diy solar

diy solar

Calculating Battery Bank Needs/Size

How is on-demand more efficient?
You have to pay for the energy to heat the water you use either way.

What fraction of total energy is lost through insulation, vs. used to heat the water in the first place?

For on-demand, you're going to get the power from grid or gas source.
With tank, you can gradually heat it from PV. A water heater is a far cheaper battery than lead-acid or lithium.

Instant-on needs low thermal resistance between heater and water, so I understand they require periodic descaling.

Use dollars to compare things.
 
Hedges said:
How is on-demand more efficient?

What fraction of total energy is lost through insulation, vs. used to heat the water in the first place?
Click to expand...
Standby loss is 1 to 2.5 kWh per day. Your choice if you find that material or not. A lot of people complaining about their inverter losses in the same range.

www.viessmann.co.uk

How long does water stay hot in a tank? | Viessmann UK

When your boiler heats up water, it stays in a hot water tank until it needs to be used. But how long can a tank keep this water warm for? Find out here.
www.viessmann.co.uk www.viessmann.co.uk
 
If I pay $0.25/kWh off-peak, 2.5 kWh/day is $0.63/day, $20/month.
Don't think you can justify price of on-demand water heater (or periodic maintenance) for that.

Since I have gas, I only pay about $8/month. Not just standby loss, but water heating and cooking too.

Inverters, it matters because it drains battery.
Even if you're off grid, get an electric storage water heater, and power it with surplus electricity during the day.
 
Hedges said:
If I pay $0.25/kWh off-peak, 2.5 kWh/day is $0.63/day, $20/month.
Don't think you can justify price of on-demand water heater (or periodic maintenance) for that.

Since I have gas, I only pay about $8/month. Not just standby loss, but water heating and cooking too.

Inverters, it matters because it drains battery.
Even if you're off grid, get an electric storage water heater, and power it with surplus electricity during the day.
Click to expand...
Where are you located? My only gas usage comes from this water heater and I pay $50-60/mo.
 
DIYrich said:
A grid connected battery charger only, with the inverter only connected to the battery, is one way to do it.
Click to expand...
Do you mean a charger connect to the grid that feeds power to a battery bank to charge the batteries when PV is low/unavailable?
 
TheHappyNomads said:
Where are you located? My only gas usage comes from this water heater and I pay $50-60/mo.
Click to expand...

San Jose.
Current gas charges $11.37

Maybe I don't bathe as often as you do? 1686520644470.png
Or you have a larger household?
 
TheHappyNomads said:
Moving away from natural gas usage. Water heater tanks are incredibly inefficient.
Click to expand...

DIYrich said:
On demand water heater can be more efficient, but you usually need natural gas or propane.
Click to expand...

Electric water heater is near 100% efficient, only I^2R losses in the wires. Tank type could have lower loss in wires than instant.
Insulation can be quite good.

Heat pump would make more hot for less electric, but costs more $$ and steals the heat from its surroundings.

But gas is still much cheaper than electricity.
 
Last edited:
TheHappyNomads said:
Thank you all for your helpful responses. I should be/should have been more clear. Our house is located in the heart of the SF Bay Area and so I'm not really able to/trying to be 100% off grid and sufficiently detached from the grid. My main goal is to use as much solar as possible to minimize my electric bills as much as possible. Our water heater and small living room fireplace insert are natural gas. I would love to replace the water heater tank with an on-demand system in the future since we only have one bathroom. After doing some research, it seems to me that it doesn't make sense to connect to the grid with the NEMA 3.0 contract in place for newcomers to the solar game. Plus, it affords the option to use non-UL listed inverters like the LV6548 and other components that I don't need/can't permit. Currently, we pay more for electric delivery services to our house from PG&E than the actual amount of electricity used. Hope this makes more sense and I apologize for the lack of clarity that led to generator/backup comments even though I greatly appreciate them.
Click to expand...
It might still be prudent to have a system capable of some grid independence. Long term grid outages from wildfires and other causes are far from unknown.
These outages may last for extended periods, the way governments everywhere seems to be descending into chaos and madness.

On demand gas water heating is ideal where there can be long periods of not requiring hot water. For example a working couple without children where the house is empty during the day. If the house is 100% occupied, say a retired couple that are home most of the time, or a young family, then an on demand gas water heater probably offers less of an advantage.
 
Last edited:
I'd say more like if you only use hot water weekly or monthly, like a get-away.
Storage water heaters don't loose all that much. Gas, I would guess more due to the flue. A few units have a thermostatic flapper. Electric can be insulated any amount.

A batteries-optional hybrid inverter is appealing. You can add batteries later if you want the backup, and you may get batteryless backup in the mean time. I think several from MPP to SolArk offer that. I'm looking forward to specs and price for SMA's version. For any of them, battery cost is key.

Discover the Sunny Boy Smart Energy | SMA America

The hybrid inverter at the heart of the SMA Energy System, with three backup options: Sunny Boy Smart Energy ► Discover now!
www.sma-america.com www.sma-america.com
 
Personally, I would start with one battery around 300ah and see how much more you need after running it for a couple weeks. As others have mentioned, battery size needs are wildly different for every household.

You may find that one battery does enough for you, and just supplement the rest with grid power for a few hours each morning.
 
You must log in or register to reply here.
Back
Top