Posted by Jim Wilkins on July 8, 2019, 6:05 pm
On Monday, 8 July 2019 04:21:21 UTC+1, ads wrote:
Thank you for your on-topic questions Ads.
There's the off-grid table for 4KWh maximum daily usage and 8% solar
The way I see it, your backup system generator should be capable of
meeting the maximum instantaneous essential power demand, such as the
fridge starting while the microwave or other short-duration, high
demand cooking appliance is on. This assumes non-essential loads like
computers are off while cooking, and your batteries are depleted and
If the generator you have isn't powerful enough for both the fridge
needs a power monitor to show when it's running, and will turn back on
immediately after being unplugged while cooking, so you know to wait
while the pressure drops. I set a wind-up timer for 5 minutes when
switching from grid to inverter power or back, unless the fridge is
off. I need a visible monitor like a Kill-A-Watt because I can't hear
the fridge running over the inverter fan.
Unless you run the generator overnight the battery bank should support
at least the fridge and heating system, I assume from dusk to dawn at
the minimum, or however long it's likely to be unattended..
If those conditions are satisfied the alternate energy source can be
any size you're willing to allocate space and pay for, and the
generator can make up the difference.
A grid-powered DC supply draws minimal wattage when diode-OR'd with a
solar system set to a higher float voltage, unless the solar output is
insufficient. The ones I've built smoothly share the power, with grid
demand falling as solar output increases. This is a DC UPS with the
two diodes handling the switching and sharing. The higher voltage
supply's diode conducts while the lower voltage supply's diode blocks.
The AC-powered supply is set to the lower end of the float voltage
range, the solar controller maybe 0.1V or 0.2V higher but below the
level of significant gassing.
When I use a Radio Shack 13.8V, 19A DC switching supply as the
AC-powered source its input wattage drops to zero on a KAW when solar
takes over. I had to experimentally confirm that it's output current
wouldn't burn out the diode if the grid powering it came back up while
the battery was discharged to 10V.
Posted by Scottish Scientist on July 8, 2019, 9:23 pm
On Monday, 8 July 2019 19:04:45 UTC+1, Jim Wilkins wrote:
My designer's "Off-grid daily usage focus table" doesn't specify peak power
output because small systems can have peak power loads which are much and
unpredictably higher than the average power.
So I leave it to all you experienced home power experts to decide how you a
re going to manage your peak power arrangements.
What my designer offers is that providing the off-grid system's energy usag
e keeps within the limits of the maximum daily energy usage which your conf
iguration specifies and providing the specified backup power starts up auto
matically when needed -
My modelling suggests that would be to power up the specified back-up power
whenever the energy store drops to 75% of maximum capacity.
- then the off-grid system ought to be able to meet the daily usage require
ments, without normally needing any additional back-up power and with the b
attery never getting flat.
Exceptionally, when energy demand exceeds the designed for performance that
the configuration specifies, when the energy store empties (flat battery,
empty pumped hydro reservoir etc) then the specified system cannot be expec
ted to meet demand and then some other external power source is required to
import power from to meet the demand.
In the case of an off-grid system that may mean starting up a legacy fossil
-fuel generator, kept on stand-by to keep the lights and everything else on
When designing for a 100% renewable energy system the designer tables speci
fy a capacity of routine back-up power that should ideally be supplied usin
g renewable energy.
So that could be a generator that routinely burns renewable energy bio-fuel
- bio-ethanol or bio-diesel for example.
So for the off-grid scenario, there could be one smaller routine back-up ge
nerator burning bio-fuels that all being well completes your 100% renewable
Then you can complete your system for all eventualities and worst-case scen
arios, with a larger emergency back-up generator on stand-by to burn fossil
The off-grid table doesn't specify the emergency generator requirement for
the same reason that it doesn't specify peak power demand. That's for you t