems?

[Vortex2]

A super-duper full-sized domestic PV system costs £32k+ in the UK .. and you still need a generator now & then.

I have a single panel system with a 100+Ah leisure battery.
This is adequate to charge phones, charge torches, run a low power LED lamp in the evenings etc.
Alos it can run a 200 watt load during a sunny day.
Better than nothing.

However I suspect that there is a cost-effective 'sweet point' where you can run, say, a washing machine on a sunny day, run some power tools, run a few lights, a PC etc in the evenings etc.

A cheap petrol generator of a few KW can be used in emergencies.

Does this make sense?

What sort of panel capacity and battery capacity would YOU regard as an optimum balance between usefulness and cost?

[adam2]

For limited use of heavy power loads I would proceed as follows.
About 1 kw of PV.
A 24 volt battery of at least 400 AH.
A 3 kw inverter.

A 3kw inverter should be just sufficient for a washing machine, or a kettle or an induction cooking ring or smaller power tools. A 5kw inverter will give a greater margin for motor starting.

The 1 kw of PV should produce about 1 kwh a day in the winter, that would run a 3 kw kettle for about 10 minutes, which is several mugs of tea.
Or boil the kettle on a fire or stove, and use a 1 kw [average loading] power tool for about half an hour, in addition to lighting.

In summer, several kwh would be produced each day, allowing a load of laundry and cooking a simple meal.

If only lighting is required, then a simple 12 volt only system with no inverter should be fine. A single 100 watt module will produce about 100 watt hours a day in winter. Several 4 watt LED lamps for hours each evening.

[Vortex2]

For limited use of heavy power loads I would proceed as follows.
About 1 kw of PV.
A 24 volt battery of at least 400 AH.
A 3 kw inverter.

Sound like roughly what I was thinking.
<sigh> Where did I put that credit card?

[clv101]

A super-duper full-sized domestic PV system costs £32k+ in the UK .. and you still need a generator now & then.

You need to get a few more quotes! Our super-duper full-sized domestic PV system was less than £10k. 10kW of PV, 20kWh lead acid battery, 6kW and 1.2kW inverters...

Haven't used a generator, ever.

[clv101]

For limited use of heavy power loads I would proceed as follows.
About 1 kw of PV.
A 24 volt battery of at least 400 AH.
A 3 kw inverter.

Sound like roughly what I was thinking.
<sigh> Where did I put that credit card?

Yes, we do have a small system with 6 panels (~1600W), 8kWh 48V lead acid and 3kWh inverter in the barn - runs a freezer in the summer and power tools.

[clv101]

The 1 kw of PV should produce about 1 kwh a day in the winter, that would run a 3 kw kettle for about 10 minutes, which is several mugs of tea.
Or boil the kettle on a fire or stove, and use a 1 kw [average loading] power tool for about half an hour, in addition to lighting.

Winter's a rough time for solar. While the text books say winter average of 1 peak sun hour, 1kWh per kW of panel... It can be *much* less, and not at random, there's strong autocorrelation. Our worst winter's day generated around 1.5kWh from 10kW of PV! We can easily see a whole week of ~2kWh per day from 10kW, so just 0.2kWh/kW.

[adam2]

1 Hour of peak sun equivalent is IME reasonable as an average over say a month in midwinter conditions. It is certainly not achieved every day. But 30 kwh in total over 30 days is probably realistic from a 1 kw system.

[clv101]

In Dec 21, we generated 150kWh over 31 days from 10kW PV. 0.5kWh/kW/day.

A few points, the 1 psh figure refers to winter average, Dec is worse than winter average. In an off-grid system you can't ever capture all the sun as on bright days your batteries will become full, additionally when the charge cycle reaches the absorption phase the rate of charge is often less than the potential available, again losing some potential energy.

The smaller the system (battery) the less optimal it will be. I wouldn't expect more than 15kWh from a 1kW off-grid system in Dec.

[BritDownUnder]

You need to get as much PV as you can as it will be a tradeable commodity in the future. And in the UK probably get a wind turbine too. And a woodburner.

[anotherexlurker]

The smallest systems I have used/use are 320W of panels, but that was/is only due to size limitations of the roof of a caravan or van.
In each case I used 4 Trojan or Crown T105 6V batteries wired as 2 sets of 2 in series to give 12V then paralleled together for approx 450Ah.

The reason I prefer Trojan 6V over 12V was based on the price and long term known history (they are used as golf cart batteries).

These small banks could happily run a small 12V compressor fridge and be large enough to take reasonably high loads such as a 12V microwave or via an inverter that could power a small kettle or small oven (I always tried to keep the max load to around 1KW).

[adam2]

Until very recently, small battery charging PV systems have been uneconomic if compared to electricity purchased from the grid. Most useful for isolated premises without grid service, and arguably worthwhile as a doom prep even in premises with utility service. Energy cycled through a battery cost about 25 pence a unit due to battery depreciation, and seemed rather pointless when it could be purchased from the grid for 15 pence.

The position has now changed, UK grid power has roughly doubled in price from about 15 pence a unit to about 30 pence a unit and is expected to increase to about 60 pence a unit. For a small user it is already well over £1 a unit due to the standing charge.

PV modules and batteries are now cheaper, and a small battery charging system might now be worthwhile even in premises with utility service, or more importantly in premises that PREVIOUSLY had mains supply but this has been cut of for non payment.