48V off-grid with AC blend?

BB. said:

Welcome to the forum Steve,

A quick question... Are you allowed to feed power back into the grid--Or does your utility prohibit power back feed (zero export to grid)?

Many hybrid inverters (Grid Tied/Off Grid capable with solar and battery bank) will feed excess solar charging power back to the utility--This ensures that all energy that you generate is used--Whether by the home or back to the utility for credit.

The zero back feed is a little bit more difficult to find.

Are you looking for solar/battery backup power in the case of utility power failure? Or just trying to save money on electric power?

Pure Grid Tied (GT inverter(s), no battery bank) is the cheapest/most efficient method of solar to grid.

Hybrid (GT+Off Grid+Solar+Battery) is much more expensive... But given where energy prices are heading in Europe for the present time--I could see the attraction...

In the US, GT Solar can generate power at $0.15 or less per kWH. Off grid/battery/hybrid is usually closer to $1.00 to $2.00 per kWH--Although it can be possible to get down towards $0.50 per kWH or less (find good prices for hardware and battery bank, manage your energy usage so you use most of the available solar harvest every day (i.e,. conserve on cloudy days, use washer/dryer/etc. on sunny days).

I am guessing you are somewhere around London UK? Just to give you an idea of what the sun looks like:

http://www.solarelectricityhandbook.com/solar-irradiance.html

London
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 38° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.27	2.04	2.76	3.67	4.17	4.20
Jul	Aug	Sep	Oct	Nov	Dec
4.25	4.16	3.26	2.41	1.53	1.05

Not that sunny during "summer", and winter is just horrible. Suggest around 3 hours a day+ is where solar can become cost effective and "useful".

Do you have an idea about how many kWatt*Hours per day you need... Just to give you some quick math--Say a relatively energy efficient home at 300 kWH per month (typical is usually around 500-1,000 kWH per month depending on lots of variables):

• 300 kWH per month / 30 days per month = 10 kWH per day = 10,000 WH per day

Pure Grid Tie:

• 10,000 WH per day * 1/0.77 GT inverter system eff * 1/3.0 hours per day sun (April thru Sept) = 4,329 Watt array (GT "summer")
• 10,000 WH per day * 1/0.52 Pure off grid + battery inverter system eff * 1/3.0 hours per day sun (April thru Sept) = 6,410 Watt array (OG "summer")

And to give you an idea of battery sizing--This is for Flooded Cell Lead Acid for 2 says of stored energy, 50% max planned discharge for longer battery life (we can talk Lithium Ion and such if you wish):

• 10,000 WH per day * 1/0.52 off grid system eff * 2 days storage * 1/0.50 max discharge * 1/48 volts = 1,603 AH @ 48 volt bank

300 kWH per month is not a lot of power for a utility connected home... For a very energy efficient off grid home/cabin usually suggest looking at 100 kwH per month (3,300 WH per day) for the basics (LED lights, refrigerator, laptop computer, etc.). And that usually assumes that heating (cooking, hot water, space heating) is managed with wood/fossil fuels and such...

Before spending money on solar--Highly suggest conservation first (turning unused appliances "off" when not in use, most efficient lighting, fridge, insulation, perhaps induction cooker, solar hot water, etc.)...

Things are getting pretty tight over in Europe right now... I guess that space heating is going to be a critical need for most of northern Europe--And Solar is really expensive for any form of heating (and A/C or cooling).

Solar water/space heating can be effective... But between plumbing issues (pumps, water leaks, freezing weather, etc.) and such--May not be practical for many people.

Another option for heating is to look for mini-split "heat pump" systems... The new units are getting 2-3x as efficient for heating vs electric resistance strip heaters (1/2 to 1/3 the amount of electricity) used. And these newer units can still be more efficient than resistance heaters below freezing. There are also heat pump water heaters too... (Note that heat pump systems are not "cheap", and being more complex can be more difficult to repair).

Of course, trying to find heat pump heating/hot water systems is going to be difficult too (China's lock downs, supply chain issues, etc.).

Getting more data on your energy usage (if you don't have that yet) is important for solar and conservation (figure out where to spend your time and money).... Things like Kill-a-Watt type energy meters (for plug-in AC loads) and whole house monitors (the whole home usage, and some have options to measure major circuits like heating, A/C, dryer, electric stove) will be a great help:

https://www.amazon.co.uk/s?k=kill+a+watt+meter+uk&sprefix=kill+a+watt,aps,362 (plug-in energy meters)
https://www.amazon.co.uk/s?k=whole+house+energy+monitors+for+home+uk&sprefix=whole+house+energy+m,aps,319 (a couple whole home monitors--I am sure there are many others)

If you have a larger property--Solar Heating can be done even in very cold climates--And can be a DIY (do it yourself) type project:

https://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

However, for many folks, solar electric+conservation+heat pump+etc. is a more practical answer.

Note that solar hot water can work pretty well with some partial shading. Solar electric panels need exposure to full sun. Even a minor bit of shading (overhead power lines, neighbor's trees, chimneys, etc.) can easily cut solar harvest by 1/2 to even zero harvest.

I wish I had "better" answers for you and your family & friends. This could be ugly for the next few years.

-Bill

PS: Obviously, I know nothing about your energy usage--The 300 kWH per month number was used to show the math for a "moderately" sized system.

Energy usage is a highly personal set of choices and what works for me may not work for you. Knowing your energy usage (and lots of conservation) will help you design a system that meets your needs.

-BB