Quick answer
Use a blended rate only for a rough first look, not the final proposal. It hides when electricity is cheap or expensive and can mix charges the battery can reduce with charges it cannot. For the client figures, use the real tariff and half-hourly load so you can show where the saving comes from.
No. A blended rate is fine for any initial calculations you run to see if a site is worth modelling, but don't build a client proposal on it. Blended rates roll everything like the client’s peak-rate electricity and your standing charges into one overall average.
That's not enough to work out what savings they’ll make because you don't know whether the battery can:
- Reduce peak demand charges
- Shift enough load out of expensive time-of-use periods
- Avoid enough import at the times when the client actually pays more for power
Use it and expect your proposal to come under proper scrutiny from an FD. Get the actual tariffs and the half-hourly data before you present any savings or payback figure to the client, so they can see where the saving comes from on their own tariff and load profile, not an average p/kWh.
Why blended rates hide where the saving comes from
The savings from batteries come from the gap between the price the client pays when the battery charges and the price they avoid when it discharges. Without being able to see that gap, the FD you’re presenting to can’t see how they’ll achieve the saving.
Let's say the client pays 12p per kWh overnight and 28p per kWh at peak. The battery charges during the cheaper overnight period and discharges during the peak period. On paper, you get a spread of up to 16p per kWh before energy losses.
But the client only gets that saving on the energy the battery can store electricity in the cheaper period and use it in a more expensive period. If there is not enough charge in the battery overnight, it might run out before the peak period is over.
Alternatively, the site might not use much power during peak periods. In these cases, there’s not much expensive grid import for the battery to replace, so it uses its energy to export at the lower export rate.
Clients need to see how you work this out and you can’t show them with a blended rate because they can’t see the mechanics behind the saving.
What to use for the final figures
Use at least 12 months of half-hourly data and model the client’s usage against the actual import rates.
You will normally need:
- Each import rate and the times it applies
- At least 12 months of half-hourly electricity data
- Battery power in kW and usable capacity in kWh
- Charge and discharge efficiency
- Minimum state of charge and any reserve
- Site import and export limits
- Controller, software and other running costs
Start by running the historical load through the tariff without putting the battery in your model, and compare the result against the client's invoices from the same period. If the difference is too large to explain with billing dates, rounding or missing line items, check the tariff bands, the meter data and the pass-through charges. This can be where you find the mistake before it gets into the proposal.
Next, run the same half-hourly data, this time putting the battery in your model. Use the battery size, efficiency, reserve and site limits you plan to quote. Compare the two results to see how much the battery saves by charging in cheaper periods and discharging in more expensive ones.
Take off the controller, software and other running costs before the saving goes into the proposal. This is your client's net annual saving.
Related questions
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