inverter size (single phase) for (UK) house

[xcentric]

Recommendations on inverter size? Large house with 3 kids and typical electronics, plus 2 adults working from home - typical use is 30kWh per day (seems a lot to me but there it is, and I get people to turn stuff off all the time ), 50 if we use the Aga on economy 7 (cheap(er) rate tariff) overnight!

Planning one array of ~4-6kW on garage roof (possibly more), and could fit another 10-16 panels on house roof, possibly more - so 4-8kW additionally. Batteries will be used to store excess and charge from Economy 7 overnight so we ideally use cheap rate and solar mostly. Any other excess can go back to grid.

Better to get 6kW inverter then a second inverter for additional panels, or getting 10kW inverter initially and using just one? Looking for cost-effective solution too..... I know inverters can be overdriven by 10-20% from solar inputs, and that woking near max power are more efficient..... Space for inverters/batteries not an issue.

 

[xcentric]

Note current recommendation from on here was a Solis 6kW inverter but I think that was specifically to replace my existing non-working 5.5kW one - I'd not considered larger inverters before I considered doing the roof install too.

I guess one advantage of more than one inverter is that it gives me more mppt's and so more flexibility if I divide to build a shed with a solar roof, for e.g. Likely? who knows!

And on that, anyone installed panels on their own roof (tiled onto battens and membrane, standard uk construction) - easy or worth paying expert to do?

 

[MichaelK]

One thing I think you will find is that once you actually make the commitment to go off-grid, your electrical consumption per day is going to drop significantly. So, that 30kWh/day consumption could easily be cut to a third to a forth, with only minor mindset changes.

For my own place, I'm finding to keep the lights, and TV on, the refrigerator running 24/7, and some internet surfing I only use about 4kWh per day. One problem though you will face is educating the other family members on how/when to consume power. (Honey, could you please wait till after 8am before starting that load of laundry?).

First thing you really need to do is make an itemized list of what it is your home is powering, and the time of day. What do you use for heating and cooling? How is cooking performed? Are you up till late at night completing company reports? Can some of the heating activities be replaced with gas?

At my own homestead, I have lots of ground space, so have primarily implemented rotating ground mounts. I did however create a rooftop array frame for my workshop for handling the base load. I constructed a 4 meter-long triangularly shaped frame out of unistruts, with the frame holding 1000W of panels at the correct orientation for maximal insolation.

I have metal roofing, so what I did was place short unistrut strips bolted to the roof, held in place with standard rubber-gasketed roofing screws. From the unistrut pieces, I have 6mm bolts projecting straight upwards, while is how I bolt the unistrut frame down on the unistrut strips. I just hoisted the frame up onto the gently sloping roof, plomped in down onto the bolts, and tightened it down. No water leaks for years now.

I would look at a single quality 6-8kW hybrid inverter that can feed to the grid, but charge your battery. I'd say your panel configuration could easily support a 500-600Ah battery at 48V. Look at brands like Outback, Schneider, and Vitron. Seeking the "cost effective" solutions, is likely to lead to failure. You get what you pay for!

 

[DIYrich]

Inverter sizing is based on maximum usage at a point in time, not average usage over a day or month. 30kwh could be 1.5kw per hour for 24 hours, or 10kw for 3 hours and nothing for the rest of the day.

I have a 15kw inverter and normal high usage is around 5kw, but much more surge when AC motor turns on.

 

[xcentric]

4kW/h tends to be peak, which is perhaps max land of 8kW for a few minutes - and 2-2.5kW/h for the rest of the time. Loads have been somewhat reduced, though some could be reduced some more I'm sure.

The aim is to be peak cost electricity free, so solar and battery in the day, with enough battery to last evening, allowing off-peak to recharge it. So it should be that a 6kW inverter will do 99% of the role it needs to. But with more solar power available (at optimal sun/angle/cleanliness) do I overpanel the 6kW one, or push the power into larger inverter? If I can get more out then the batteries will be more fully charged. Certainly if I want to run the 30A Aga overnight I'm going to need over 8kW as the inverter - though I don't think I'll get enough battery and solar to be able to run that for the 5 hours it needs to heat up to make it worth using it.

 

[xcentric]

Put it a different way - would you run one 8kW inverter or 2 4kW ones?
one 8kW or over panel one 6kW?

 

[DIYrich]

If running off battery during peak times is your goal:
1) how much power do you need during peak cost time. Get 1.5x in batteries for that.
2) inverter sized to provide your peak power needs and to charge the battery over 5 hours overnight.