Charging solar batteries from the grid during free power hours

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aaristotle

142 posts

Master Geek

#318621 4-Feb-2025 17:06

With our power KWh rates increasing 50% over the last 6 months I am looking at solar to see if it would be cost effective.

I've used the RoarPower.nz solar system calculator which uses the Niwa Solarview data to model local solar generation, along with a year of my half hourly power usage readings to get the monthly usage and time of day averages, solar system specs, then it calculates overall cost of grid power.

The majority of my load is or can be shifted to the middle of the day, so it looks like a solar only system would have about a 4 year payback (Based on Micromall kits + $3k install), but savings after adding a 10KWh battery would take ~10 years to pay off the battery.

I see that some of the inverter/battery systems can be charged from the grid but I can't find any specification of what maximum rate the batteries would be charged at. Does anyone know what the maximum rate might be (ie compared to inverter size)?

Is it feasible to charge the batteries from the grid during free hour(s) of power and use this especially during the winter months when solar generation is lower and usage higher and most of the generation goes to self consumption?

Do any of the power suppliers have any policies/restrictions on doing this?

SomeoneSomewhere

1736 posts

Uber Geek

Lifetime subscriber

#3339207 4-Feb-2025 17:25

Inverters will generally specify maximum power input from the grid. It's typically pretty much nameplate power.

Putting cycles on your batteries unnecessarily is usually not a great idea; the cost per kWh transferred through the battery is usually upper single-digit cents, and you will end up having to replace the battery sooner if you cycled it daily. This is made even worse when you are trying to charge the battery at maximum rate.

More solar is likely to be significantly more cost effective, unless your lines company has a low cap on maximum export.

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aaristotle

142 posts

Master Geek

#3339220 4-Feb-2025 18:09

I had worked out if I could grid charge 15KWh (3phase x 5KW over a free hour of power) I could save about $750 annually, but didn't take into account probably adding 40% more charging cycles during the winter months, and shortening the life of the battery.

I'm in the Northpower area who have a $100 fee for up to 10KW, or $500 up to 100KW.

Using the RoarPower model 17KW of panels appears to break even on the power bill, however I will need to look at other suppliers prices to see if the buyback and usage charges make any difference to the model.

SomeoneSomewhere

1736 posts

Uber Geek

Lifetime subscriber

#3339222 4-Feb-2025 18:19

Three phase makes it easier to hit the required power quantities, but beware that if you import on one phase while exporting on another, the NZ metering code requires your lines company to charge you for the import and reimburse you (typically at a lower rate) on the export. That can significantly screw up plans for net metering if you don't have an inverter that can provide an unbalanced output to zero out unbalanced loads.

If you're going to consider near 100% offset and a battery large enough to handle that over the winter months, you've essentially gone off grid but spend ~$700/year for the privilege of having the grid there for emergencies.

In Northland I would expect that there wouldn't be substantially less solar in winter, and your demands would likely be higher in summer (aircon, not much need for heating).

Be wary of designing and investing for a provider's specific plan. The free-hour plans might not last that much longer if the providers keep making losses, and they will bake the energy costs for the free hour into the base plan.

aaristotle

142 posts

Master Geek

#3339330 4-Feb-2025 19:46

SomeoneSomewhere:

In Northland I would expect that there wouldn't be substantially less solar in winter, and your demands would likely be higher in summer (aircon, not much need for heating).

We don't air condition even though it has been getting up to around 25-27C most days this year, just open the windows and let the breeze blow through!

Based on the Solarview data for here, daily KWh generated in the winter are about 50% less than summer, so quite significant. Our winter usage is about 40% higher than summer, as even though we are known as the Winterless North it tends to drop to less than 10C on many nights and we do have the odd frosty 1C morning. We have a spa pool and swim spa, plus electric hot water and heat pump which are on timers so the free power climbs to about 35% over winter.

If it wasn't for the significant free power savings Electric Kiwi wouldn't be the cheapest supplier in our scenario.

johno1234

2612 posts

Uber Geek

#3339335 4-Feb-2025 20:11

LFP batteries should be good for 6000 cycles or 10 years. So you could cycle once per day and still reach the ten years well ahead of the 6000 cycles.

tweake

2225 posts

Uber Geek

#3339336 4-Feb-2025 20:13

while it rarely ever snows in northland, it still gets below freezing. typically work on a cold temp of 4c.

my heating bill is a whole lot higher than my aircon bill, and my heat pumps run almost every day of the year.