Community & shared solar

Subscription size kWh calculator

Find the exact subscription size in kWh you need to hit your target offset, with allocation losses and a safety margin built in.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your usage
Find monthly usage on a recent bill, or annual usage from a 12-month summary.
3 Offset target and losses
100% aims to fully offset your usage. Some subscribers choose 50% to lower cost.
Transmission and accounting loss between the array and your meter. Ask your provider, or use 3% if unknown.
4 Safety margin and growth Optional
Extra buffer for usage growth or array underperformance. 5% to 10% is common.
Rough usage increase to account for upcoming electric vehicle or heat pump additions.

How it works

Three steps to your subscription size

Step 1

Enter your usage and target offset

Use your actual annual usage from a bill summary if you have it. Most subscribers target 100%, though some choose a partial offset. See the 50% offset calculator if you're weighing a partial subscription.

Step 2

Add the allocation loss factor

Ask your provider for their specific loss factor, or use 3% as a reasonable default if you don't have one yet. This step is what prevents under-subscribing without realizing it.

Step 3

Check the size against provider limits

Some providers cap how large or small a subscription can be. Compare your result against the minimum and maximum subscription size tools before enrolling.

Understanding subscription sizing

Why the target kWh isn't the same as the subscription kWh

Your goal is usually to offset a percentage of your actual usage, but the subscription size you sign up for needs to be somewhat larger than that target because of allocation losses between the shared array and your meter. A household targeting a full 10,800 kWh offset with a 3% loss factor needs to subscribe to roughly 11,134 kWh to actually net that amount in credits. Once you know that number, check it against your expected bill impact with the subscription savings calculator. A rooftop system's sizing follows the same annual-usage logic without the allocation loss step, covered by the system size by annual kWh usage tool.

What the safety margin actually protects against

A safety margin covers two separate risks: your usage creeping up over time, and the shared array underperforming its projected output in a weaker sun year. A 5% to 10% margin is common practice, though it does mean paying for slightly more subscription than your current usage strictly requires. See how that translates into monthly credits with the credit allocation estimator. The Department of Energy's community solar basics guide covers the underlying allocation and credit mechanism this margin is built to protect against.

Planning around a future EV or heat pump

Adding an electric vehicle typically raises household usage by 2,500 to 4,000 kWh a year depending on driving distance, while a heat pump conversion can add 20% or more to annual usage in colder climates. If either is on your near-term plans, size your subscription with that future load in mind rather than resizing later, since some contracts lock the subscription for the full term. The DSIRE database publishes the current rules, so confirm the details there before you file anything.

What to do with the number this tool gives you

Bring your recommended subscription size to the provider during enrollment and confirm it fits within their available capacity. If your number lands close to their minimum or maximum threshold, ask about adjusting your safety margin slightly to fit cleanly within their program's limits, using the minimum subscription size tool as a reference point. Your target offset percentage feeds into this same calculation too, covered by the bill offset percentage calculator if you want to double check that piece separately. Check the DSIRE database for your state's typical subscription size rules too, since minimum and maximum thresholds vary by program.

Questions

Frequently asked

Multiply your annual electricity usage in kWh by your target offset percentage, then add a small margin for allocation losses, usually 2% to 5%. A household using 10,800 kWh a year targeting a 100% offset should subscribe to roughly 11,100 to 11,700 kWh depending on the provider's loss factor.

Some energy is lost between the shared solar farm and your meter through transmission and allocation accounting, typically 2% to 8% depending on the utility and provider. Subscribing to slightly more than your target usage accounts for this gap so your actual credited amount matches your goal.

Subscribing to more than your usage still credits your bill for every kWh, but unused credit either rolls over or, in rare cases, goes unused depending on your utility's rules. Subscribing to less than your usage simply means a smaller share of your bill gets offset, which lowers your savings but isn't a problem.

Many providers allow you to adjust your subscription size once or twice a year, though some contracts lock the size for the full term. Check your enrollment agreement, and ask the provider directly if you expect your usage to change significantly, such as after adding an EV or a heat pump.

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