Sizing PV and Battery Under a No-Export Rule at a Warehouse
A 250,000 ft² distribution warehouse has a 1.2 MW peak demand, 4,000 MWh annual consumption, and a load profile that peaks between 15:00 and 18:00 on weekdays. The utility demand charge is $16/kW with an 80% ratchet for 12 months, and energy costs $0.08/kWh. The utility does not allow export. A developer proposes a 500 kW roof-mounted PV array and a 250 kW/500 kWh battery. The warehouse minimum daytime load is about 400 kW, and weekend load is 150 kW. The roof has 60,000 ft² of usable area. The owner wants to cut demand charges and has also received a proposal for LED lighting and dock-door seals. The battery would be charged from PV and, if needed, off-peak grid energy. The owner asks whether the PV should be sized at 500 kW and how the battery should be dispatched.
1. Should the PV array be sized at 500 kW? Use the load data and the no-export rule to justify the recommendation.
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Use 15-minute load data for at least a year. The no-export rule means PV output must stay below the site load at every interval. The weekday minimum is about 400 kW, but the weekend minimum is 150 kW, and PV output can exceed that on a clear weekend day. A 500 kW array will therefore export or require curtailment unless the inverter is limited or a battery absorbs the excess.
Size the array to the reliable minimum load with a margin, or use a 500 kW array with an export-limiting controller and a battery charging strategy. Check roof area: 60,000 ft² may fit 500 kW, but leave access and structural margins. The better first answer is not 500 kW; it is a load-following size, perhaps 350-400 kW, plus a curtailment plan. Confirm the interconnection rule and the utility's treatment of inadvertent export.
2. Design a battery dispatch strategy for demand reduction. What are the capacity limits and ratchet risks?
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A 250 kW/500 kWh battery can shave 200 kW for 2.5 hours or about 167 kW for 3 hours, ignoring efficiency. The 15:00-18:00 peak is three hours, so a 200 kW shave would need 600 kWh; either accept a shorter shave or buy more energy capacity. Dispatch should forecast the peak, charge from PV midday or off-peak, and reserve enough energy to cover the peak plus a safety margin. It must not charge during the peak and must not export.
Demand savings at 200 kW are about 200 x $16 x 12 = $38,400 per year, but the 80% ratchet is the risk: one missed peak can set a high demand for 12 months. Use controls with a demand forecast, a state-of-charge reserve, and a manual override. Meter and verify the demand reduction at the utility meter.
3. Screen the PV, battery, and efficiency options economically. What risks and sequencing would you present to the owner?
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Screen efficiency first. LED lighting and dock-door seals usually have simple paybacks under three years and reduce load before generation is sized. For PV, 500 kW at about $1.20/W is $600,000; at about 1,800 kWh/kW-year it produces roughly 900 MWh/year, worth $72,000/year at $0.08/kWh, an 8-year simple payback before incentives and before curtailment. If the no-export limit cuts output, the payback stretches. For the battery, $250,000 installed with $38,000/year demand savings is about a 6.5-year payback, but that assumes perfect dispatch every month.
Include degradation, O&M, inverter replacement, tariff changes, and demand-ratchet risk. Compare the portfolio, not just the PV. A smaller PV array plus battery and efficiency measures may capture more demand savings with less curtailment risk than a 500 kW array alone.